Declaring a global variable

The rule of the thumb is avoiding define entities with linkage in the header file. Since the header can be include by multiple cpp file, then compiler will complain the multiple definitions;

For example a head file a.hpp

int foo;

and this file will be included by two cpp files, then at link time compiler complains multiple definitions of foo; The problem roots in that ``int foo'' stands for both the declaration and definition;

The correct way is using ``extern'' to tell compiler it is just declaration , not a definition, which will be put in the cpp file. For example a.hpp contains

extern int foo;

and in one of the cpp file (viewed as an implementation file) put

int foo;

Then we just one implementation with multiple declaration;

Adding static qualifier before the declaration seems can have compiler work without any error.

static int foo;

but actually it is very dangerous since if two cpp files include the head file they actually have their own copy of integer foo rather than a global foo;

If the value is const, then in the head file

static const int foo = 9;
/* 
or
const int foo =9;
*/

It is fine since compiler will recognize it is unnecessary to keep multiple copies of foo for each cpp file;