Reason

It's a longstanding convention. But consistency is more important, so if your project uses something else, follow that.

Note

This convention reflects a common use pattern: Headers are more often shared with C to compile as both C++ and C, which typically uses .h, and it's easier to name all headers .h instead of having different extensions for just those headers that are intended to be shared with C. On the other hand, implementation files are rarely shared with C and so should typically be distinguished from .c files, so it's normally best to name all C++ implementation files something else (such as .cpp).

The specific names .h and .cpp are not required (just recommended as a default) and other names are in widespread use. Examples are .hh, .C, and .cxx. Use such names equivalently. In this document, we refer to .h and .cpp as a shorthand for header and implementation files, even though the actual extension might be different.

Your IDE (if you use one) might have strong opinions about suffixes.

Example

// foo.h:
extern int a;   // a declaration
extern void foo();

// foo.cpp:
int a;   // a definition
void foo() { ++a; }

foo.h provides the interface to foo.cpp. Global variables are best avoided.

Example, bad

// foo.h:
int a;   // a definition
void foo() { ++a; }

#include <foo.h> twice in a program and you get a linker error for two one-definition-rule violations.

Enforcement

• Flag non-conventional file names.
• Check that .h and .cpp (and equivalents) follow the rules below.

FAQ: Answers to frequently asked questions

This section covers answers to frequently asked questions about these guidelines.