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Kaleidoscope
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This document is currently a work in progress. While you certainly won't be penalized for following the style described herein, it's still a moving target as of June 20, 2017.
Our style guide is based on the Google C++ style guide which was current as of June 2, 2017, but has been modified to better reflect the constraints of embedded development and the peculiarities of an Arduino-compatible environment.
C++ is the language of Arduino libraries, and as a consequence, in which Kaleidoscope was written in. As every C++ programmer knows, the language has many powerful features, but this power brings with it complexity, which in turn can make code more bug-prone and harder to read and maintain.
The goal of this guide is to manage this complexity by describing in detail the dos and don'ts of writing C++ code. These rules exist to keep the code base manageable while still allowing coders to use C++ language features productively.
Style, also known as readability, is what we call the conventions that govern our C++ code. The term Style is a bit of a misnomer, since these conventions cover far more than just source file formatting.
Note that this guide is not a C++ tutorial: we assume that the reader is familiar with the language.
Why do we have this document?
There are a few core goals that we believe this guide should serve. These are the fundamental **why**s that underlie all of the individual rules. By bringing these ideas to the fore, we hope to ground discussions and make it clearer to our broader community why the rules are in place and why particular decisions have been made. If you understand what goals each rule is serving, it should be clearer to everyone when a rule may be waived (some can be), and what sort of argument or alternative would be necessary to change a rule in the guide.
The goals of the style guide as we currently see them are as follows:
The benefit of a style rule must be large enough to justify asking all of our engineers to remember it. The benefit is measured relative to the codebase we would get without the rule, so a rule against a very harmful practice may still have a small benefit if people are unlikely to do it anyway. This principle mostly explains the rules we don't have, rather than the rules we do: for example, goto contravenes many of the following principles, but is already vanishingly rare, so the Style Guide doesn't discuss it.
Our codebase (and most individual components submitted to it) is expected to continue for quite some time. As a result, more time will be spent reading most of our code than writing it. We explicitly choose to optimize for the experience of our average software engineer reading, maintaining, and debugging code in our codebase rather than ease when writing said code. "Leave a trace for the reader" is a particularly common sub-point of this principle: When something surprising or unusual is happening in a snippet of code (for example, transfer of pointer ownership), leaving textual hints for the reader at the point of use is valuable (std::unique_ptr demonstrates the ownership transfer unambiguously at the call site). This is doubly true for libraries meant to be used by end-users, where the target audience is not software engineers, but novice users of the Arduino ecosystem. People for whom their Sketch may be the first program they ever write.
Using one style consistently through our codebase lets us focus on other (more important) issues. Consistency also allows for automation: tools that format your code or adjust your #includes only work properly when your code is consistent with the expectations of the tooling. In many cases, rules that are attributed to "Be Consistent" boil down to "Just pick one and stop worrying about it"; the potential value of allowing flexibility on these points is outweighed by the cost of having people argue over them.
Consistency with the way other Arduino libraries use C++ has value for the same reasons as consistency within our code base. Our libraries do not function in isolation, and they may very well be used together with libraries by the broader Arduino community. However, sometimes the idioms, or style used in other libraries do not fit the needs of our codebase. In those cases, it is appropriate to apply our own rules. Similarly, if a feature in the C++ standard solves a problem, or if some idiom is widely known and accepted, that's an argument for using it. However, sometimes standard features and idioms are flawed, or were just designed without our codebase's needs in mind. In those cases (as described below) it's appropriate to constrain or ban standard features.
C++ has features that are more surprising or dangerous than one might think at a glance. Some style guide restrictions are in place to prevent falling into these pitfalls. There is a high bar for style guide waivers on such restrictions, because waiving such rules often directly risks compromising program correctness.
C++ has features that may not be generally appropriate because of the complexity they introduce to the code. In widely used code, it may be more acceptable to use trickier language constructs, because any benefits of more complex implementation are multiplied widely by usage, and the cost in understanding the complexity does not need to be paid again when working with new portions of the codebase. When in doubt, waivers to rules of this type can be sought by asking your project leads. This is specifically important for our codebase because code ownership and team membership changes over time: even if everyone that works with some piece of code currently understands it, such understanding is not guaranteed to hold a few years from now. Not to mention that our target audience are average people, not average C++ programmers.
With a codebase spanning many plugins, and even more when we consider the broader Arduino ecosystem, some mistakes and simplifications for one engineer can become costly for many. For instance it's particularly important to avoid polluting the global namespace: name collisions across a codebase of dozens of repositories are difficult to work with and hard to avoid if everyone puts things into the global namespace.
Performance optimizations can sometimes be necessary and appropriate, even when they conflict with the other principles of this document.
The intent of this document is to provide maximal guidance with reasonable restriction. As always, common sense and good taste should prevail. By this we specifically refer to the established conventions of the entire community (both Kaleidoscope and Arduino communities), not just your personal preferences or those of your team. Be skeptical about and reluctant to use clever or unusual constructs: the absence of a prohibition is not the same as a license to proceed. Use your judgment, and if you are unsure, please don't hesitate to ask, to get additional input.
Before looking at the coding style guidelines, we must first talk about libraries. Every Kaleidoscope plugin is also an Arduino library. The core firmware is an Arduino library too. As such, libraries should follow the Arduino library specification (revision 2.1 or later), with a few additional recommendations:
We want to make it as easy for the consumers of the plugins to use them as possible. A big part of this is updating to a newer version of it. Using semantic versioning makes it easier for our users to keep up with the libraries.
It does not matter much what hardware the example is for, as long as there is one. Because the Arduino IDE will offer examples, having one is of great benefit: it is a starting point. An easily accessible starting point.
Documenting the interfaces of the library, how to use it, its dependencies, in a way that is meaningful for a novice user is a very strong recommendation. The README.md in a library should target a novice audience. Would one want or need to document parts of the code that is only meaningful for more advanced programmers, do so in the code.
The Kaleidoscope-Plugin repository is there to aid plugin and library developers. Use the tools it provides. They make everyone's life easier, and help maintain consistency across the board.
In general, every .cpp file should have an associated .h file. There are some common exceptions, such as unittests and examples.
Correct use of header files can make a huge difference to the readability, size and performance of your code.
The following rules will guide you through the various pitfalls of using header files.
Header files should be self-contained (compile on their own) and end in
.h. Non-header files that are meant for inclusion should end in.incand be used sparingly.
All header files should be self-contained. Users and refactoring tools should not have to adhere to special conditions to include the header. Specifically, a header should have header guards and include all other headers it needs.
Prefer placing the definitions for template and inline functions in the same file as their declarations. The definitions of these constructs must be included into every .cpp file that uses them, or the program may fail to link in some build configurations. If declarations and definitions are in different files, including the former should transitively include the latter. Do not move these definitions to separately included header files (-inl.h).
As an exception, a template that is explicitly instantiated for all relevant sets of template arguments, or that is a private implementation detail of a class, is allowed to be defined in the one and only .cpp file that instantiates the template.
There are rare cases where a file designed to be included is not self-contained. These are typically intended to be included at unusual locations, such as the middle of another file. They might not use header guards, and might not include their prerequisites. Name such files with the .inc extension. Use sparingly, and prefer self-contained headers when possible.
All header files should have #pragma once guards at the top to prevent multiple inclusion.
Avoid using forward declarations where possible. Just
#includethe headers you need.
Definition
A *"forward declaration"* is a declaration of a class, function, or template without an associated definition.
Pros
#includes force the compiler to open more files and process more input.#includes can force your code to be recompiled more often, due to unrelated changes in the header.Cons
#include is needed. Replacing an #include with a forward declaration can silently change the meaning of code.#includeing the header.Decision
#include that header.#include its header file.Please see Names and Order of Includes for rules about when to #include a header.
Define inline functions when required to do so in order to get the compiler to generate more compact code.
Definition
You can declare functions in a way that allows the compiler to expand them inline rather than calling them through the usual function call mechanism. Sometimes the compiler does this automatically, sometimes we need to instruct it explicitly for the sake of either performance, or code size.
Pros
Inlining a function can generate more efficient object code, as long as the inlined function is small. Feel free to inline accessors and mutators, and other short, performance-critical functions.
Cons
Overuse of inlining can actually make programs slower. Depending on a function's size, inlining it can cause the code size to increase or decrease. Inlining a very small accessor function will usually decrease code size while inlining a very large function can dramatically increase code size.
Decision
A decent rule of thumb is to not inline a function if it is more than 10 lines long. Beware of destructors, which are often longer than they appear because of implicit member- and base-destructor calls!
Another useful rule of thumb: it's typically not cost effective to inline functions with loops or switch statements (unless, in the common case, the loop or switch statement is never executed).
It is important to know that functions are not always inlined even if they are declared as such; for example, virtual and recursive functions are not normally inlined. Usually recursive functions should not be inline. The main reason for making a virtual function inline is to place its definition in the class, either for convenience or to document its behavior, e.g., for accessors and mutators.
Use standard order for readability and to avoid hidden dependencies: Related header, Arduino libraries, other libraries'
.h, your project's.h.
All libraries must have at least one header in their top-level src/ directory, to be included without any path components. This is the way Arduino finds libraries, and a limitation we must adhere to. These headers should - in general - include any other headers they may need, so that the consumer of the library only has to include one header. The name of this header must be the same as the name of the library.
The recommended naming is to prefix the library name with Kaleidoscope-.
If there is more than one header, they should be listed as descendants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory), and live in a Kaleidoscope subdirectory. For example, if we have a plugin named Kaleidoscope-Something, which has an additional header file other than Kaleidoscope-Something.h, it should be in src/Kaleidoscope/Something-Other.h, and be included as:
Having more than one level of subdirectories is not recommended.
In dir/foo.cpp or dir/foo_test.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:
dir2/foo2.h.h files..h files.With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cpp or dir/foo_test.cpp will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages.
dir/foo.cc and dir2/foo2.h are usually in the same directory (e.g. Kaleidoscope/Something_test.cpp and Kaleidoscope/Something.h), but may sometimes be in different directories too.
Within each section the includes should be ordered alphabetically.
