Mir C++ style guide

Revision 4.2

Tim Penhey Neil J. Patel Thomas Voss

Background

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.

One way in which we keep the code base manageable is by enforcing consistency. It is very important that any programmer be able to look at another’s code and quickly understand it. Maintaining a uniform style and following conventions means that we can more easily use “pattern-matching” to infer what various symbols are and what invariants are true about them. Creating common, required idioms and patterns makes code much easier to understand. In some cases there might be good arguments for changing certain style rules, but we nonetheless keep things as they are in order to preserve consistency.

Another issue this guide addresses is that of C++ feature bloat. C++ is a huge language with many advanced features. In some cases we constrain, or even ban, use of certain features. We do this to keep code simple and to avoid the various common errors and problems that these features can cause. This guide lists these features and explains why their use is restricted.

Note that this guide is not a C++ tutorial: we assume that the reader is familiar with the language.

Header files

In general, every .cpp file should have an associated .h file. There are some common exceptions, such as unit tests and small .cpp files containing just a main() function.

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.

The #define guard

All header files should have #define guards to prevent multiple inclusion. The format of the symbol name should be <PROJECT>_<PATH>_<FILE>_H_.

To guarantee uniqueness, they should be based on the full path in a project’s source tree. For example, the file foo/src/bar/baz.h in project foo should have the following guard:

#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_

...

#endif  // FOO_BAR_BAZ_H_

Header file dependencies

Don’t use an #include when a forward declaration would suffice.

When you include a header file you introduce a dependency that will cause your code to be recompiled whenever the header file changes. If your header file includes other header files, any change to those files will cause any code that includes your header to be recompiled. Therefore, we prefer to minimize includes, particularly includes of header files in other header files.

You can significantly reduce the number of header files you need to include in your own header files by using forward declarations. For example, if your header file uses the File class in ways that do not require access to the declaration of the File class, your header file can just forward declare class File; instead of having to #include "file/base/file.h".

How can we use a class Foo in a header file without access to its definition?

• We can declare data members of type Foo* or Foo&.

• We can declare (but not define) functions with arguments, and/or return values, of type Foo. (One exception is if an argument Foo or Foo const& has a non-explicit, one-argument constructor, in which case we need the full definition to support automatic type conversion.)

• We can declare static data members of type Foo. This is because static data members are defined outside the class definition.

On the other hand, you must include the header file for Foo if your class subclasses Foo or has a data member of type Foo.

Sometimes it makes sense to have pointer (or better, unique_ptr) members instead of object members. However, this complicates code readability and imposes a performance penalty, so avoid doing this transformation if the only purpose is to minimize includes in header files.

Of course, .cpp files typically do require the definitions of the classes they use, and usually have to include several header files.

Note: If you use a symbol Foo in your source file, you should bring in a definition for Foo yourself, either via an #include or via a forward declaration. Do not depend on the symbol being brought in transitively via headers not directly included. One exception is if Foo is used in myfile.cpp, it’s ok to #include (or forward-declare) Foo in myfile.h, instead of myfile.cpp.

Inline functions

Define functions inline only when they are small, say, 10 lines or less.

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.

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. On modern processors smaller code usually runs faster due to better use of the instruction cache.

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.

The -inl.h files

You may use file names with a -inl.h suffix to define complex inline functions when needed.

The definition of an inline function needs to be in a header file, so that the compiler has the definition available for inlining at the call sites. However, implementation code properly belongs in .cpp files, and we do not like to have much actual code in .h files unless there is a readability or performance advantage.

If an inline function definition is short, with very little, if any, logic in it, you should put the code in your .h file. For example, accessors and mutators should certainly be inside a class definition. More complex inline functions may also be put in a .h file for the convenience of the implementer and callers, though if this makes the .h file too unwieldy you can instead put that code in a separate -inl.h file. This separates the implementation from the class definition, while still allowing the implementation to be included where necessary.

Another use of -inl.h files is for definitions of function templates. This can be used to keep your template definitions easy to read.

Do not forget that a -inl.h file requires a #define guard just like any other header file.

Function parameter ordering

When defining a function, parameter order is: outputs, then inputs.

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 non-const references or pointers to non-const. When ordering function parameters, put all output parameters before any input-only parameters. In particular, do not add new parameters to the end of the function just because they are new; place new output parameters before the input-only 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.

Names and order of includes

Use standard order for readability and to avoid hidden dependencies: your project’s public .h, your project’s private .h, other libraries’ .h, .C library, C++ library,

All of a project’s header files should be listed as descendants of the project’s source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, my-awesome-project/src/base/logging.h should be included as

#include "base/logging.h"

In dir/foo.cpp or dir/foo_test.cpp, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:

1. dir2/foo2.h (preferred location — see details below).

2. Your project’s public .h files.

3. Your project’s private .h files.

4. Other libraries’ .h files.

5. C system files.

6. C++ system files.

The preferred ordering reduces hidden dependencies. We want every header file to be compilable on its own. The easiest way to achieve this is to make sure that every one of them is the first .h file #included in some .cpp.

dir/foo.cpp and dir2/foo2.h are often in the same directory (e.g. base/test_basictypes.cpp and base/basictypes.h), but can be in different directories too.

Within each section it is nice to order the includes alphabetically.

For example, the includes in my-awesome-project/src/foo/internal/fooserver.cpp might look like this:

#include "foo/public/fooserver.h"  // Preferred location.

#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/public/bar.h"

#include <sys/types.h>
#include <unistd.h>

#include <hash_map>
#include <vector>

Scoping

Namespaces

Unnamed namespaces in .cpp files are encouraged. With named namespaces, choose the name based on the project, and possibly its path. Do not use a using-directive in a header file.

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 (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes.

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.

Cons:

Namespaces can be confusing, because they provide an additional (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes.

Use of unnamed spaces in header files can easily cause violations of the C++ One Definition Rule (ODR).

Decision:

Use namespaces according to the policy described below.

Unnamed Namespaces

• Unnamed namespaces are allowed and even encouraged in .cpp files, to avoid runtime naming conflicts:

  namespace                           // This is in a .cpp file.
  {
  // The content of a namespace is not indented
  enum { UNUSED, EOF, ERROR };       // Commonly used tokens.
  bool AtEof() { return pos_ == EOF; }  // Uses our namespace's EOF.
  
