Keyword 'extern' is used to specify external linkage. It is implicit in function declarations written at global and namespace scope and should not be used in such declarations. Global and namespace scope const objects and typedefs have internal linkage. Recommended practice is to define all const objects with internal linkage in header files only. Hence extern qualification is only necessary when declaring data objects with external linkage.

   
   // Header file:
   const float s = 3.0E8F;    // internal linkage constant definition
   extern int a;              // external linkage object declaration
   int foo( int );            // external linkage function declaration
   
   // Implementation file:
   int a = 2;                 // external linkage object definition

Rule 11.2

Stroustrup;