You should include all the headers that define the symbols you rely upon, except in the unusual case of forward declarations . If you rely on symbols from bar.h, don't count on the fact that you included foo.h which (currently) includes bar.h: include bar.h yourself, unless foo.h explicitly demonstrates its intent to provide you the symbols of bar.h. However, any includes present in the related header do not need to be included again in the related cc (i.e., foo.cc can rely on foo.h's includes).
For example, the includes in Kaleidoscope-Something/src/Kaleidoscope/Something.cpp might look like this:
Exception
Sometimes, system-specific code needs conditional includes. Such code can put conditional includes after other includes. Of course, keep your system-specific code small and localized. Example:
With few exceptions, place code in a namespace. Namespaces should have unique names based on the project name, and possibly its path. Do not use using-directives (e.g.
using namespace foo). Do not use inline namespaces. For unnamed namespaces, see Unnamed Namespaces and Static Variables.
Definition
Namespaces subdivide the global scope into distinct, named scopes, and so are useful for preventing name collisions in the global scope.
Pros
Namespaces provide a method for preventing name conflicts in large programs while allowing most code to use reasonably short names.
For example, if two different projects have a class Foo in the global scope, these symbols may collide at compile time or at runtime. If each project places their code in a namespace, project1::Foo and project2::Foo are now distinct symbols that do not collide, and code within each project's namespace can continue to refer to Foo without the prefix.
Inline namespaces automatically place their names in the enclosing scope. Consider the following snippet, for example:
The expressions X::Y::foo() and X::foo() are interchangeable. Inline namespaces are primarily intended for ABI compatibility across versions.
Cons
Namespaces can be confusing, because they complicate the mechanics of figuring out what definition a name refers to.
Inline namespaces, in particular, can be confusing because names aren't actually restricted to the namespace where they are declared. They are only useful as part of some larger versioning policy.
In some contexts, it's necessary to repeatedly refer to symbols by their fully-qualified names. For deeply-nested namespaces, this can add a lot of clutter.
Decision
Namespaces should be used as follows:
Namespaces wrap the entire source file after includes, gflags definitions/declarations and forward declarations of classes from other namespaces.
```c++ // In the .h file namespace mynamespace {
// All declarations are within the namespace scope. // Notice the lack of indentation. class MyClass { public: ... void Foo(); };
} // namespace mynamespace ```
```c++ // In the .cc file namespace mynamespace {
// Definition of functions is within scope of the namespace. void MyClass::Foo() { ... }
} // namespace mynamespace ```
More complex .cpp files might have additional details, like flags or using-declarations.
```c++ #include "a.h"
DEFINE_FLAG(bool, someflag, false, "dummy flag");
namespace a {
using ::foo::bar;
...code for a... // Code goes against the left margin.
} // namespace a ```
std, including forward declarations of standard library classes. Declaring entities in namespace std is undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file.You may not use a using-directive to make all names from a namespace available.
```c++ // Forbidden – This pollutes the namespace. using namespace foo; ```
Do not use Namespace aliases at namespace scope in header files except in explicitly marked internal-only namespaces, because anything imported into a namespace in a header file becomes part of the public API exported by that file.
```c++ // Shorten access to some commonly used names in .cc files. namespace baz = ::foo::bar::baz; ```
```c++ // Shorten access to some commonly used names (in a .h file). namespace librarian { namespace impl { // Internal, not part of the API. namespace sidetable = ::pipeline_diagnostics::sidetable; } // namespace impl
inline void my_inline_function() { // namespace alias local to a function (or method). namespace baz = ::foo::bar::baz; ... } } // namespace librarian ```
When definitions in a
.cppfile do not need to be referenced outside that file, place them in an unnamed namespace or declare themstatic. Do not use either of these constructs in.hfiles.
Definition
All declarations can be given internal linkage by placing them in unnamed namespaces, and functions and variables can be given internal linkage by declaring them static. This means that anything you're declaring can't be accessed from another file. If a different file declares something with the same name, then the two entities are completely independent.
Decision
Use of internal linkage in .cc files is encouraged for all code that does not need to be referenced elsewhere. Do not use internal linkage in .h files.
Format unnamed namespaces like named namespaces. In the terminating comment, leave the namespace name empty:
Prefer placing nonmember functions in a namespace; use completely global functions rarely. Prefer grouping functions with a namespace instead of using a class as if it were a namespace. Static methods of a class should generally be closely related to instances of the class or the class's static data.
Pros
Nonmember and static member functions can be useful in some situations. Putting nonmember functions in a namespace avoids polluting the global namespace.
Cons
Nonmember and static member functions may make more sense as members of a new class, especially if they access external resources or have significant dependencies.
Decision
Sometimes it is useful to define a function not bound to a class instance. Such a function can be either a static member or a nonmember function. Nonmember functions should not depend on external variables, and should nearly always exist in a namespace. Rather than creating classes only to group static member functions which do not share static data, use namespaces instead. For a header myproject/foo_bar.h, for example, write
instead of
If you define a nonmember function and it is only needed in its .cpp file, use internal linkage to limit its scope.
Place a function's variables in the narrowest scope possible, and initialize variables in the declaration.
C++ allows you to declare variables anywhere in a function. We encourage you to declare them in as local a scope as possible, and as close to the first use as possible. This makes it easier for the reader to find the declaration and see what type the variable is and what it was initialized to. In particular, initialization should be used instead of declaration and assignment, e.g.:
Variables needed for if, while and for statements should normally be declared within those statements, so that such variables are confined to those scopes. E.g.:
There is one caveat: if the variable is an object, its constructor is invoked every time it enters scope and is created, and its destructor is invoked every time it goes out of scope.
It may be more efficient to declare such a variable used in a loop outside that loop:
Variables of class type with static storage duration are forbidden: they cause hard-to-find bugs due to indeterminate order of construction and destruction. However, such variables are allowed if they are
constexpr: they have no dynamic initialization or destruction.
Objects with static storage duration, including global variables, static variables, static class member variables, and function static variables, must be Plain Old Data (POD): only ints, chars, floats, or pointers, or arrays/structs of POD.
The order in which class constructors and initializers for static variables are called is only partially specified in C++ and can even change from build to build, which can cause bugs that are difficult to find. Therefore in addition to banning globals of class type, we do not allow non-local static variables to be initialized with the result of a function, unless that function (such as getenv(), or getpid()) does not itself depend on any other globals. However, a static POD variable within function scope may be initialized with the result of a function, since its initialization order is well-defined and does not occur until control passes through its declaration.
Likewise, global and static variables are destroyed when the program terminates, regardless of whether the termination is by returning from main() or by calling exit(). The order in which destructors are called is defined to be the reverse of the order in which the constructors were called. Since constructor order is indeterminate, so is destructor order. For example, at program-end time a static variable might have been destroyed, but code still running - perhaps in another thread - tries to access it and fails. Or the destructor for a static string variable might be run prior to the destructor for another variable that contains a reference to that string.
One way to alleviate the destructor problem is to terminate the program by calling quick_exit() instead of exit(). The difference is that quick_exit() does not invoke destructors and does not invoke any handlers that were registered by calling atexit(). If you have a handler that needs to run when a program terminates via quick_exit() (flushing logs, for example), you can register it using at_quick_exit(). (If you have a handler that needs to run at both exit() and quick_exit(), you need to register it in both places.)
As a result we only allow static variables to contain POD data. This rule completely disallows std::vector (use C arrays instead), or string (use const char []).
If you need a static or global variable of a class type, consider initializing a pointer (which will never be freed), from either your main() function or from pthread_once(). Note that this must be a raw pointer, not a "smart" pointer, since the smart pointer's destructor will have the order-of-destructor issue that we are trying to avoid.
Classes are the fundamental unit of code in C++. Naturally, we use them extensively. This section lists the main dos and don'ts you should follow when writing a class.
Avoid virtual method calls in constructors, and avoid initialization that can fail if you can't signal an error.
Definition
It is possible to perform arbitrary initialization in the body of the constructor.
Pros
const and may also be easier to use with standard containers or algorithms.Cons
bool IsValid() state checking mechanism (or similar) which is easy to forget to call.Decision
Constructors should never call virtual functions. If appropriate for your code , terminating the program may be an appropriate error handling response. Otherwise, consider a factory function or Init() method. Avoid Init() methods on objects with no other states that affect which public methods may be called (semi-constructed objects of this form are particularly hard to work with correctly).
Do not define implicit conversions. Use the
explicitkeyword for conversion operators and single-argument constructors.
Definition
Implicit conversions allow an object of one type (called the source type) to be used where a different type (called the destination type) is expected, such as when passing an int argument to a function that takes a double parameter.
In addition to the implicit conversions defined by the language, users can define their own, by adding appropriate members to the class definition of the source or destination type. An implicit conversion in the source type is defined by a type conversion operator named after the destination type (e.g. operator bool()). An implicit conversion in the destination type is defined by a constructor that can take the source type as its only argument (or only argument with no default value).
The explicit keyword can be applied to a constructor or (since C++11) a conversion operator, to ensure that it can only be used when the destination type is explicit at the point of use, e.g. with a cast. This applies not only to implicit conversions, but to C++11's list initialization syntax:
This kind of code isn't technically an implicit conversion, but the language treats it as one as far as explicit is concerned.
Pros
Cons
explicit, there's no reliable way to tell whether it's intended to define an implicit conversion, or the author simply forgot to mark it.Decision
Type conversion operators, and constructors that are callable with a single argument, must be marked explicit in the class definition. As an exception, copy and move constructors should not be explicit, since they do not perform type conversion. Implicit conversions can sometimes be necessary and appropriate for types that are designed to transparently wrap other types. In that case, contact your project leads to request a waiver of this rule.
Constructors that cannot be called with a single argument should usually omit explicit. Constructors that take a single std::initializer_list parameter should also omit explicit, in order to support copy-initialization (e.g. MyType m = {1, 2};).
Support copying and/or moving if these operations are clear and meaningful for your type. Otherwise, disable the implicitly generated special functions that perform copies and moves.
