(C++) extern

 

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

 

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#include <cassert> #include "Unit1.h" #include "Unit2.h" int main() {   assert(GetX() == 42);   assert(GetY() == 42); }

 

 

 

 

 

Unit1.h

 

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#ifndef Unit1H #define Unit1H int GetX(); #endif

 

 

 

 

 

Unit1.cpp

 

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#include "Unit1.h" int x = 42; int GetX() { return x; }

 

 

 

 

 

Unit2.h

 

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#ifndef Unit2H #define Unit2H int GetY(); #endif

 

 

 

 

 

Unit2.cpp

 

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#include "Unit2.h" extern int x; int y = x; //Seems risky, dependent on module process order int GetY() { return y; }

 

 

 

 

 

References

 

1. 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'
2. 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'