  }  // namespace
  

  However, file-scope declarations that are associated with a particular class may be declared in that class as types, static data members or static member functions rather than as members of an unnamed namespace. Terminate the unnamed namespace as shown, with a comment // namespace.

• Do not use unnamed namespaces in .h files.

Named Namespaces

Named 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:

  // 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
  

  // In the .cpp file
  namespace mynamespace
  {
  
  // Definition of functions is within scope of the namespace.
  void MyClass::foo()
  {
      ...
  }
  
  }  // namespace mynamespace
  

  The typical .cpp file might have more complex detail, including the need to reference classes in other namespaces.

  #include "a.h"
  
  DEFINE_BOOL(someflag, false, "placeholder flag");
  
  class C;  // Forward declaration of class C in the global namespace.
  namespace a { class A; }  // Forward declaration of a::A.
  
  namespace b
  {
  
  ...code for b...         // Code goes against the left margin.
  
  }  // namespace b
  

• Do not declare anything in namespace std, not even 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 use a using-directive to make all names from a namespace available, but only in a source file.

• You may use a using-declaration anywhere in a .cpp file, and in functions, methods or classes in .h files.

  // OK in .cpp files.
  // Must be in a function, method or class in .h files.
  using ::foo::bar;
  

• Namespace aliases are allowed anywhere in a .cpp file, anywhere inside the named namespace that wraps an entire .h file, and in functions and methods.

  // Shorten access to some commonly used names in .cpp files.
  namespace fbz = ::foo::bar::baz;
  
  // Shorten access to some commonly used names (in a .h file).
  namespace librarian
  {
  // The following alias is available to all files including
  // this header (in namespace librarian):
  // alias names should therefore be chosen consistently
  // within a project.
  namespace pd_s = ::pipeline_diagnostics::sidetable;
  
  inline void my_inline_function()
  {
      // namespace alias local to a function (or method).
      namespace fbz = ::foo::bar::baz;
      ...
  }
  }  // namespace librarian
  

  Note that an alias in a .h file is visible to everyone #including that file, so public headers (those available outside a project) and headers transitively #included by them, should avoid defining aliases, as part of the general goal of keeping public APIs as small as possible.

Nested classes

Although you may use public nested classes when they are part of an interface, consider a namespace to keep declarations out of the global scope.

Definition:

A class can define another class within it; this is also called a member class.

class Foo
{
private:
    // Bar is a member class, nested within Foo.
    class Bar
    {
      ...
    };

};

Pros:

This is useful when the nested (or member) class is only used by the enclosing class; making it a member puts it in the enclosing class scope rather than polluting the outer scope with the class name. Nested classes can be forward declared within the enclosing class and then defined in the .cpp file to avoid including the nested class definition in the enclosing class declaration, since the nested class definition is usually only relevant to the implementation.

Cons:

Nested classes can be forward-declared only within the definition of the enclosing class. Thus, any header file manipulating a Foo::Bar* pointer will have to include the full class declaration for Foo.

Decision:

Do not make nested classes public unless they are actually part of the interface, e.g., a class that holds a set of options for some method.

Nonmember, static member, and global functions

Prefer nonmember functions within a namespace or static member functions to global functions; use completely global functions rarely.

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, or even necessary, 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.

Functions defined in the same compilation unit as production classes may introduce unnecessary coupling and link-time dependencies when directly called from other compilation units; static member functions are particularly susceptible to this. Consider extracting a new class, or placing the functions in a namespace possibly in a separate library.

If you must define a nonmember function and it is only needed in its .cpp file, use an unnamed namespace or static linkage (eg static int foo() {...}) to limit its scope.

Local variables

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.

int i;
i = f();      // Bad -- initialization separate from declaration.

int j = g();  // Good -- declaration has initialization.

Note that gcc implements for (int i = 0; i < 10; ++i) correctly (the scope of i is only the scope of the for loop), so you can then reuse i in another for loop in the same scope. It also correctly scopes declarations in if and while statements, e.g.

while (char const* p = strchr(str, '/')) str = p + 1;

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.

// Inefficient implementation:
for (int i = 0; i < 1000000; ++i)
{
    Foo f;  // My ctor and dtor get called 1000000 times each.
    f.do_something(i);
}

It may be more efficient to declare such a variable used in a loop outside that loop:

Foo f;  // My ctor and dtor get called once each.
for (int i = 0; i < 1000000; ++i)
{
    f.do_something(i);
}

Classes

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.

Constructors

The purpose of a constructor is to initialise a class so that its invariants hold. For value classes it is worth having a cheap default constructor.

Definition:

It is possible to perform initialization in the body of the constructor.

Pros:

Convenience in typing. No need to worry about whether the class has been initialized or not.

Cons:

The problems with doing work in constructors are:

• If the work calls virtual functions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly introduce this problem even if your class is not currently subclassed, causing much confusion.

• If someone creates a global variable of this type (which is against the rules, but still), the constructor code will be called before main(), possibly breaking some implicit assumptions in the constructor code.

Decision:

Constructors should not make virtual calls to functions, access potentially uninitialized global variables, etc.

Default constructors

You must define a default constructor if your class defines member variables of POD types and has no other constructors. Otherwise the compiler will do it for you, badly.

Definition:

The default constructor is called when we create a class object with no arguments. It is always called when calling new[] (for arrays).

Pros:

Initializing structures by default makes debugging much easier.

Cons:

Extra work for you, the code writer.

Decision:

If your class defines POD member variables and has no other constructors you must define a default constructor (one that takes no arguments). It should initialize the object in such a way that its internal state is consistent and valid.

The reason for this is that if you have no other constructors and do not define a default constructor, the compiler will generate one for you. This compiler generated constructor may not initialize your object sensibly.

If your class is composed from and/or inherits from an existing class or classes but you add no new member variables, you are not required to have a default constructor.

If your class has value semantics then consider making the class invariants such that the default constructor is cheap. For example, initialising member pointers to nullptr and allocating on first use.

Explicit constructors

Use the C++ keyword explicit for constructors with one argument.