Definition
A copyable type allows its objects to be initialized or assigned from any other object of the same type, without changing the value of the source. For user-defined types, the copy behavior is defined by the copy constructor and the copy-assignment operator. string is an example of a copyable type.
A movable type is one that can be initialized and assigned from temporaries (all copyable types are therefore movable). std::unique_ptr<int> is an example of a movable but not copyable type. For user-defined types, the move behavior is defined by the move constructor and the move-assignment operator.
The copy/move constructors can be implicitly invoked by the compiler in some situations, e.g. when passing objects by value.
Pros
Objects of copyable and movable types can be passed and returned by value, which makes APIs simpler, safer, and more general. Unlike when passing objects by pointer or reference, there's no risk of confusion over ownership, lifetime, mutability, and similar issues, and no need to specify them in the contract. It also prevents non-local interactions between the client and the implementation, which makes them easier to understand, maintain, and optimize by the compiler. Further, such objects can be used with generic APIs that require pass-by-value, such as most containers, and they allow for additional flexibility in e.g., type composition.
Copy/move constructors and assignment operators are usually easier to define correctly than alternatives like Clone(), CopyFrom() or Swap(), because they can be generated by the compiler, either implicitly or with = default. They are concise, and ensure that all data members are copied. Copy and move constructors are also generally more efficient, because they don't require heap allocation or separate initialization and assignment steps, and they're eligible for optimizations such as copy elision.
Move operations allow the implicit and efficient transfer of resources out of rvalue objects. This allows a plainer coding style in some cases.
Cons
Some types do not need to be copyable, and providing copy operations for such types can be confusing, nonsensical, or outright incorrect. Types representing singleton objects (Registerer), objects tied to a specific scope (Cleanup), or closely coupled to object identity (Mutex) cannot be copied meaningfully. Copy operations for base class types that are to be used polymorphically are hazardous, because use of them can lead to object slicing. Defaulted or carelessly-implemented copy operations can be incorrect, and the resulting bugs can be confusing and difficult to diagnose.
Copy constructors are invoked implicitly, which makes the invocation easy to miss. This may cause confusion for programmers used to languages where pass-by-reference is conventional or mandatory. It may also encourage excessive copying, which can cause performance problems.
Decision
Provide the copy and move operations if their meaning is clear to a casual user and the copying/moving does not incur unexpected costs. If you define a copy or move constructor, define the corresponding assignment operator, and vice-versa. If your type is copyable, do not define move operations unless they are significantly more efficient than the corresponding copy operations. If your type is not copyable, but the correctness of a move is obvious to users of the type, you may make the type move-only by defining both of the move operations.
If your type provides copy operations, it is recommended that you design your class so that the default implementation of those operations is correct. Remember to review the correctness of any defaulted operations as you would any other code, and to document that your class is copyable and/or cheaply movable if that's an API guarantee.
Due to the risk of slicing, avoid providing an assignment operator or public copy/move constructor for a class that's intended to be derived from (and avoid deriving from a class with such members). If your base class needs to be copyable, provide a public virtual Clone() method, and a protected copy constructor that derived classes can use to implement it.
If you do not want to support copy/move operations on your type, explicitly disable them using = delete in the public: section:
Use a
structonly for passive objects that carry data; everything else is aclass.
The struct and class keywords behave almost identically in C++. We add our own semantic meanings to each keyword, so you should use the appropriate keyword for the data-type you're defining.
structs should be used for passive objects that carry data, and may have associated constants, but lack any functionality other than access/setting the data members. The accessing/setting of fields is done by directly accessing the fields rather than through method invocations. Methods should not provide behavior but should only be used to set up the data members, e.g., constructor, destructor, Initialize(), Reset(), Validate().
If more functionality is required, a class is more appropriate. If in doubt, make it a class.
For consistency with STL, you can use struct instead of class for functors and traits.
Note that member variables in structs and classes have different naming rules.
Composition is often more appropriate than inheritance. When using inheritance, make it
public.
Definition
When a sub-class inherits from a base class, it includes the definitions of all the data and operations that the parent base class defines. In practice, inheritance is used in two major ways in C++: implementation inheritance, in which actual code is inherited by the child, and interface inheritance , in which only method names are inherited.
Pros
Implementation inheritance reduces code size by re-using the base class code as it specializes an existing type. Because inheritance is a compile-time declaration, you and the compiler can understand the operation and detect errors. Interface inheritance can be used to programmatically enforce that a class expose a particular API. Again, the compiler can detect errors, in this case, when a class does not define a necessary method of the API.
Cons
For implementation inheritance, because the code implementing a sub-class is spread between the base and the sub-class, it can be more difficult to understand an implementation. The sub-class cannot override functions that are not virtual, so the sub-class cannot change implementation. The base class may also define some data members, so that specifies physical layout of the base class.
Decision
All inheritance should be public. If you want to do private inheritance, you should be including an instance of the base class as a member instead.
Do not overuse implementation inheritance. Composition is often more appropriate. Try to restrict use of inheritance to the *"is-a"* case: Bar subclasses Foo if it can reasonably be said that Bar "is a kind of" Foo.
Make your destructor virtual if necessary. If your class has virtual methods, its destructor should be virtual.
Limit the use of protected to those member functions that might need to be accessed from subclasses. Note that data members should be private.
Explicitly annotate overrides of virtual functions or virtual destructors with an override or (less frequently) final specifier. Older (pre-C++11) code will use the virtual keyword as an inferior alternative annotation. For clarity, use exactly one of override, final, or virtual when declaring an override. Rationale: A function or destructor marked override or final that is not an override of a base class virtual function will not compile, and this helps catch common errors. The specifiers serve as documentation; if no specifier is present, the reader has to check all ancestors of the class in question to determine if the function or destructor is virtual or not.
Only very rarely is multiple implementation inheritance actually useful. We allow multiple inheritance only when at most one of the base classes has an implementation; all other base classes must be pure interface classes tagged with the
Interfacesuffix.
Definition
Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are pure interfaces and those that have an implementation.
Pros
Multiple implementation inheritance may let you re-use even more code than single inheritance (see Inheritance).
Cons
Only very rarely is multiple implementation inheritance actually useful. When multiple implementation inheritance seems like the solution, you can usually find a different, more explicit, and cleaner solution.
Decision
Multiple inheritance is allowed only when all superclasses, with the possible exception of the first one, are pure interfaces. In order to ensure that they remain pure interfaces, they must end with the Interface suffix.
Classes that satisfy certain conditions are allowed, but not required, to end with an
Interfacesuffix.
Definition
A class is a pure interface if it meets the following requirements:
Interface suffix.An interface class can never be directly instantiated because of the pure virtual method(s) it declares. To make sure all implementations of the interface can be destroyed correctly, the interface must also declare a virtual destructor (in an exception to the first rule, this should not be pure). See Stroustrup, <cite>The C++ Programming Language</cite>, 3rd edition, section 12.4 for details.
Pros
Tagging a class with the Interface suffix lets others know that they must not add implemented methods or non static data members. This is particularly important in the case of multiple inheritance. Additionally, the interface concept is already well-understood by Java programmers.
Cons
The Interface suffix lengthens the class name, which can make it harder to read and understand. Also, the interface property may be considered an implementation detail that shouldn't be exposed to clients.
Decision
A class may end with Interface only if it meets the above requirements. We do not require the converse, however: classes that meet the above requirements are not required to end with Interface.
Overload operators judiciously. Do not create user-defined literals.
Definition
C++ permits user code to declare overloaded versions of the built-in operators using the operator keyword, so long as one of the parameters is a user-defined type. The operator keyword also permits user code to define new kinds of literals using operator"", and to define type-conversion functions such as operator bool().
Pros
Operator overloading can make code more concise and intuitive by enabling user-defined types to behave the same as built-in types. Overloaded operators are the idiomatic names for certain operations (e.g. ==, <, =, and <<), and adhering to those conventions can make user-defined types more readable and enable them to interoperate with libraries that expect those names.
User-defined literals are a very concise notation for creating objects of user-defined types.
Cons
foo < bar may do one thing, while &foo < &bar does something totally different.& can cause the same code to have different meanings depending on whether the overload declaration is visible. Overloads of &&, ||, and , (comma) cannot match the evaluation-order semantics of the built-in operators.Decision
Define overloaded operators only if their meaning is obvious, unsurprising, and consistent with the corresponding built-in operators. For example, use | as a bitwise- or logical-or, not as a shell-style pipe.
Define operators only on your own types. More precisely, define them in the same headers, .cpp files, and namespaces as the types they operate on. That way, the operators are available wherever the type is, minimizing the risk of multiple definitions. If possible, avoid defining operators as templates, because they must satisfy this rule for any possible template arguments. If you define an operator, also define any related operators that make sense, and make sure they are defined consistently. For example, if you overload <, overload all the comparison operators, and make sure < and > never return true for the same arguments.
Prefer to define non-modifying binary operators as non-member functions. If a binary operator is defined as a class member, implicit conversions will apply to the right-hand argument, but not the left-hand one. It will confuse your users if a < b compiles but b < a doesn't.
Don't go out of your way to avoid defining operator overloads. For example, prefer to define ==, =, and <<, rather than Equals(), CopyFrom(), and PrintTo(). Conversely, don't define operator overloads just because other libraries expect them. For example, if your type doesn't have a natural ordering, but you want to store it in a std::set, use a custom comparator rather than overloading <.
Do not overload &&, ||, , (comma), or unary &. Do not overload operator"", i.e. do not introduce user-defined literals.
Type conversion operators are covered in the section on implicit conversions. The = operator is covered in the section on copy constructors . Overloading << for use with streams is covered in the section on streams. See also the rules on function overloading , which apply to operator overloading as well.
Make data members
private, unless they arestatic const(and follow the naming convention for constants). <strike>For technical reasons, we allow data members of a test fixture class to beprotectedwhen using Google Test).</strike>
Group similar declarations together, placing public parts earlier.
A class definition should usually start with a public: section, followed by protected:, then private:. Omit sections that would be empty.
