Keyword to make a variable known over multiple units, but keeping the declaration and initialization local to a file (probably an implementation file (.cpp))

Example

In the example below there are two integer globals (note: avoid using global data [1,2]). The int x is declared and initialized in unit1.cpp, the int y in declared in unit2.cpp and initialized by the locally unknown int x. To read the values of both integers, two getters are put in the header files.

UnitMain.cpp


#include <cassert>

#include "Unit1.h"

#include "Unit2.h"

 

int main()

{

  assert(GetX() == 42);

  assert(GetY() == 42);

}

Unit1.h


#ifndef Unit1H

#define Unit1H

 

int GetX();

 

#endif

Unit1.cpp


#include "Unit1.h"



int x = 42;



int GetX() { return x; }

Unit2.h


#ifndef Unit2H

#define Unit2H



int GetY();



#endif

Unit2.cpp


#include "Unit2.h"



extern int x;



int y = x; //Seems risky, dependent on module process order



int GetY() { return y; }

References

Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Item 10: 'Minimize global and shared data'
Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Item 18: 'Declare variables as locally as possible'

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