MISRA C++:2008 Rule 3-3-1

Objects or functions with external linkage shall be declared in a header file

Description

Rule Definition

Objects or functions with external linkage shall be declared in a header file.

Rationale

If you declare a function or object in a header file, it is clear that the function or object is meant to be accessed in multiple translation units. If you intend to access the function or object from a single translation unit, declare it static or in an unnamed namespace.

Troubleshooting

If you expect a rule violation but do not see it, refer to Coding Standard Violations Not Displayed.

Examples

Declaration in Header File Missing

This example uses two files:

• decls.h:

  extern int x;

• file.cpp:

  #include "decls.h"
  
  int x = 0;
  int y = 0; //Noncompliant
  static int z = 0;

In this example, the variable x is declared in a header file but the variable y is not. The variable z is also not declared in a header file but it is declared with the static specifier and does not have external linkage.

Check Information

Group: Basic Concepts

Category: Required