Within each section, generally prefer grouping similar kinds of declarations together, and generally prefer the following order: types (including typedef, using, and nested structs and classes), constants, factory functions, constructors, assignment operators, destructor, all other methods, data members.
Do not put large method definitions inline in the class definition. Usually, only trivial or performance-critical, and very short, methods may be defined inline. See Inline Functions for more details.
When defining a function, parameter order is: inputs, then outputs.
Parameters to C/C++ functions are either input to the function, output from the function, or both. Input parameters are usually values or const references, while output and input/output parameters will be pointers to non-const. When ordering function parameters, put all input-only parameters before any output parameters. In particular, do not add new parameters to the end of the function just because they are new; place new input-only parameters before the output parameters.
This is not a hard-and-fast rule. Parameters that are both input and output (often classes/structs) muddy the waters, and, as always, consistency with related functions may require you to bend the rule.
Prefer small and focused functions.
We recognize that long functions are sometimes appropriate, so no hard limit is placed on functions length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.
Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code.
You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code: if working with such a function proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the function into smaller and more manageable pieces.
All parameters passed by reference must be labeled
const.
Definition
In C, if a function needs to modify a variable, the parameter must use a pointer, eg int foo(int *pval). In C++, the function can alternatively declare a reference parameter: int foo(int &val).
Pros
Defining a parameter as reference avoids ugly code like (*pval)++. Necessary for some applications like copy constructors. Makes it clear, unlike with pointers, that a null pointer is not a possible value.
Cons
References can be confusing, as they have value syntax but pointer semantics.
Decision
Within function parameter lists all references must be const:
In fact it is a very strong convention in Google code that input arguments are values or const references while output arguments are pointers. Input parameters may be const pointers, but we never allow non-const reference parameters except when required by convention, e.g., swap().
However, there are some instances where using const T* is preferable to const T& for input parameters. For example:
Remember that most of the time input parameters are going to be specified as const T&. Using const T* instead communicates to the reader that the input is somehow treated differently. So if you choose const T* rather than const T&, do so for a concrete reason; otherwise it will likely confuse readers by making them look for an explanation that doesn't exist.
Use overloaded functions (including constructors) only if a reader looking at a call site can get a good idea of what is happening without having to first figure out exactly which overload is being called.
Definition
You may write a function that takes a const string& and overload it with another that takes const char*.
Pros
Overloading can make code more intuitive by allowing an identically-named function to take different arguments. It may be necessary for templatized code, and it can be convenient for Visitors.
Cons
If a function is overloaded by the argument types alone, a reader may have to understand C++'s complex matching rules in order to tell what's going on. Also many people are confused by the semantics of inheritance if a derived class overrides only some of the variants of a function.
Decision
If you want to overload a function, consider qualifying the name with some information about the arguments, e.g., AppendString(), AppendInt() rather than just Append(). If you are overloading a function to support variable number of arguments of the same type, consider making it take a std::vector so that the user can use an initializer list to specify the arguments.
Default arguments are allowed on non-virtual functions when the default is guaranteed to always have the same value. Follow the same restrictions as for function overloading, and prefer overloaded functions if the readability gained with default arguments doesn't outweigh the downsides below.
Pros
Often you have a function that uses default values, but occasionally you want to override the defaults. Default parameters allow an easy way to do this without having to define many functions for the rare exceptions. Compared to overloading the function, default arguments have a cleaner syntax, with less boilerplate and a clearer distinction between 'required' and 'optional' arguments.
Cons
Defaulted arguments are another way to achieve the semantics of overloaded functions, so all the reasons not to overload functions apply.
The defaults for arguments in a virtual function call are determined by the static type of the target object, and there's no guarantee that all overrides of a given function declare the same defaults.
Default parameters are re-evaluated at each call site, which can bloat the generated code. Readers may also expect the default's value to be fixed at the declaration instead of varying at each call.
Function pointers are confusing in the presence of default arguments, since the function signature often doesn't match the call signature. Adding function overloads avoids these problems.
Decision
Default arguments are banned on virtual functions, where they don't work properly, and in cases where the specified default might not evaluate to the same value depending on when it was evaluated. (For example, don't write void f(int n = counter++);.)
In some other cases, default arguments can improve the readability of their function declarations enough to overcome the downsides above, so they are allowed. When in doubt, use overloads.
Use trailing return types only where using the ordinary syntax (leading return types) is impractical or much less readable.
Definition
C++ allows two different forms of function declarations. In the older form, the return type appears before the function name. For example:
The new form, introduced in C++11, uses the auto keyword before the function name and a trailing return type after the argument list. For example, the declaration above could equivalently be written:
The trailing return type is in the function's scope. This doesn't make a difference for a simple case like int but it matters for more complicated cases, like types declared in class scope or types written in terms of the function parameters.
Pros
Trailing return types are the only way to explicitly specify the return type of a lambda expression. In some cases the compiler is able to deduce a lambda's return type, but not in all cases. Even when the compiler can deduce it automatically, sometimes specifying it explicitly would be clearer for readers.
Sometimes it's easier and more readable to specify a return type after the function's parameter list has already appeared. This is particularly true when the return type depends on template parameters. For example:
versus
Cons
Trailing return type syntax is relatively new and it has no analogue in C++-like languages like C and Java, so some readers may find it unfamiliar.
Existing code bases have an enormous number of function declarations that aren't going to get changed to use the new syntax, so the realistic choices are using the old syntax only or using a mixture of the two. Using a single version is better for uniformity of style.
Decision
In most cases, continue to use the older style of function declaration where the return type goes before the function name. Use the new trailing-return-type form only in cases where it's required (such as lambdas) or where, by putting the type after the function's parameter list, it allows you to write the type in a much more readable way. The latter case should be rare; it's mostly an issue in fairly complicated template code, which is discouraged in most cases.
There are various tricks and utilities that we use to make C++ code more robust, and various ways we use C++ that may differ from what you see elsewhere.
Prefer to have single, fixed owners for dynamically allocated objects. Prefer to transfer ownership with smart pointers.
Definition
"Ownership" is a bookkeeping technique for managing dynamically allocated memory (and other resources). The owner of a dynamically allocated object is an object or function that is responsible for ensuring that it is deleted when no longer needed. Ownership can sometimes be shared, in which case the last owner is typically responsible for deleting it. Even when ownership is not shared, it can be transferred from one piece of code to another.
"Smart" pointers are classes that act like pointers, e.g. by overloading the * and -> operators. Some smart pointer types can be used to automate ownership bookkeeping, to ensure these responsibilities are met. std::unique_ptr is a smart pointer type introduced in C++11, which expresses exclusive ownership of a dynamically allocated object; the object is deleted when the std::unique_ptr goes out of scope. It cannot be copied, but can be moved to represent ownership transfer. std::shared_ptr is a smart pointer type that expresses shared ownership of a dynamically allocated object. std::shared_ptrs can be copied; ownership of the object is shared among all copies, and the object is deleted when the last std::shared_ptr is destroyed.
Pros
Cons
std::unique_ptr expresses ownership transfer using C++11's move semantics, which are relatively new and may confuse some programmers.Decision
If dynamic allocation is necessary, prefer to keep ownership with the code that allocated it. If other code needs access to the object, consider passing it a copy, or passing a pointer or reference without transferring ownership. Prefer to use std::unique_ptr to make ownership transfer explicit. For example:
Do not design your code to use shared ownership without a very good reason. One such reason is to avoid expensive copy operations, but you should only do this if the performance benefits are significant, and the underlying object is immutable (i.e. std::shared_ptr<const Foo>>). If you do use shared ownership, prefer to use std::shared_ptr.
Never use std::auto_ptr. Instead, use std::unique_ptr.
Use
cpplint.pyto detect style errors.
cpplint.py is a tool that reads a source file and identifies many style errors. It is not perfect, and has both false positives and false negatives, but it is still a valuable tool. False positives can be ignored by putting // NOLINT at the end of the line or // NOLINTNEXTLINE in the previous line.
Some projects have instructions on how to run cpplint.py from their project tools. If the project you are contributing to does not, you can download cpplint.py separately.
Use rvalue references only to define move constructors and move assignment operators, or for perfect forwarding.
Definition
Rvalue references are a type of reference that can only bind to temporary objects. The syntax is similar to traditional reference syntax. For example, void f(string&& s); declares a function whose argument is an rvalue reference to a string.
Pros
v1 is a std::vector<string>, for example, then auto v2(std::move(v1)) will probably just result in some simple pointer manipulation instead of copying a large amount of data. In some cases this can result in a major performance improvement.std::move is necessary to make effective use of some standard-library types, such as std::unique_ptr.Cons
Decision
Use rvalue references only to define move constructors and move assignment operators (as described in Copyable and Movable Types and, in conjunction with std::forward, to support perfect forwarding. You may use std::move to express moving a value from one object to another rather than copying it.
We allow use of
friendclasses and functions, within reason.
Friends should usually be defined in the same file so that the reader does not have to look in another file to find uses of the private members of a class. A common use of friend is to have a FooBuilder class be a friend of Foo so that it can construct the inner state of Foo correctly, without exposing this state to the world. In some cases it may be useful to make a unittest class a friend of the class it tests.
Friends extend, but do not break, the encapsulation boundary of a class. In some cases this is better than making a member public when you want to give only one other class access to it. However, most classes should interact with other classes solely through their public members.
We do not use C++ exceptions.
Pros
Init() method, but these require heap allocation or a new "invalid" state, respectively.Cons
throw statement to an existing function, you must examine all of its transitive callers. Either they must make at least the basic exception safety guarantee, or they must never catch the exception and be happy with the program terminating as a result. For instance, if f() calls g() calls h(), and h throws an exception that f catches, g has to be careful or it may not clean up properly.Decision
On their face, the benefits of using exceptions outweigh the costs, especially in new projects. However, for existing code, the introduction of exceptions has implications on all dependent code. If exceptions can be propagated beyond a new project, it also becomes problematic to integrate the new project into existing exception-free code. Because most existing C++ code at Google is not prepared to deal with exceptions, it is comparatively difficult to adopt new code that generates exceptions.