Definition:

Normally, if a constructor takes one argument, it can be used as a conversion. For instance, if you define Foo::Foo(string name) and then pass a string to a function that expects a Foo, the constructor will be called to convert the string into a Foo and will pass the Foo to your function for you. This can be convenient but is also a source of trouble when things get converted and new objects created without you meaning them to. Declaring a constructor explicit prevents it from being invoked implicitly as a conversion.

Pros:

Avoids undesirable conversions.

Cons:

Avoids desirable conversions.

Decision:

We require all single argument constructors to be explicit. Always put explicit in front of one-argument constructors in the class definition: explicit Foo(string name);

The exception is copy constructors, which, in the rare cases when we allow them, should probably not be explicit. Classes that are intended to be transparent wrappers around other classes are also exceptions. Such exceptions should be clearly marked with comments.

Copy constructors

Provide a copy constructor and assignment operator only when necessary. Otherwise, disable them with the help of = delete;.

Definition:

The copy constructor and assignment operator are used to create copies of objects. The copy constructor is implicitly invoked by the compiler in some situations, e.g. passing objects by value.

Pros:

Copy constructors make it easy to copy objects. STL containers require that all contents be copyable and assignable. Copy constructors can be more efficient than CopyFrom()-style workarounds because they combine construction with copying, the compiler can elide them in some contexts, and they make it easier to avoid heap allocation.

Cons:

Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It also reduces readability, as it becomes hard to track which objects are being passed around by value as opposed to by reference, and therefore where changes to an object are reflected.

Decision:

Few classes need to be copyable. Most should have neither a copy constructor nor an assignment operator. In many situations, a pointer or reference will work just as well as a copied value, with better performance. For example, you can pass function parameters by reference or pointer instead of by value, and you can store pointers rather than objects in an STL container.

If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or Clone(), rather than a copy constructor, because such methods cannot be invoked implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or because your class needs to be stored by value in an STL container), provide both a copy constructor and assignment operator.

If your class does not need a copy constructor or assignment operator, you must explicitly disable them.

Structs vs. classes

Use a struct only for passive objects that carry data; everything else is a class.

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.

Inheritance

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.

When redefining an inherited virtual method (both pure and non-pure), explicitly declare it override in the declaration of the derived class. Rationale: using override allows the compiler to consistently detect attempts to override methods that have been changed or completely removed. It also makes it straightforward for a reader to determine if a method is virtual or not.

Multiple inheritance

Only very rarely is multiple inheritance of implementation actually useful. We allow multiple inheritance only when at most one of the base classes has an implementation; all other base classes must be interface classes.

Definition:

Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are 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 interfaces.

Interfaces

Classes that satisfy certain conditions are interfaces.

Definition:

A class is an interface if it meets the following requirements:

• It has only public pure virtual (”= 0”) methods and static methods (but see below for destructor).

• It may not have non-static data members.

• It need not have any constructors defined. If a constructor is provided, it must take no arguments and it must be protected.

• If it is a subclass, it may only be derived from classes that satisfy these conditions.

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, they must also declare a virtual, or protected, destructor (in an exception to the first rule, this should not be pure). See Stroustrup, The C++ Programming Language, 3rd edition, section 12.4 for details.

Operator overloading

Overload operators where appropriate.

Definition:

A class can define that operators such as + and / operate on the class as if it were a built-in type.

Pros:

Can make code appear more intuitive because a class will behave in the same way as built-in types (such as int). Overloaded operators are more playful names for functions that are less-colorfully named, such as equals() or add(). For some template functions to work correctly, you may need to define operators.

Cons:

While operator overloading can make code more intuitive, it has several drawbacks:

• It can fool our intuition into thinking that expensive operations are cheap, built-in operations.

• It is much harder to find the call sites for overloaded operators. Searching for Equals() is much easier than searching for relevant invocations of ==.

• Some operators work on pointers too, making it easy to introduce bugs. Foo + 4 may do one thing, while &Foo + 4 does something totally different. The compiler does not complain for either of these, making this very hard to debug.

Overloading also has surprising ramifications. For instance, if a class overloads unary operator&, it cannot safely be forward-declared.

Decision:

In general, do overload operators where appropriate.

See also Copy Constructors and Function Overloading.

Access control

Make data members private, and provide access to them through accessor functions as needed (for technical reasons, we allow data members of a test fixture class to be protected when using Google Test). Typically a variable would be called foo and the accessor function get_foo(). You may also want a mutator function set_foo(). Exception: static const data members need not be private.

The definitions of accessors are usually inlined in the header file.

See also Inheritance and Function Names.

Declaration order

Use the specified order of declarations within a class: public: before private:, methods before data members (variables), etc.

Your class definition should start with its public: section, followed by its protected: section and then its private: section. If any of these sections are empty, omit them.

Within each section, the declarations generally should be in the following order:

• Typedefs and Enums

• Constants (static const data members)

• Constructors

• Destructor

• Methods, including static methods

• Data Members (except static const data members)

Friend declarations should always be in the private section, and the DISALLOW_COPY_AND_ASSIGN macro invocation should be at the end of the private: section. It should be the last thing in the class. See Copy Constructors.

Method definitions in the corresponding .cpp file should be the same as the declaration order, as much as possible.

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.

Write short functions

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.

Other C++ features

Reference arguments

Most parameters passed by reference should 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 NULL 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:

void foo(string const& in, string* out);

In fact it is a very strong convention in Unity code that input arguments are values or const references while output arguments are pointers. Input parameters may be const pointers. Non-const reference parameters are allowed but there must be a valid reason for it, a strong preference is given to const reference parameters.

One case when you might want an input parameter to be a const pointer is if you want to emphasize that the argument is not copied, so it must exist for the lifetime of the object; it is usually best to document this in comments as well. STL adapters such as bind2nd and mem_fun do not permit reference parameters, so you must declare functions with pointer parameters in these cases, too.

Function overloading

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 string const& and overload it with another that takes char const*.

class MyClass {
public:
    void analyze(string const& text);
    void analyze(char const* text, size_t textlen);
};

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., append_string(), AppendInt() rather than just append().

Default arguments

We do not allow default function parameters, except in a few uncommon situations explained below.

Pros:

Often you have a function that uses lots of 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.

Cons:

People often figure out how to use an API by looking at existing code that uses it. Default parameters are more difficult to maintain because copy-and-paste from previous code may not reveal all the parameters. Copy-and-pasting of code segments can cause major problems when the default arguments are not appropriate for the new code.

