Firstly, you can't just use 'extern' to allow you to put a variable in a header file. The way you need to do it, is to put some normal variable definitions in a CPP file. Doesn't matter which, but it makes sense to put them in something like globals.cpp. Then, you put references to those global variables in the header file globals.h. This is what makes them global. The globals themselves are not in the header file: only the extern references.
Example:
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That should fix any such problems you have regarding duplicated variables when it comes to link-time.
Secondly, the #pragma once directive can't stop a header being used more than once in a program. It just prevents it being used more than once for each compilation unit... ie. once for each .C or .CPP file you compile. Remember that each CPP file compiles independently to begin with, using whichever header files they need. It is only at the linker stage that they are combined into one... and therefore, any variables that are declared in header files may end up in more than one compiled CPP file, and therefore when the linker comes to put it all together, it finds duplicates. That is why you only put the actual variable definitions in one CPP file, and just put the extern declarations in the headers, which is enough to keep the compiler happy.
The #pragma once directive is roughly equivalent to using the standard inclusion guards on a header file, but is less portable. Therefore I recommend you get into the habit of putting your own inclusion guards in your own headers, like so:
#ifndef GLOBALS_H#define GLOBALS_H// ... the content of your header file here...#endif // GLOBALS_H Edited by - Kylotan on January 19, 2001 7:50:09 PM