Given that Google's existing code is not exception-tolerant, the costs of using exceptions are somewhat greater than the costs in a new project. The conversion process would be slow and error-prone. We don't believe that the available alternatives to exceptions, such as error codes and assertions, introduce a significant burden.
Our advice against using exceptions is not predicated on philosophical or moral grounds, but practical ones. Because we'd like to use our open-source projects at Google and it's difficult to do so if those projects use exceptions, we need to advise against exceptions in Google open-source projects as well. Things would probably be different if we had to do it all over again from scratch.
This prohibition also applies to the exception-related features added in C++11, such as noexcept, std::exception_ptr, and std::nested_exception.
Avoid using Run Time Type Information (RTTI).
Definition
RTTI allows a programmer to query the C++ class of an object at run time. This is done by use of typeid or dynamic_cast.
Cons
Querying the type of an object at run-time frequently means a design problem. Needing to know the type of an object at runtime is often an indication that the design of your class hierarchy is flawed.
Undisciplined use of RTTI makes code hard to maintain. It can lead to type-based decision trees or switch statements scattered throughout the code, all of which must be examined when making further changes.
Pros
The standard alternatives to RTTI (described below) require modification or redesign of the class hierarchy in question. Sometimes such modifications are infeasible or undesirable, particularly in widely-used or mature code.
RTTI can be useful in some unit tests. For example, it is useful in tests of factory classes where the test has to verify that a newly created object has the expected dynamic type. It is also useful in managing the relationship between objects and their mocks.
RTTI is useful when considering multiple abstract objects. Consider
Decision
RTTI has legitimate uses but is prone to abuse, so you must be careful when using it. You may use it freely in unittests, but avoid it when possible in other code. In particular, think twice before using RTTI in new code. If you find yourself needing to write code that behaves differently based on the class of an object, consider one of the following alternatives to querying the type:
When the logic of a program guarantees that a given instance of a base class is in fact an instance of a particular derived class, then a dynamic_cast may be used freely on the object. Usually one can use a static_cast as an alternative in such situations.
Decision trees based on type are a strong indication that your code is on the wrong track.
Code such as this usually breaks when additional subclasses are added to the class hierarchy. Moreover, when properties of a subclass change, it is difficult to find and modify all the affected code segments.
Do not hand-implement an RTTI-like workaround. The arguments against RTTI apply just as much to workarounds like class hierarchies with type tags. Moreover, workarounds disguise your true intent.
Use C++-style casts like
static_cast<float>(double_value), or brace initialization for conversion of arithmetic types likeint64 y = int64{1} << 42. Do not use cast formats likeint y = (int)xorint y = int(x)(but the latter is okay when invoking a constructor of a class type).
Definition
C++ introduced a different cast system from C that distinguishes the types of cast operations.
Pros
The problem with C casts is the ambiguity of the operation; sometimes you are doing a conversion (e.g., (int)3.5) and sometimes you are doing a cast (e.g., (int)"hello"). Brace initialization and C++ casts can often help avoid this ambiguity. Additionally, C++ casts are more visible when searching for them.
Cons The C++-style cast syntax is verbose and cumbersome.
Decision
Do not use C-style casts. Instead, use these C++-style casts when explicit type conversion is necessary.
int64{x}). This is the safest approach because code will not compile if conversion can result in information loss. The syntax is also concise.static_cast as the equivalent of a C-style cast that does value conversion, when you need to explicitly up-cast a pointer from a class to its superclass, or when you need to explicitly cast a pointer from a superclass to a subclass. In this last case, you must be sure your object is actually an instance of the subclass.const_cast to remove the const qualifier (see const).reinterpret_cast to do unsafe conversions of pointer types to and from integer and other pointer types. Use this only if you know what you are doing and you understand the aliasing issues.See the RTTI section for guidance on the use of dynamic_cast.
Use streams where appropriate, and stick to "simple"
usages.
Definition
Streams are the standard I/O abstraction in C++, as exemplified by the standard header <iostream>. They are widely used in Google code, but only for debug logging and test diagnostics.
Pros
The <<and>>stream operators provide an API for formatted I/O that is easily learned, portable, reusable, and extensible.printf, by contrast, doesn't even supportstring, to say nothing of user-defined types, and is very difficult to use portably.printf` also obliges you to choose among the numerous slightly different versions of that function, and navigate the dozens of conversion specifiers.
Streams provide first-class support for console I/O via std::cin, std::cout, std::cerr, and std::clog. The C APIs do as well, but are hampered by the need to manually buffer the input.
Cons
<< operators interferes with internationalization, because it bakes word order into the code, and streams' support for localization is flawed.<< is extremely costly for the compiler. When used pervasively in a large code base, it can consume as much as 20% of the parsing and semantic analysis time.Decision
Use streams only when they are the best tool for the job. This is typically the case when the I/O is ad-hoc, local, human-readable, and targeted at other developers rather than end-users. Be consistent with the code around you, and with the codebase as a whole; if there's an established tool for your problem, use that tool instead.
Avoid using streams for I/O that faces external users or handles untrusted data. Instead, find and use the appropriate templating libraries to handle issues like internationalization, localization, and security hardening.
If you do use streams, avoid the stateful parts of the streams API (other than error state), such as imbue(), xalloc(), and register_callback(). Use explicit formatting functions rather than stream manipulators or formatting flags to control formatting details such as number base, precision, or padding.
Overload << as a streaming operator for your type only if your type represents a value, and << writes out a human-readable string representation of that value. Avoid exposing implementation details in the output of <<; if you need to print object internals for debugging, use named functions instead (a method named DebugString() is the most common convention).
Use prefix form (
++i) of the increment and decrement operators with iterators and other template objects.
Definition
When a variable is incremented (++i or i++) or decremented (--i or i--) and the value of the expression is not used, one must decide whether to preincrement (decrement) or postincrement (decrement).
Pros
When the return value is ignored, the "pre" form (++i) is never less efficient than the "post" form (i++), and is often more efficient. This is because post-increment (or decrement) requires a copy of i to be made, which is the value of the expression. If i is an iterator or other non-scalar type, copying i could be expensive. Since the two types of increment behave the same when the value is ignored, why not just always pre-increment?
Cons
The tradition developed, in C, of using post-increment when the expression value is not used, especially in for loops. Some find post-increment easier to read, since the "subject" (i) precedes the "verb" (++), just like in English.
Decision
For simple scalar (non-object) values there is no reason to prefer one form and we allow either. For iterators and other template types, use pre-increment.
Use
constwhenever it makes sense. With C++11,constexpris a better choice for some uses of const.
Definition
Declared variables and parameters can be preceded by the keyword const to indicate the variables are not changed (e.g., const int foo). Class functions can have the const qualifier to indicate the function does not change the state of the class member variables (e.g., class Foo { int Bar(char c) const; };).
Pros
Easier for people to understand how variables are being used. Allows the compiler to do better type checking, and, conceivably, generate better code. Helps people convince themselves of program correctness because they know the functions they call are limited in how they can modify your variables. Helps people know what functions are safe to use without locks in multi-threaded programs.
Cons
const is viral: if you pass a const variable to a function, that function must have const in its prototype (or the variable will need a const_cast). This can be a particular problem when calling library functions.
Decision
const variables, data members, methods and arguments add a level of compile-time type checking; it is better to detect errors as soon as possible. Therefore we strongly recommend that you use const whenever it makes sense to do so:
const T&) or pointer-to-const (const T*), respectively.const whenever possible. Accessors should almost always be const. Other methods should be const if they do not modify any data members, do not call any non-const methods, and do not return a non-const pointer or non-const reference to a data member.const whenever they do not need to be modified after construction.The mutable keyword is allowed but is unsafe when used with threads, so thread safety should be carefully considered first.
Some people favor the form int const *foo to const int* foo. They argue that this is more readable because it's more consistent: it keeps the rule that const always follows the object it's describing. However, this consistency argument doesn't apply in codebases with few deeply-nested pointer expressions since most const expressions have only one const, and it applies to the underlying value. In such cases, there's no consistency to maintain. Putting the const first is arguably more readable, since it follows English in putting the "adjective" (const) before the "noun" (int).
That said, while we encourage putting const first, we do not require it. But be consistent with the code around you!
In C++11, use
constexprto define true constants or to ensure constant initialization.
Definition
Some variables can be declared constexpr to indicate the variables are true constants, i.e. fixed at compilation/link time. Some functions and constructors can be declared constexpr which enables them to be used in defining a constexpr variable.
Pros
Use of constexpr enables definition of constants with floating-point expressions rather than just literals; definition of constants of user-defined types; and definition of constants with function calls.
Cons
Prematurely marking something as constexpr may cause migration problems if later on it has to be downgraded. Current restrictions on what is allowed in constexpr functions and constructors may invite obscure workarounds in these definitions.
Decision
constexpr definitions enable a more robust specification of the constant parts of an interface. Use constexpr to specify true constants and the functions that support their definitions. Avoid complexifying function definitions to enable their use with constexpr. Do not use constexpr to force inlining.
Of the built-in C++ integer types, the only one used is int. If a program needs a variable of a different size, use a precise-width integer type from <stdint.h>, such as int16_t. If your variable represents a value that could ever be greater than or equal to 2^31 (2GiB), use a 64-bit type such as int64_t. Keep in mind that even if your value won't ever be too large for an int, it may be used in intermediate calculations which may require a larger type. When in doubt, choose a larger type.
Definition
C++ does not specify the sizes of its integer types. Typically people assume that short is 16 bits, int is 32 bits, long is 32 bits and long long is 64 bits.
Pros
Uniformity of declaration.
Cons
The sizes of integral types in C++ can vary based on compiler and architecture.
Decision
<stdint.h> defines types like int16_t, uint32_t, int64_t, etc. You should always use those in preference to short, unsigned long long and the like, when you need a guarantee on the size of an integer. Of the C integer types, only int should be used. When appropriate, you are welcome to use standard types like size_t and ptrdiff_t.
We use int very often, for integers we know are not going to be too big, e.g., loop counters. Use plain old int for such things. You should assume that an int is at least 32 bits, but don't assume that it has more than 32 bits. If you need a 64-bit integer type, use int64_t or uint64_t.