Decision:

Except as described below, we require all arguments to be explicitly specified, to force programmers to consider the API and the values they are passing for each argument rather than silently accepting defaults they may not be aware of.

One specific exception is when default arguments are used to simulate variable-length argument lists.

// Support up to 4 params by using a default empty AlphaNum.
string str_cat(AlphaNum const& a,
               AlphaNum const& b = gEmptyAlphaNum,
               AlphaNum const& c = gEmptyAlphaNum,
               AlphaNum const& d = gEmptyAlphaNum);

Variable-length arrays and alloca()

We do not allow variable-length arrays or alloca().

Pros:

Variable-length arrays have natural-looking syntax. Both variable-length arrays and alloca() are very efficient.

Cons:

Variable-length arrays and alloca are not part of Standard C++. More importantly, they allocate a data-dependent amount of stack space that can trigger difficult-to-find memory overwriting bugs: “It ran fine on my machine, but dies mysteriously in production”.

Decision:

Use a safe allocator instead, such as unique_ptr.

Friends

We allow use of friend classes 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 unit test 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.

Casting

Use C++ casts like static_cast<>(). Do not use other cast formats like int y = (int)x; or int y = int(x);.

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"); C++ casts avoid this. Additionally C++ casts are more visible when searching for them.

Cons:

The syntax is nasty.

Decision:

Do not use C-style casts. Instead, use these C++-style casts.

• Use static_cast as the equivalent of a C-style cast that does value conversion, or when you need to explicitly up-cast a pointer from a class to its superclass.

• Use const_cast to remove the const qualifier (see const).

• Use 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.

• Do not use dynamic_cast except in test code. If you need to know type information at runtime in this way outside of a unit test, you probably have a design flaw.

Streams

Use streams only for logging.

Definition:

Streams are a replacement for printf() and scanf().

Pros:

With streams, you do not need to know the type of the object you are printing. You do not have problems with format strings not matching the argument list. (Though with gcc, you do not have that problem with printf either.) Streams have automatic constructors and destructors that open and close the relevant files.

Cons:

Streams make it difficult to do functionality like pread(). Some formatting (particularly the common format string idiom %.*s) is difficult if not impossible to do efficiently using streams without using printf-like hacks. Streams do not support operator reordering (the %1s directive), which is helpful for internationalization.

Decision:

Do not use streams, except where required by a logging interface. Use printf-like routines instead.

There are various pros and cons to using streams, but in this case, as in many other cases, consistency trumps the debate. Do not use streams in your code.

Extended Discussion

There has been debate on this issue, so this explains the reasoning in greater depth. Recall the Only One Way guiding principle: we want to make sure that whenever we do a certain type of I/O, the code looks the same in all those places. Because of this, we do not want to allow users to decide between using streams or using printf plus Read/Write/etc. Instead, we should settle on one or the other. We made an exception for logging because it is a pretty specialized application, and for historical reasons.

Proponents of streams have argued that streams are the obvious choice of the two, but the issue is not actually so clear. For every advantage of streams they point out, there is an equivalent disadvantage. The biggest advantage is that you do not need to know the type of the object to be printing. This is a fair point. But, there is a downside: you can easily use the wrong type, and the compiler will not warn you. It is easy to make this kind of mistake without knowing when using streams.

cout << this;  // Prints the address
cout << *this;  // Prints the contents

The compiler does not generate an error because << has been overloaded. We discourage overloading for just this reason.

Some say printf formatting is ugly and hard to read, but streams are often no better. Consider the following two fragments, both with the same typo. Which is easier to discover?

cerr << "Error connecting to '" << foo->bar()->hostname.first
     << ":" << foo->bar()->hostname.second << ": " << strerror(errno);

fprintf(stderr, "Error connecting to '%s:%u: %s",
        foo->bar()->hostname.first, foo->bar()->hostname.second,
        strerror(errno));

And so on and so forth for any issue you might bring up. (You could argue, “Things would be better with the right wrappers,” but if it is true for one scheme, is it not also true for the other? Also, remember the goal is to make the language smaller, not add yet more machinery that someone has to learn.)

Either path would yield different advantages and disadvantages, and there is not a clearly superior solution. The simplicity doctrine mandates we settle on one of them though, and the majority decision was on printf + read/write.

Preincrement and predecrement

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 of const

We strongly recommend that you use const whenever it makes sense to do so.

Definition:

Declared variables and parameters can be preceded by the keyword const to indicate the variables are not changed (e.g., int const 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:

• If a function does not modify an argument passed by reference or by pointer, that argument should be const.

• Declare methods to be 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.

• Consider making data members const whenever they do not need to be modified after construction.

However, do not go crazy with const. Something like int const* const* const x; is likely overkill, even if it accurately describes how const x is. Focus on what’s really useful to know: in this case, int const** x is probably sufficient.

The mutable keyword is allowed but is unsafe when used with threads, so thread safety should be carefully considered first.

Where to put the const

We favor the form int const* foo to const int* foo. This keeps the const with the type modifier (& or *).

That said, while we encourage putting const after the type, we do not require it. But be consistent with the code around you!

Integer types

Use built-in C++ integer types, both signed and unsigned int. Use more specific types like size_t where appropriate. If a program needs a variable of a different size, use a precise-width integer type from <cstdint>, such as int16_t.

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:

<cstdint> 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. When appropriate, you are welcome to use standard types like size_t and ptrdiff_t.

For integers we know can be “big”, use int64_t.

64-bit portability

Code should be 64-bit and 32-bit friendly. Bear in mind problems of printing, comparisons, and structure alignment.

