A header file must not contain object definitions or non-inline function definitions

Reason

Including entities subject to the one-definition rule leads to linkage errors.

Example

// file.h:
namespace Foo {
    int x = 7;
    int xx() { return x+x; }
}

// file1.cpp:
#include <file.h>
// ... more ...

 // file2.cpp:
#include <file.h>
// ... more ...

Linking file1.cpp and file2.cpp will give two linker errors.

Alternative formulation: A header file must contain only:

• #includes of other header files (possibly with include guards)
• templates
• class definitions
• function declarations
• extern declarations
• inline function definitions
• constexpr definitions
• const definitions
• using alias definitions
• ???

Enforcement

Check the positive list above.