For integers we know can be "big", use int64_t.
You should not use the unsigned integer types such as uint32_t, unless there is a valid reason such as representing a bit pattern rather than a number, or you need defined overflow modulo 2^N. In particular, do not use unsigned types to say a number will never be negative. Instead, use assertions for this.
If your code is a container that returns a size, be sure to use a type that will accommodate any possible usage of your container. When in doubt, use a larger type rather than a smaller type.
Use care when converting integer types. Integer conversions and promotions can cause non-intuitive behavior.
Some people, including some textbook authors, recommend using unsigned types to represent numbers that are never negative. This is intended as a form of self-documentation. However, in C, the advantages of such documentation are outweighed by the real bugs it can introduce. Consider:
This code will never terminate! Sometimes gcc will notice this bug and warn you, but often it will not. Equally bad bugs can occur when comparing signed and unsigned variables. Basically, C's type-promotion scheme causes unsigned types to behave differently than one might expect.
So, document that a variable is non-negative using assertions. Don't use an unsigned type.
Avoid defining macros, especially in headers; prefer inline functions, enums, and
constvariables. Name macros with a project-specific prefix. Do not use macros to define pieces of a C++ API.
Macros mean that the code you see is not the same as the code the compiler sees. This can introduce unexpected behavior, especially since macros have global scope.
The problems introduced by macros are especially severe when they are used to define pieces of a C++ API, and still more so for public APIs. Every error message from the compiler when developers incorrectly use that interface now must explain how the macros formed the interface. Refactoring and analysis tools have a dramatically harder time updating the interface. As a consequence, we specifically disallow using macros in this way. For example, avoid patterns like:
Luckily, macros are not nearly as necessary in C++ as they are in C. Instead of using a macro to inline performance-critical code, use an inline function. Instead of using a macro to store a constant, use a const variable. Instead of using a macro to "abbreviate" a long variable name, use a reference. Instead of using a macro to conditionally compile code ... well, don't do that at all. It makes testing much more difficult.
Macros can do things these other techniques cannot, and you do see them in the codebase, especially in the lower-level libraries. And some of their special features (like stringifying, concatenation, and so forth) are not available through the language proper. But before using a macro, consider carefully whether there's a non-macro way to achieve the same result. If you need to use a macro to define an interface, contact your project leads to request a waiver of this rule.
The following usage pattern will avoid many problems with macros; if you use macros, follow it whenever possible:
.h file.#define macros right before you use them, and #undef them right after.#undef an existing macro before replacing it with your own; instead, pick a name that's likely to be unique.## to generate function/class/variable names.Exporting macros from headers (i.e. defining them in a header without #undefing them before the end of the header) is extremely strongly discouraged. If you do export a macro from a header, it must have a globally unique name. To achieve this, it must be named with a prefix consisting of your project's namespace name (but upper case).
Use
0for integers,0.0for reals,nullptr(orNULL) for pointers, and `'\0'` for chars.
Use 0 for integers and 0.0 for reals. This is not controversial.
For pointers (address values), there is a choice between 0, NULL, and nullptr. For projects that allow C++11 features, use nullptr. For C++03 projects, we prefer NULL because it looks like a pointer. In fact, some C++ compilers provide special definitions of NULL which enable them to give useful warnings, particularly in situations where sizeof(NULL) is not equal to sizeof(0).
Use `'\0'` for chars. This is the correct type and also makes code more readable.
Prefer
sizeof(varname)tosizeof(type).
Use sizeof(varname) when you take the size of a particular variable. sizeof(varname) will update appropriately if someone changes the variable type either now or later. You may use sizeof(type) for code unrelated to any particular variable, such as code that manages an external or internal data format where a variable of an appropriate C++ type is not convenient.
Good code
Bad code
Good code
Use
autoto avoid type names that are noisy, obvious, or unimportant - cases where the type doesn't aid in clarity for the reader. Continue to use manifest type declarations when it helps readability.
Pros
Cons
Sometimes code is clearer when types are manifest, especially when a variable's initialization depends on things that were declared far away. In expressions like:
it may not be obvious what the resulting types are if the type of y isn't very well known, or if y was declared many lines earlier.
Programmers have to understand the difference between auto and const auto& or they'll get copies when they didn't mean to.
If an auto variable is used as part of an interface, e.g. as a constant in a header, then a programmer might change its type while only intending to change its value, leading to a more radical API change than intended.
Decision
auto is permitted when it increases readability, particularly as described below. Never initialize an auto-typed variable with a braced initializer list.
Specific cases where auto is allowed or encouraged:
find, begin, or end for instance).new commonly falls into this category, as does use of auto in a range-based loop over a container whose type is spelled out nearby.(Encouraged) When iterating over a map with a range-based loop (because it is often assumed that the correct type is std::pair<KeyType, ValueType> whereas it is actually std::pair<const KeyType, ValueType>). This is particularly well paired with local key and value aliases for .first and .second (often const-ref).
```c++ for (const auto& item : some_map) { const KeyType& key = item.first; const ValType& value = item.second; // The rest of the loop can now just refer to key and value, // a reader can see the types in question, and we've avoided // the too-common case of extra copies in this iteration. } ```
You may use braced initializer lists.
In C++03, aggregate types (arrays and structs with no constructor) could be initialized with braced initializer lists.
In C++11, this syntax was generalized, and any object type can now be created with a braced initializer list, known as a braced-init-list in the C++ grammar. Here are a few examples of its use.
A user-defined type can also define a constructor and/or assignment operator that take std::initializer_list<T>, which is automatically created from braced-init-list:
Finally, brace initialization can also call ordinary constructors of data types, even if they do not have std::initializer_list<T> constructors.
Never assign a braced-init-list to an auto local variable. In the single element case, what this means can be confusing.
Bad code
Good code
See Braced Initializer List Format for formatting.
Use lambda expressions where appropriate. Prefer explicit captures when the lambda will escape the current scope.
Definition
Lambda expressions are a concise way of creating anonymous function objects. They're often useful when passing functions as arguments. For example:
They further allow capturing variables from the enclosing scope either explicitly by name, or implicitly using a default capture. Explicit captures require each variable to be listed, as either a value or reference capture:
Default captures implicitly capture any variable referenced in the lambda body, including this if any members are used:
Lambdas were introduced in C++11 along with a set of utilities for working with function objects, such as the polymorphic wrapper std::function.
Pros
std::function, and std::bind can be used in combination as a general purpose callback mechanism; they make it easy to write functions that take bound functions as arguments.Cons
Decision
Prefer explicit captures if the lambda may escape the current scope. For example, instead of:
```c++ { Foo foo; ... executor->Schedule([&] { Frobnicate(foo); }) ... } // BAD! The fact that the lambda makes use of a reference to foo and // possibly this (if Frobnicate is a member function) may not be // apparent on a cursory inspection. If the lambda is invoked after // the function returns, that would be bad, because both foo // and the enclosing object could have been destroyed. ```
prefer to write:
```c++ { Foo foo; ... executor->Schedule([&foo] { Frobnicate(foo); }) ... } // BETTER - The compile will fail if Frobnicate is a member // function, and it's clearer that foo is dangerously captured by // reference.
Like other declarations, aliases declared in a header file are part of that header's public API unless they're in a function definition, in the private portion of a class, or in an explicitly-marked internal namespace. Aliases in such areas or in .cc files are implementation details (because client code can't refer to them), and are not restricted by this rule.
Pros
Cons
Decision
Don't put an alias in your public API just to save typing in the implementation; do so only if you intend it to be used by your clients.
When defining a public alias, document the intent of the new name, including whether it is guaranteed to always be the same as the type it's currently aliased to, or whether a more limited compatibility is intended. This lets the user know whether they can treat the types as substitutable or whether more specific rules must be followed, and can help the implementation retain some degree of freedom to change the alias.
Don't put namespace aliases in your public API. (See also Namespaces).
For example, these aliases document how they are intended to be used in client code:
These aliases don't document intended use, and half of them aren't meant for client use:
However, local convenience aliases are fine in function definitions, private sections of classes, explicitly marked internal namespaces, and in .cpp files:
The most important consistency rules are those that govern naming. The style of a name immediately informs us what sort of thing the named entity is: a type, a variable, a function, a constant, a macro, etc., without requiring us to search for the declaration of that entity. The pattern-matching engine in our brains relies a great deal on these naming rules.
Naming rules are pretty arbitrary, but we feel that consistency is more important than individual preferences in this area, so regardless of whether you find them sensible or not, the rules are the rules.
Names should be descriptive; avoid abbreviation.
Give as descriptive a name as possible, within reason. Do not worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. Do not use abbreviations that are ambiguous or unfamiliar to readers outside your project, and do not abbreviate by deleting letters within a word.
Bad code
Note that certain universally-known abbreviations are OK, such as i for an iteration variable and T for a template parameter.
Template parameters should follow the naming style for their category: type template parameters should follow the rules for type names, and non-type template parameters should follow the rules for variable names.
Filenames should be all lowercase and can include underscores (
_) or dashes (-). Follow the convention that your project uses. If there is no consistent local pattern to follow, prefer "_".
Examples of acceptable file names:
my_useful_class.cppmy-useful-class.cppmyusefullclass.cppmyusefulclass_test.cppC++ files should end in .cpp and header files should end in .h. Files that rely on being textually included at specific points should end in .inc (see also the section on self-contained headers).
Do not use filenames that already exist in /usr/include, such as db.h.
In general, make your filenames very specific. For example, use http_server_logs.h rather than logs.h. A very common case is to have a pair of files called, e.g., foo_bar.h and foo_bar.cpp, defining a class called FooBar.
Inline functions must be in a .h file. If your inline functions are very short, they should go directly into your .h file.
Type names start with a capital letter and have a capital letter for each new word, with no underscores:
MyExcitingClass,MyExcitingEnum.