• printf() specifiers for some types are not cleanly portable between 32-bit and 64-bit systems. C99 defines some portable format specifiers. Unfortunately, MSVC 7.1 does not understand some of these specifiers and the standard is missing a few, so we have to define our own ugly versions in some cases (in the style of the standard include file inttypes.h):

  // printf macros for size_t, in the style of inttypes.h
  #ifdef _LP64
  #define __PRIS_PREFIX "z"
  #else
  #define __PRIS_PREFIX
  #endif
  
  // Use these macros after a % in a printf format string
  // to get correct 32/64 bit behavior, like this:
  // size_t size = records.size();
  // printf("%"PRIuS"\n", size);
  
  #define PRIdS __PRIS_PREFIX "d"
  #define PRIxS __PRIS_PREFIX "x"
  #define PRIuS __PRIS_PREFIX "u"
  #define PRIXS __PRIS_PREFIX "X"
  #define PRIoS __PRIS_PREFIX "o"
  

  Type	DO NOT use	DO use	Notes
  void* (or any pointer)	%lx	%p
  int64_t	%qd, %lld	%"PRId64"
  uint64_t	%qu, %llu, %llx	%"PRIu64", %"PRIx64"
  size_t	%u	%"PRIuS", %"PRIxS"	C99 specifies %zu
  ptrdiff_t	%d	%"PRIdS"	C99 specifies %zd

  Note that the PRI* macros expand to independent strings which are concatenated by the compiler. Hence if you are using a non-constant format string, you need to insert the value of the macro into the format, rather than the name. It is still possible, as usual, to include length specifiers, etc., after the % when using the PRI* macros. So, e.g. printf("x = %30"PRIuS"\n", x) would expand on 32-bit Linux to printf("x = %30" "u" "\n", x), which the compiler will treat as printf("x = %30u\n", x).

• Remember that sizeof(void*) != sizeof(int). Use intptr_t if you want a pointer-sized integer.

• You may need to be careful with structure alignments, particularly for structures being stored on disk. Any class/structure with a int64_t/uint64_t member will by default end up being 8-byte aligned on a 64-bit system. If you have such structures being shared on disk between 32-bit and 64-bit code, you will need to ensure that they are packed the same on both architectures. Most compilers offer a way to alter structure alignment. For gcc, you can use __attribute__((packed)). MSVC offers #pragma pack() and __declspec(align()).

• Use the LL or ULL suffixes as needed to create 64-bit constants. For example:

  int64_t my_value = 0x123456789LL;
  uint64_t my_mask = 3ULL << 48;
  

• If you really need different code on 32-bit and 64-bit systems, use #ifdef _LP64 to choose between the code variants. (But please avoid this if possible, and keep any such changes localized.)

Preprocessor macros

Be very cautious with macros. Prefer inline functions, enums, and const variables to macros.

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.

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 (except, of course, for the #define guards to prevent double inclusion of header files). 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.

The following usage pattern will avoid many problems with macros; if you use macros, follow it whenever possible:

• Don’t define macros in a .h file.

• #define macros right before you use them, and #undef them right after.

• Do not just #undef an existing macro before replacing it with your own; instead, pick a name that’s likely to be unique.

• Try not to use macros that expand to unbalanced C++ constructs, or at least document that behavior well.

• Prefer not using ## to generate function/class/variable names.

0 and NULL

Use 0 for integers, 0.0 for reals, nullptr for pointers, and '\0' for chars.

Use 0 for integers and 0.0 for reals. This is not controversial.

For pointers (address values), C++11 added the nullptr construct. This allows the compiler to do additional checks, and is the preferred NULL pointer value.

Use '\0' for chars. This is the correct type and also makes code more readable.

sizeof

Use sizeof(varname) instead of sizeof(type) whenever possible.

Use sizeof(varname) because it will update appropriately if the type of the variable changes. sizeof(type) may make sense in some cases, but should generally be avoided because it can fall out of sync if the variable’s type changes.

Struct data;
memset(&data, 0, sizeof(data));

memset(&data, 0, sizeof(Struct));

C++11

Use C++11 features wherever appropriate.

Definition:

C++11 is the current ISO C++ standard. It contains significant changes both to the language and libraries from the older standard.

Naming

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.

General naming rules

Function names, variable names, and filenames should be descriptive; eschew abbreviation. Types and variables should be nouns, while functions should be “command” verbs.

How to Name

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. Examples of well-chosen names:

int num_errors;                  // Good.
int num_completed_connections;   // Good.

Poorly-chosen names use ambiguous abbreviations or arbitrary characters that do not convey meaning:

int n;                           // Bad - meaningless.
int nerr;                        // Bad - ambiguous abbreviation.
int n_comp_conns;                // Bad - ambiguous abbreviation.

Type and variable names should typically be nouns: e.g., FileOpener, num_errors.

Function names should typically be imperative (that is they should be commands): e.g., open_file(), set_num_errors(). There is an exception for accessors, which, described more completely in Function Names, should be named the same as the variable they access.

Abbreviations

Do not use abbreviations unless they are extremely well known outside your project. For example:

// Good
// These show proper names with no abbreviations.
int num_dns_connections;  // Most people know what "DNS" stands for.
int price_count_reader;   // OK, price count. Makes sense.

// Bad!
// Abbreviations can be confusing or ambiguous outside a small group.
int wgc_connections;  // Only your group knows what this stands for.
int pc_reader;        // Lots of things can be abbreviated "pc".

Never abbreviate by leaving out letters:

int error_count;  // Good.

int error_cnt;    // Bad.

File 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.cpp my-useful-class.cpp myusefulclass.cpp test_myusefulclass.cpp // _unittest and _regtest are deprecated.

C++ files should end in .cpp and header files should end in .h.

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. However, if your inline functions include a lot of code, they may go into a third file that ends in -inl.h. In a class with a lot of inline code, your class could have three files:

url_table.h      // The class declaration.
url_table.cpp     // The class definition.
url_table-inl.h  // Inline functions that include lots of code.

See also the section -inl.h Files

Type names

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, typedefs, and enums — 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:

// classes and structs
class UrlTable  ...
class UrlTableTester  ...
struct UrlTableProperties  ...

// typedefs
typedef hash_map<UrlTableProperties*, string> PropertiesMap;

// enums
enum UrlTableErrors ...

Variable names

Variable names are all lowercase, with underscores between words. Class member variables follow this convention. For instance: my_exciting_local_variable, my_exciting_member_variable.

Common Variable names

For example:

string table_name;  // OK - uses underscore.
string tablename;   // OK - all lowercase.

string tableName;   // Bad - mixed case.

Class Data Members

Data members (also called instance variables or member variables) are lowercase with optional underscores like regular variable names.

string table_name;  // OK - underscore at end.
string tablename;   // OK.

Struct Variables

Data members in structs should be named like regular variables.

struct UrlTableProperties
{
    string name;
    int num_entries;
}

See Structs vs. Classes for a discussion of when to use a struct versus a class.

