Programming Forums - View Single Post - static class members and libraries (linking question)

Eggbert · Jun 22nd, 2005, 4:31 PM

Good question! The answer is that the library has no choice but to guarantee that the definition of a static member is performed only once. If a static member is defined multiple times, the behavior is undefined. As with anything falling under the One Definition Rule, the easiest fix is to provide a header with your declarations for inclusion in however many files you want:

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#ifndef C_H
#define C_H

class C {
  ...
  static int data;
  ...
};

#endif

And then a source file that is linked with the project once and provides definitions for declared names in the header file:

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#include "c.h"

int C::data = 0;

This is a safe and easy way to avoid multiple definitions, especially for situations where an error or warning is not required, such as this one.

Jun 22nd, 2005, 4:31 PM	#2
Eggbert Professional Programmer Join Date: Nov 2004 Posts: 250 Rep Power: 5	Good question! The answer is that the library has no choice but to guarantee that the definition of a static member is performed only once. If a static member is defined multiple times, the behavior is undefined. As with anything falling under the One Definition Rule, the easiest fix is to provide a header with your declarations for inclusion in however many files you want: (Toggle Plain Text) #ifndef C_H #define C_H class C { ... static int data; ... }; #endif #ifndef C_H #define C_H class C { ... static int data; ... }; #endif And then a source file that is linked with the project once and provides definitions for declared names in the header file: (Toggle Plain Text) #include "c.h" int C::data = 0; #include "c.h" int C::data = 0; This is a safe and easy way to avoid multiple definitions, especially for situations where an error or warning is not required, such as this one.