The names of all types - classes, structs, type aliases, enums, and type template parameters - have the same naming convention. Type names should start with a capital letter and have a capital letter for each new word. No underscores. For example:
The names of variables (including function parameters) and data members are all lowercase, with underscores between words. Data members of classes (but not structs) additionally have trailing underscores. For instance:
a_local_variable,a_struct_data_member,a_class_data_member_.
For example:
Data members of classes, both static and non-static, are named like ordinary nonmember variables, but with a trailing underscore.
Data members of structs, both static and non-static, are named like ordinary nonmember variables. They do not have the trailing underscores that data members in classes have.
See Structs vs. Classes for a discussion of when to use a struct versus a class.
Variables declared constexpr or const, and whose value is fixed for the duration of the program, are named with a leading "k" followed by mixed case. For example:
All such variables with static storage duration (i.e. statics and globals, see Storage Duration for details) should be named this way. This convention is optional for variables of other storage classes, e.g. automatic variables, otherwise the usual variable naming rules apply.
Regular functions have mixed case; accessors and mutators may be named like variables.
Ordinarily, functions should start with a lower-case letter and have a capital letter for each new word (a.k.a. "[Camel Case][wikipedia:camel-case]" or "Pascal case"). Such names should not have underscores. Prefer to capitalize acronyms as single words (i.e. startRpc(), not startRPC()).
(The same naming rule applies to class- and namespace-scope constants that are exposed as part of an API and that are intended to look like functions, because the fact that they're objects rather than functions is an unimportant implementation detail.)
Accessors and mutators (get and set functions) may be named like variables. These often correspond to actual member variables, but this is not required. For example, int count() and void set_count(int count).
Namespace names are all lower-case. Top-level namespace names are based on the project name . Avoid collisions between nested namespaces and well-known top-level namespaces.
The name of a top-level namespace should usually be the name of the project or team whose code is contained in that namespace. The code in that namespace should usually be in a directory whose basename matches the namespace name (or subdirectories thereof).
Keep in mind that the rule against abbreviated names applies to namespaces just as much as variable names. Code inside the namespace seldom needs to mention the namespace name, so there's usually no particular need for abbreviation anyway.
Avoid nested namespaces that match well-known top-level namespaces. Collisions between namespace names can lead to surprising build breaks because of name lookup rules. In particular, do not create any nested std namespaces. Prefer unique project identifiers (websearch::index, websearch::index_util) over collision-prone names like websearch::util.
For internal namespaces, be wary of other code being added to the same internal namespace causing a collision (internal helpers within a team tend to be related and may lead to collisions). In such a situation, using the filename to make a unique internal name is helpful (websearch::index::frobber_internal for use in frobber.h)
Enumerators (for both scoped and unscoped enums) should be named either like constants or like macros: either
kEnumNameorENUM_NAME.
Preferably, the individual enumerators should be named like constants. However, it is also acceptable to name them like macros. The enumeration name, UrlTableErrors (and AlternateUrlTableErrors), is a type, and therefore mixed case.
You're not really going to [define a macro](#preprocessor-macros, are you? If you do, they're like this:
MY_MACRO_THAT_SCARES_SMALL_CHILDREN.
Please see the description of macros; in general macros should not be used. However, if they are absolutely needed, then they should be named with all capitals and underscores.
If you are naming something that is analogous to an existing C or C++ entity then you can follow the existing naming convention scheme.
bigopen() function name, follows form of open()
uint typedef
bigpos struct or class, follows form of pos
sparse_hash_map STL-like entity; follows STL naming conventions
LONGLONG_MAX INT_MAX Though a pain to write, comments are absolutely vital to keeping our code readable. The following rules describe what you should comment and where. But remember: while comments are very important, the best code is self-documenting. Giving sensible names to types and variables is much better than using obscure names that you must then explain through comments.
When writing your comments, write for your audience: the next contributor who will need to understand your code. Be generous - the next one may be you!
Use either the
//or/* */syntax, as long as you are consistent.
You can use either the // or the /* */ syntax; however, // is much more common. Be consistent with how you comment and what style you use where.
Start each file with license boilerplate.
File comments describe the contents of a file. If a file declares, implements, or tests exactly one abstraction that is documented by a comment at the point of declaration, file comments are not required. All other files must have file comments.
Every file should contain license boilerplate. Choose the appropriate boilerplate for the license used by the project (for example, Apache 2.0, BSD, LGPL, GPL).
If you make significant changes to a file with an author line, consider deleting the author line.
If a .h declares multiple abstractions, the file-level comment should broadly describe the contents of the file, and how the abstractions are related. A 1 or 2 sentence file-level comment may be sufficient. The detailed documentation about individual abstractions belongs with those abstractions, not at the file level.
Do not duplicate comments in both the .h and the .cpp. Duplicated comments diverge.
Every non-obvious class declaration should have an accompanying comment that describes what it is for and how it should be used.
The class comment should provide the reader with enough information to know how and when to use the class, as well as any additional considerations necessary to correctly use the class. Document the synchronization assumptions the class makes, if any. If an instance of the class can be accessed by multiple threads, take extra care to document the rules and invariants surrounding multithreaded use.
The class comment is often a good place for a small example code snippet demonstrating a simple and focused usage of the class.
When sufficiently separated (e.g. .h and .cpp files), comments describing the use of the class should go together with its interface definition; comments about the class operation and implementation should accompany the implementation of the class's methods.
Declaration comments describe use of the function (when it is non-obvious); comments at the definition of a function describe operation.
Almost every function declaration should have comments immediately preceding it that describe what the function does and how to use it. These comments may be omitted only if the function is simple and obvious (e.g. simple accessors for obvious properties of the class). These comments should be descriptive ("Opens the file") rather than imperative ("Open the file"); the comment describes the function, it does not tell the function what to do. In general, these comments do not describe how the function performs its task. Instead, that should be left to comments in the function definition.
Types of things to mention in comments at the function declaration:
Here is an example:
However, do not be unnecessarily verbose or state the completely obvious. Notice below that it is not necessary to say "returns false otherwise" because this is implied.
When documenting function overrides, focus on the specifics of the override itself, rather than repeating the comment from the overridden function. In many of these cases, the override needs no additional documentation and thus no comment is required.
When commenting constructors and destructors, remember that the person reading your code knows what constructors and destructors are for, so comments that just say something like "destroys this object" are not useful. Document what constructors do with their arguments (for example, if they take ownership of pointers), and what cleanup the destructor does. If this is trivial, just skip the comment. It is quite common for destructors not to have a header comment.
If there is anything tricky about how a function does its job, the function definition should have an explanatory comment. For example, in the definition comment you might describe any coding tricks you use, give an overview of the steps you go through, or explain why you chose to implement the function in the way you did rather than using a viable alternative. For instance, you might mention why it must acquire a lock for the first half of the function but why it is not needed for the second half.
Note you should not just repeat the comments given with the function declaration, in the .h file or wherever. It's okay to recapitulate briefly what the function does, but the focus of the comments should be on how it does it.
In general the actual name of the variable should be descriptive enough to give a good idea of what the variable is used for. In certain cases, more comments are required.
The purpose of each class data member (also called an instance variable or member variable) must be clear. If there are any invariants (special values, relationships between members, lifetime requirements) not clearly expressed by the type and name, they must be commented. However, if the type and name suffice (int num_events_;), no comment is needed.
In particular, add comments to describe the existence and meaning of sentinel values, such as nullptr or -1, when they are not obvious. For example:
All global variables should have a comment describing what they are, what they are used for, and (if unclear) why it needs to be global. For example:
In your implementation you should have comments in tricky, non-obvious, interesting, or important parts of your code.
Tricky or complicated code blocks should have comments before them. Example:
Also, lines that are non-obvious should get a comment at the end of the line. These end-of-line comments should be separated from the code by 2 spaces. Example:
Note that there are both comments that describe what the code is doing, and comments that mention that an error has already been logged when the function returns.
If you have several comments on subsequent lines, it can often be more readable to line them up:
When the meaning of a function argument is nonobvious, consider one of the following remedies:
bool argument with an enum argument. This will make the argument values self-describing.Consider the following example:
versus:
Do not state the obvious. In particular, don't literally describe what code does, unless the behavior is nonobvious to a reader who understands C++ well. Instead, provide higher level comments that describe why the code does what it does, or make the code self describing.
Compare this:
To this:
Self-describing code doesn't need a comment. The comment from the example above would be obvious:
Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.
Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style.
Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.
Use
TODOcomments for code that is temporary, a short-term solution, or good-enough but not perfect.
TODOs should include the string TODO in all caps, followed by the name, e-mail address, bug ID, or other identifier of the person or issue with the best context about the problem referenced by the TODO. The main purpose is to have a consistent TODO that can be searched to find out how to get more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO with a name, it is almost always your name that is given.
If your TODO is of the form "At a future date do something" make sure that you either include a very specific date ("Fix by November 2005") or a very specific event ("Remove this code when all clients can handle XML responses.").
Mark deprecated interface points with
DEPRECATEDcomments.
You can mark an interface as deprecated by writing a comment containing the word DEPRECATED in all caps. The comment goes either before the declaration of the interface or on the same line as the declaration.
After the word DEPRECATED, write your name, e-mail address, or other identifier in parentheses.
A deprecation comment must include simple, clear directions for people to fix their callsites. In C++, you can implement a deprecated function as an inline function that calls the new interface point.
Marking an interface point DEPRECATED will not magically cause any callsites to change. If you want people to actually stop using the deprecated facility, you will have to fix the callsites yourself or recruit a crew to help you.
New code should not contain calls to deprecated interface points. Use the new interface point instead. If you cannot understand the directions, find the person who created the deprecation and ask them for help using the new interface point.
Coding style and formatting are pretty arbitrary, but a project is much easier to follow if everyone uses the same style. Individuals may not agree with every aspect of the formatting rules, and some of the rules may take some getting used to, but it is important that all project contributors follow the style rules so that they can all read and understand everyone's code easily.
To help you format code correctly, we use "Artistic Style" 3.0. The make astyle target is available in the Kaleidoscope and plugin Makefiles. Our CI infrastructure enforces astyle's decisions.
Each line of text in your code should be at most 80 characters long.