Global Variables

There are no special requirements for global variables, which should be rare in any case, but if you use one, consider prefixing it with g_ or some other marker to easily distinguish it from local variables.

Constant names

Name constants like other variables, using all lowercase, with underscores between words. default_width.

It is distracting that whether a variable is “const” affects the name.

auto const match = map.find(value);
int const width{1024};
int height{900};

Function names

Regular functions, accessors, and mutators are all lowercase, with underscores between words.

Regular Functions

Functions and accessors are all lowercase, with underscores between words.

If your function crashes upon an error, you should append _or_die to the function name. This only applies to functions which could be used by production code and to errors that are reasonably likely to occur during normal operation.

add_table_entry()
delete_url()
open_file_or_die()

Accessors and Mutators

Accessors and mutators (get and set functions) should follow the naming conventions for regular functions.

class MyClass
{
public:
    ...
    int get_num_entries() const { return num_entries; }
    void set_num_entries(int value) { num_entries = value; }

private:
    int num_entries;
};

Namespace names

Namespace names are all lower-case, and based on project names and possibly their directory structure: my_awesome_project.

See Namespaces for a discussion of namespaces and how to name them.

Enumerator names

Enumerators should be named like member variables: out_of_memory, enclosed within an enum class.

Preferably, the individual enumerators should be named like class data members. The enumeration name, UrlTableErrors, is a type, and therefore mixed case.

enum class UrlTableErrors
{
    ok,
    out_of_memory,
    malformed_input,
};

Macro names

You’re not really going to define a macro, 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.

#define ROUND(x) ...
#define PI_ROUNDED 3.0

Exceptions to naming rules

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

a constant, as in INT_MAX

Comments

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!

Comment style

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.

File comments

Start each file with a copyright notice, followed by a description of the contents of the file.

Legal Notice and Author Line

Every file should contain the following items, in order:

• a optional mode line, // -*- Mode: C++; indent-tabs-mode: nil; tab-width: 2 -*-

• a copyright statement (for example, Copyright (C) 2011 Canonical Ltd)

• the license boilerplate.

• an author line to identify the original author of the file

If you make significant changes to a file that someone else originally wrote, add yourself to the author line. This can be very helpful when another contributor has questions about the file and needs to know whom to contact about it.

File Contents

Every file should have a comment at the top, below the copyright notice and author line, that describes the contents of the file.

Generally a .h file will describe the classes that are declared in the file with an overview of what they are for and how they are used. A .cpp file should contain more information about implementation details or discussions of tricky algorithms. If you feel the implementation details or a discussion of the algorithms would be useful for someone reading the .h, feel free to put it there instead, but mention in the .cpp that the documentation is in the .h file.

Do not duplicate comments in both the .h and the .cpp. Duplicated comments diverge.

Class comments

Every class definition should have an accompanying comment that describes what it is for and how it should be used.

// Iterates over the contents of a GargantuanTable.  Sample usage:
//    GargantuanTableIterator* iter = table->new_iterator();
//    for (iter->seek("foo"); !iter->done(); iter->next())
//    {
//        process(iter->key(), iter->value());
//    }
//    delete iter;
class GargantuanTableIterator
{
  ...
};

If you have already described a class in detail in the comments at the top of your file feel free to simply state “See comment at top of file for a complete description”, but be sure to have some sort of comment.

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.

Function comments

Declaration comments describe use of the function; comments at the definition of a function describe operation.

Function Declarations

Every function declaration should have comments immediately preceding it that describe what the function does and how to use it. 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:

• What the inputs and outputs are.

• For class member functions: whether the object remembers reference arguments beyond the duration of the method call, and whether it will free them or not.

• If the function allocates memory that the caller must free.

• Whether any of the arguments can be NULL.

• If there are any performance implications of how a function is used.

• If the function is re-entrant. What are its synchronization assumptions?

Here is an example:

// Returns an iterator for this table.  It is the client's
// responsibility to delete the iterator when it is done with it,
// and it must not use the iterator once the GargantuanTable object
// on which the iterator was created has been deleted.
//
// The iterator is initially positioned at the beginning of the table.
//
// This method is equivalent to:
//    Iterator* iter = table->new_iterator();
//    iter->seek("");
//    return iter;
// If you are going to immediately seek to another place in the
// returned iterator, it will be faster to use new_iterator()
// and avoid the extra seek.
Iterator* get_iterator() const;

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.

// Returns true if the table cannot hold any more entries.
bool is_table_full();

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.

Function Definitions

Each function definition should have a comment describing what the function does if there’s anything tricky about how it does its job. 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.

Variable comments

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.

Class Data Members

Each class data member (also called an instance variable or member variable) should have a comment describing what it is used for. If the variable can take sentinel values with special meanings, such as NULL or -1, document this. For example:

private:
    // Keeps track of the total number of entries in the table.
    // Used to ensure we do not go over the limit. -1 means
    // that we don't yet know how many entries the table has.
    int num_total_entries;

Global Variables

As with data members, all global variables should have a comment describing what they are and what they are used for. For example:

// The total number of tests cases that we run through in this regression test.
const int number_of_test_cases = 6;

Implementation comments

In your implementation you should have comments in tricky, non-obvious, interesting, or important parts of your code.

Class Data Members

Tricky or complicated code blocks should have comments before them. Example:

// Divide result by two, taking into account that x
// contains the carry from the add.
for (int i = 0; i < result->size(); i++)
{
    x = (x << 8) + (*result)[i];
    (*result)[i] = x >> 1;
    x &= 1;
}

Line Comments

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:

// If we have enough memory, mmap the data portion too.
mmap_budget = max<int64>(0, mmap_budget - index_->length());
if (mmap_budget >= data_size_ && !mmap_data(mmap_chunk_bytes, mlock))
    return;  // Error already logged.

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:

do_something();                      // Comment here so the comments line up.
do_Something_else_that_is_longer();  // Comment here so there are two spaces between
                                     // the code and the comment.
{   // Three space before comment when opening a new scope is allowed,
    // thus the comment lines up with the following comments and code.
    do_something_else();  // Two spaces before line comments normally.
}

nullptr, true/false, 1, 2, 3…

When you pass in nullptr, boolean, or literal integer values to functions, you should consider adding a comment about what they are, or make your code self-documenting by using constants. For example, compare:

bool success = calculate_something(interesting_value,
                                   10,
                                   false,
                                   nullptr);  // What are these arguments??

versus:

bool success = calculate_something(interesting_value,
                                   10,        // Default base value.
                                   false,     // Not the first time we're calling this.
                                   nullptr);  // No callback.

