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.¹

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, reduce its visibility in other translation units by declaring the object as static or in an unnamed namespace.

Polyspace Implementation

Polyspace^® reports a violation of this rule if you declare an object or function with external linkage outside of a header file. These objects and functions have external linkage when they are declared at the namespace level, unless they are part of an explicitly unnamed namespace:

Functions not declared static
Non-const variables not declared static
Variables declared extern
Enumerations
Names of classes, class member functions, static data members
Names of nested class and enumerations
Names of friend functions declared within the class body

Functions and extern objects declared at a block scope also have external linkage.

For details about which objects and functions have external linkage, see Storage class specifiers.

Troubleshooting

If you expect a rule violation but Polyspace does not report it, see Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

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Variable with External Linkage Declared Outside Header File

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;

The variables x and y have external linkage, but only x is declared in a header file. Polyspace reports a violation for the declaration of the external variable y outside of a header file.

The variable z is declared with the static specifier and does not have external linkage. Declaring it outside of a header file is compliant with this rule.

Check Information

Group: Basic Concepts

Category: Required

Version History

Introduced in R2013b