We recognize that this rule is controversial, but so much existing code already adheres to it, and we feel that consistency is important.
Pros
Those who favor this rule argue that it is rude to force them to resize their windows and there is no need for anything longer. Some folks are used to having several code windows side-by-side, and thus don't have room to widen their windows in any case. People set up their work environment assuming a particular maximum window width, and 80 columns has been the traditional standard. Why change it?
Cons
Proponents of change argue that a wider line can make code more readable. The 80-column limit is an hidebound throwback to 1960s mainframes; modern equipment has wide screens that can easily show longer lines.
Decision
80 characters is the maximum.
Exception
Comment lines can be longer than 80 characters if it is not feasible to split them without harming readability, ease of cut and paste or auto-linking – e.g. if a line contains an example command or a literal URL longer than 80 characters.
Exception
A raw-string literal may have content that exceeds 80 characters. Except for test code, such literals should appear near the top of a file.
Exception
An #include statement with a long path may exceed 80 columns.
Non-ASCII characters should be rare, and must use UTF-8 formatting.
You shouldn't hard-code user-facing text in source, even English, so use of non-ASCII characters should be rare. However, in certain cases it is appropriate to include such words in your code. For example, if your code parses data files from foreign sources, it may be appropriate to hard-code the non-ASCII string(s) used in those data files as delimiters. More commonly, unittest code (which does not need to be localized) might contain non-ASCII strings. In such cases, you should use UTF-8, since that is an encoding understood by most tools able to handle more than just ASCII.
Hex encoding is also OK, and encouraged where it enhances readability - for example, "\xEF\xBB\xBF", or, even more simply, u8"\uFEFF", is the Unicode zero-width no-break space character, which would be invisible if included in the source as straight UTF-8.
Use the u8 prefix to guarantee that a string literal containing \uXXXX escape sequences is encoded as UTF-8. Do not use it for strings containing non-ASCII characters encoded as UTF-8, because that will produce incorrect output if the compiler does not interpret the source file as UTF-8.
You shouldn't use the C++11 char16_t and char32_t character types, since they're for non-UTF-8 text. For similar reasons you also shouldn't use wchar_t (unless you're writing code that interacts with the Windows API, which uses wchar_t extensively).
Use only spaces, and indent 2 spaces at a time.
We use spaces for indentation. Do not use tabs in your code. You should set your editor to emit spaces when you hit the tab key.
Return type on the same line as function name, parameters on the same line if they fit. Wrap parameter lists which do not fit on a single line as you would wrap arguments in a function call.
Functions look like this:
If you have too much text to fit on one line:
or if you cannot fit even the first parameter:
Some points to note:
Unused parameters that are obvious from context may be omitted:
Unused parameters that might not be obvious should comment out the variable name in the function definition:
Attributes, and macros that expand to attributes, appear at the very beginning of the function declaration or definition, before the return type:
Format parameters and bodies as for any other function, and capture lists like other comma-separated lists.
For by-reference captures, do not leave a space between the ampersand (&) and the variable name.
Short lambdas may be written inline as function arguments.
Either write the call all on a single line, wrap the arguments at the parenthesis, or start the arguments on a new line indented by four spaces and continue at that 4 space indent. In the absence of other considerations, use the minimum number of lines, including placing multiple arguments on each line where appropriate.
Function calls have the following format:
If the arguments do not all fit on one line, they should be broken up onto multiple lines, with each subsequent line aligned with the first argument. Do not add spaces after the open paren or before the close paren:
Arguments may optionally all be placed on subsequent lines with a four space indent:
Put multiple arguments on a single line to reduce the number of lines necessary for calling a function unless there is a specific readability problem. Some find that formatting with strictly one argument on each line is more readable and simplifies editing of the arguments. However, we prioritize for the reader over the ease of editing arguments, and most readability problems are better addressed with the following techniques.
If having multiple arguments in a single line decreases readability due to the complexity or confusing nature of the expressions that make up some arguments, try creating variables that capture those arguments in a descriptive name:
Or put the confusing argument on its own line with an explanatory comment:
If there is still a case where one argument is significantly more readable on its own line, then put it on its own line. The decision should be specific to the argument which is made more readable rather than a general policy.
Sometimes arguments form a structure that is important for readability. In those cases, feel free to format the arguments according to that structure:
Format a braced initializer list exactly like you would format a function call in its place.
If the braced list follows a name (e.g. a type or variable name), format as if the {} were the parentheses of a function call with that name. If there is no name, assume a zero-length name.
Prefer no spaces inside parentheses. The
ifandelsekeywords belong on separate lines.
There are two acceptable formats for a basic conditional statement. One includes spaces between the parentheses and the condition, and one does not.
The most common form is without spaces. Either is fine, but be consistent. If you are modifying a file, use the format that is already present. If you are writing new code, use the format that the other files in that directory or project use. If in doubt and you have no personal preference, do not add the spaces.
If you prefer you may add spaces inside the parentheses:
Note that in all cases you must have a space between the if and the open parenthesis. You must also have a space between the close parenthesis and the curly brace, if you're using one.
Short conditional statements may be written on one line if this enhances readability. You may use this only when the line is brief and the statement does not use the else clause.
This is not allowed when the if statement has an else:
In general, curly braces are not required for single-line statements, but they are allowed if you like them; conditional or loop statements with complex conditions or statements may be more readable with curly braces. Some projects require that an if must always always have an accompanying brace.
However, if one part of an if-else statement uses curly braces, the other part must too:
Switch statements may use braces for blocks. Annotate non-trivial fall-through between cases. Braces are optional for single-statement loops. Empty loop bodies should use empty braces or
continue.
case blocks in switch statements can have curly braces or not, depending on your preference. If you do include curly braces they should be placed as shown below.
If not conditional on an enumerated value, switch statements should always have a default case (in the case of an enumerated value, the compiler will warn you if any values are not handled). If the default case should never execute, simply assert:
Braces are optional for single-statement loops.
Empty loop bodies should use an empty pair of braces or continue, but not a single semicolon.
No spaces around period or arrow. Pointer operators do not have trailing spaces.
The following are examples of correctly-formatted pointer and reference expressions:
Note that:
* or &.When declaring a pointer variable or argument, you may place the asterisk adjacent to either the type or to the variable name:
It is allowed (if unusual) to declare multiple variables in the same declaration, but it is disallowed if any of those have pointer or reference decorations. Such declarations are easily misread.
You should do this consistently within a single file, so, when modifying an existing file, use the style in that file.
When you have a boolean expression that is longer than the standard line length , be consistent in how you break up the lines.
In this example, the logical AND operator is always at the end of the lines:
Note that when the code wraps in this example, both of the && logical AND operators are at the end of the line. This is more common in Google code, though wrapping all operators at the beginning of the line is also allowed. Feel free to insert extra parentheses judiciously because they can be very helpful in increasing readability when used appropriately. Also note that you should always use the punctuation operators, such as && and ~ , rather than the word operators, such as and and compl.
Do not needlessly surround the
returnexpression with parentheses.
Use parentheses in return expr; only where you would use them in x = expr;.
Your choice of
=,(), or{}.
You may choose between =, (), or {}; the following are all correct:
Be careful when using a braced initialization list {...} on a type with an std::initializer_list constructor. A nonempty braced-init-list prefers the std::initializer_list constructor whenever possible. Note that empty braces {} are special, and will call a default constructor if available. To force the non-std::initializer_list constructor, use parentheses instead of braces.
Also, the brace form prevents narrowing of integral types. This can prevent some types of programming errors.
The hash mark that starts a preprocessor directive should always be at the beginning of the line.
Even when preprocessor directives are within the body of indented code, the directives should start at the beginning of the line.
Sections in
public,protectedandprivateorder, each indented one space.
The basic format for a class definition (lacking the comments, see class comments for a discussion of what comments are needed) is:
Things to note:
public:, protected:, and private: keywords should be indented one space.public section should be first, followed by the protected and finally the private section.Constructor initializer lists can be all on one line or with subsequent lines indented four spaces.
The acceptable formats for initializer lists are:
The contents of namespaces are not indented.
Namespaces do not add an extra level of indentation. For example, use:
Do not indent within a namespace:
When declaring nested namespaces, put each namespace on its own line.
Use of horizontal whitespace depends on location. Never put trailing whitespace at the end of a line.
Adding trailing whitespace can cause extra work for others editing the same file, when they merge, as can removing existing trailing whitespace. So: Don't introduce trailing whitespace. Remove it if you're already changing that line, or do it in a separate clean-up operation (preferably when no-one else is working on the file).
Minimize use of vertical whitespace.
This is more a principle than a rule: don't use blank lines when you don't have to. In particular, don't put more than one or two blank lines between functions, resist starting functions with a blank line, don't end functions with a blank line, and be discriminating with your use of blank lines inside functions.
The basic principle is: The more code that fits on one screen, the easier it is to follow and understand the control flow of the program. Of course, readability can suffer from code being too dense as well as too spread out, so use your judgement. But in general, minimize use of vertical whitespace.
Some rules of thumb to help when blank lines may be useful:
The coding conventions described above are mandatory. However, like all good rules, these sometimes have exceptions, which we discuss here.
You may diverge from the rules when dealing with code that does not conform to this style guide.
If you find yourself modifying code that was written to specifications other than those presented by this guide, you may have to diverge from these rules in order to stay consistent with the local conventions in that code. If you are in doubt about how to do this, ask the original author or the person currently responsible for the code. Remember that consistency includes local consistency, too.
Use common sense and BE CONSISTENT.
If you are editing code, take a few minutes to look at the code around you and determine its style. If they use spaces around their if clauses, you should, too. If their comments have little boxes of stars around them, make your comments have little boxes of stars around them too.
The point of having style guidelines is to have a common vocabulary of coding so people can concentrate on what you are saying, rather than on how you are saying it. We present global style rules here so people know the vocabulary. But local style is also important. If code you add to a file looks drastically different from the existing code around it, the discontinuity throws readers out of their rhythm when they go to read it. Try to avoid this.
OK, enough writing about writing code; the code itself is much more interesting. Have fun!
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