Or alternatively, constants or self-describing variables:

int const default_base_value = 10;
bool const first_time_calling = false;
Callback* null_callback = nullptr;
bool success = calculate_something(interesting_value,
                                   default_base_value,
                                   first_time_calling,
                                   null_callback);

Don’ts

Note that you should never describe the code itself. Assume that the person reading the code knows C++ better than you do, even though he or she does not know what you are trying to do:

// Now go through the b array and make sure that if i occurs,
// the next element is i+1.
...        // Geez.  What a useless comment.

Punctuation, spelling and grammar

Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.

Comments should usually be written as complete sentences with proper capitalization and periods at the end. 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. Complete sentences are more readable, and they provide some assurance that the comment is complete and not an unfinished thought.

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.

TODO comments

Use TODO comments 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, or other identifier of the person who can best provide context about the problem referenced by the TODO. A colon is optional. The main purpose is to have a consistent TODO format that can be searched to find the person who can provide more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO, it is almost always your name that is given.

// TODO([email protected]): Use a "*" here for concatenation operator.
// TODO(Zeke) change this to use relations.

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.”).

Deprecation comments

Mark deprecated interface points with DEPRECATED comments.

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.

Formatting

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.

Line length

Each line of text in your code should be at most 120 characters long.

We recognize that this rule is controversial, with so much existing code already adhering to a maximum line length of 80 characters. However, we favor sensible naming of variables and functions over the limit of 80 characters.

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:

120 characters is the maximum.

Non-ASCII characters

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, unit test 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" is the Unicode zero-width no-break space character, which would be invisible if included in the source as straight UTF-8.

Spaces vs. tabs

Use only spaces, and indent 4 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.

Function declarations and definitions

void or auto on the same line as function name, parameters and return type on the same line if they fit.

Functions look like this:

auto ClassName::function_name(Type par_name1, Type par_name2) -> ReturnType
{
    do_something();
    ...
}

or:

auto ClassName::long_function_name(Type par_name1, Type par_name2, Type par_name3)
-> ReturnType
{
    do_something();
    ...
}

or:

auto ClassName::really_really_really_long_function_name(
    Type par_name1,
    Type par_name2,
    Type par_name3) -> ReturnType
{
    do_something();
    ...
}

Some points to note:

• void or auto is always on the same line as the function name.

• The open parenthesis is always on the same line as the function name.

• There is never a space between the function name and the open parenthesis.

• There is never a space between the parentheses and the parameters.

• The open curly brace is always on the line following the last parameter or return type.

• The close curly brace is either on the last line by itself or (if other style rules permit) on the same line as the open curly brace.

• All parameters should be named, with identical names in the declaration and implementation. (Except where unused parameters are suppressed in the implementation.)

• All parameters should be in a single line if possible, otherwise:

  • Place each parameter in a separate line and indent each with 4 spaces.

  • Wrap groups of parameters into the next line, and indent each with 4 spaces.

If your function is const, the const keyword should be on the same line as the last parameter:

// Everything in this function signature fits on a single line
auto function_name(Type par) const -> ReturnType
{
    ...
}

// This function signature requires multiple lines, but
// the const keyword is on the line with the last parameter.
void really_long_function_name(
    Type par1,
    Type par2) const
{
    ...
}

If some parameters are unused, comment out the variable name in the function definition:

// Always have named parameters in interfaces.
class Shape
{
public:
    virtual void rotate(double radians) = 0;
}

// Always have named parameters in the declaration.
class Circle : public Shape
{
public:
    virtual void rotate(double radians);
}

// Comment out unused named parameters in definitions.
void Circle::rotate(double /*radians*/) {}

// Bad - if someone wants to implement later, it's not clear what the
// variable means.
void Circle::rotate(double) {}

Function calls

On one line if it fits; otherwise, wrap arguments at the parenthesis.

Function calls have the following format:

bool retval = do_something(argument1, argument2, argument3);

If the arguments do not all fit on one line, they should be broken up onto multiple lines, with each subsequent line indented four spaced. Do not add spaces after the open paren or before the close paren:

bool retval = do_something(
    averyveryveryverylongargument1,
    argument2, argument3);

If the function has many arguments, consider having one per line if this makes the code more readable:

bool retval = do_something(
    argument1,
    argument2,
    argument3,
    argument4);

Conditionals

Prefer no spaces inside parentheses. The else keyword belongs on a new line.

if (condition)  // no spaces inside parentheses
{
    ...  // 4 space indent.
}
else  // The else goes on a new line.
{
    ...
}

Note that in all cases you must have a space between the if and the open parenthesis.

if(condition)     // Bad - space missing after IF.

if (condition)   // Good - proper space after IF.

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.

if (x == foo) return new Foo();
if (x == bar) return new Bar();

This is not allowed when the if statement has an else:

// Not allowed - IF statement on one line when there is an ELSE clause
if (x) do_this();
else do_that();

Curly braces are preferred if the statement is on a different line than the condition.

if (condition)
{
    do_something();  // 4 space indent.
}

// Not allowed - single line IF statements without curly braces
if (condition)
    foo;

if (condition)
    foo;
else
    bar;

Loops and switch statements

Switch statements may use braces for blocks. Empty loop bodies should use {} 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:

switch (var)
{
case 0:      // no indent
    ...      // 4 space indent
    break;
case 1:
    {
        ...
        break;
    }
default:
    assert(false);
}

Empty loop bodies should use {} or continue, but not a single semicolon.

while (condition)
{
    // Repeat test until it returns false.
}
for (int i = 0; i < some_number_with_descriptive_name; ++i) {}  // Good - empty body.
while (condition) continue;  // Good - continue indicates no logic.

while (condition);  // Bad - looks like part of do/while loop.

Pointer and reference expressions

No spaces around period or arrow. Pointer operators do not have trailing spaces.

The following are examples of correctly-formatted pointer and reference expressions:

x = *p;
p = &x;
x = r.y;
x = r->y;

Note that:

• There are no spaces around the period or arrow when accessing a member.

