That's the problem with C's flat namespace. C++ namespaces address that, but at significant cost.

One possibility is to add something like:

#if defined( _WINREG_H ) && !defined( _PROJECT_H )
typedef PVALUEA PVALUE;
#elif defined( _PROJECT_H ) &&  !defined( _WINREG_H )
typedef union { double  f; int  i; char *s; } PVALUE;
#else
assert( "File " __FILE__ "requires PVALUE defined from both winreg.h a
+nd project.h" == NULL );
#endif
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That probably doesn't compile anywhere, but you get the idea.