Unofficial Autosar C++ Guidelines Reference

Rule A3-3-1 (required, implementation, automated)

Objects or functions with external linkage (including members of named namespaces) shall be declared in a header file.

Rationale

Placing the declarations of objects and functions with external linkage in a header file means that they are intended to be accessible from other translation units. If external linkage is not needed, then the object or function is supposed to be either declared in an unnamed namespace or declared static in the implementation file. This reduces the visibility of objects and functions, which allows to reach a higher encapsulation and isolation. Note that members of named namespace are by default external linkage objects.

Exception

This rule does not apply to main, or to members of unnamed namespaces.

Example

//% $Id: A3-3-1.hpp 289436 2017-10-04 10:45:23Z michal.szczepankiewicz $
#include <cstdint>
extern std::int32_t a1;
extern void F4();

namespace n
{
void F2();
std::int32_t a5; // Compliant, external linkage
}

```cpp
//% $Id: A3-3-1.cpp 289436 2017-10-04 10:45:23Z michal.szczepankiewicz $
#include "A3-3-1.hpp"
std::int32_t
a1 = 0;
// Compliant, external linkage
std::int32_t
a2 = 0;
// Non-compliant, static keyword not used
static std::int32_t a3 = 0; // Compliant, internal linkage
namespace
{
std::int32_t a4 = 0; // Compliant by exception
9 void F1()
// Compliant by exception
{
}
}
namespace n
{
void F2() // Compliant, external linkage
{
}
std::int32_t a6 = 0; // Non-compliant, external linkage
}
extern std::int32_t a7; // Non-compliant, extern object declared in .cpp file
static void F3()// Compliant, static keyword used
{
}
void F4() // Compliant, external linkage
{
a1 = 1;
a2 = 1;
a3 = 1;
a4 = 1;
n::a5 = 1;
n::a6 = 1;
a7 = 1;
}
void F5() // Non-compliant, static keyword not used
{
a1 = 2;
a2 = 2;
a3 = 2;
a4 = 2;
n::a5 = 2;
n::a6 = 2;
a7 = 2;
}

int main(int, char**) // Compliant by exception

{

F1();

n::F2();
F3();
F4();
F5();

}

See also

MISRA C++ 2008 [7]: Rule 3-3-1 Objects or functions with external linkage shall be declared in a header file.