• Pointer operators have no space after the * or &.

When declaring a pointer variable or argument, you should place the asterisk adjacent to the type:

// These are fine
char* c;
string const& str;

char * c;  // Bad - spaces on both sides of *
char *c ;  // Bad - * next to variable name
string const & str;  // Bad - spaces on both sides of &

You should do this consistently within a single file, so, when modifying an existing file, use the style in that file.

Boolean expressions

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:

if (this_one_thing > this_other_thing &&
    a_third_thing == a_fourth_thing &&
    yet_another && last_one)
{
    ...
}

Note that when the code wraps in this example, both of the && logical AND operators are at the end of the line. 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.

Return values

Do not needlessly surround the return expression with parentheses.

Use parentheses in return expr; only where you would use them in x = expr;.

return result;                  // No parentheses in the simple case.
return (some_long_condition &&  // Parentheses ok to make a complex
        another_condition);     //     expression more readable.

return (value);                // You wouldn't write var = (value);
return(result);                // return is not a function!

Variable and array initialization

Use {}.

You may choose between = and (); the following are all correct:

 //C++11: default initialization using {}
    int n{5}; //zero initialization: n is initialized to 5
    int* p{}; //initialized to nullptr
    double d{}; //initialized to 0.0
    char s[12]{}; //all 12 chars are initialized to '\0'
    string s{}; //same as: string s;
    string name{"Some Name"};
    char* p=new char [5]{}; // all five chars are initialized to '\0'

Preprocessor directives

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.

// Good - directives at beginning of line
if (lopsided_score)
{
#if DISASTER_PENDING      // Correct -- Starts at beginning of line
    drop_everything();
# if NOTIFY               // OK but not required -- Spaces after #
    notify_client();
# endif
#endif
    back_to_normal();
}

// Bad - indented directives
if (lopsided_score)
{
    #if DISASTER_PENDING  // Wrong!  The "#if" should be at beginning of line
    drop_everything();
    #endif                // Wrong!  Do not indent "#endif"
    back_to_normal();
}

Class format

Sections in public, protected and private order.

The basic format for a class declaration (lacking the comments, see Class Comments for a discussion of what comments are needed) is:

      class MyClass : public OtherClass
      {
      public:
          MyClass();  // Regular 4 space indent.
          explicit MyClass(int var);
          ~MyClass() {}

          void some_function();
          void some_function_that_does_nothing() {}

          void set_some_var(int var) { some_var_ = var; }
          int some_var() const { return some_var_; }

      private:
          MyClass(MyClass const&) = delete;
          MyClass& operator=(MyClass const&) = delete;
          bool some_internal_function();

          int some_var_;
          int some_other_var;
      };

Things to note:

• Any base class name should be on the same line as the subclass name, subject to the 80-column limit.

• The public:, protected:, and private: keywords are not indented.

• Except for the first instance, these keywords should be preceded by a blank line. This rule is optional in small classes.

• Do not leave a blank line after these keywords.

• The public section should be first, followed by the protected and finally the private section.

• See Declaration Order for rules on ordering declarations within each of these sections.

Constructor initializer lists

Constructor initializer lists can be all on one line or with subsequent lines indented four spaces.

There are three acceptable formats for initializer lists:

// When it all fits on one line:
MyClass::MyClass(int var) : some_var_(var), some_other_var_(var + 1) {}

or

// When it requires multiple lines, indent 4 spaces, putting the colon on
// the constructor declaration line and commas at the end of the line.
MyClass::MyClass(int var) :
    some_var_(var),            // 4 space indent
    some_other_var_(var + 1)
{
    ...
    do_something();
    ...
}

or

// When it requires multiple lines, indent 4 spaces, putting the colon on
// the first initializer line and commas at the end of the line.
MyClass::MyClass(int var)
    : some_var_(var),            // 4 space indent
      some_other_var_(var + 1)
{
    ...
    do_something();
    ...
}

Namespace formatting

The contents of namespaces are not indented.

Namespaces do not add an extra level of indentation. For example, use:

namespace
{

void foo()  // Correct.  No extra indentation within namespace.
{
    ...
}

}  // namespace

Do not indent within a namespace:

namespace
{

  // Wrong.  Indented when it should not be.
  void foo()
  {
    ...
  }

}  // namespace

When declaring nested namespaces, put each namespace on its own line, with the opening brace on the line following.

namespace foo
{
namespace bar
{

Horizontal whitespace

Use of horizontal whitespace depends on location. Never put trailing whitespace at the end of a line.

General

int i = 0;        // Semicolons usually have no space before them.
int x[] = { 0 };  // Spaces inside braces for array initialization are
int x[] = {0};    // optional.  If you use them, put them on both sides!

// Spaces around the colon in inheritance and initializer lists.
class Foo : public Bar
{
public:
    // For inline function implementations, put spaces between the braces
    // and the implementation itself.
    Foo(int b) : Bar(), baz_(b) {}  // No spaces inside empty braces.
    void reset() { baz_ = 0; }  // Spaces separating braces from implementation.
    ...

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).

Operators

x = 0;              // Assignment operators always have spaces around
                    // them.
x = -5;             // No spaces separating unary operators and their
++x;                // arguments.
if (x && !y)
  ...
v = w * x + y / z;  // Binary operators usually have spaces around them,
v = w*x + y/z;      // but it's okay to remove spaces around factors.
v = w * (x + z);    // Parentheses should have no spaces inside them.

Templates and Casts

vector<string> x;           // No spaces inside the angle
y = static_cast<char*>(x);  // brackets (< and >), before
                            // <, or between >( in a cast.
vector<char*> x;            // Spaces between type and pointer are
                            // okay, but be consistent.
set<list<string>> x;        // C++11 now allows >> to close templates.
set<list<string> > x;       // Older C++ requiree a space in > >, and is allowed.

Vertical whitespace

Minimize use of vertical whitespace but apply it to enhance readability.

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:

• Blank lines at the beginning or end of a function very rarely help readability.

• Blank lines inside a chain of if-else blocks may well help readability.

Exceptions to the rules

The coding conventions described above are mandatory. However, like all good rules, these sometimes have exceptions, which we discuss here.

Existing non-conformant code

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.

Parting words

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!