Integer constant expression macros with comptime assertions in them

Question

Some of my C macros, which need to expand to integer constant expressions, have compile time assertions in them based on:

#define stc_assert_expr(CexprB) sizeof(struct{int _:(CexprB)?1:-1;})

which could be alternatively spelled as

#include <assert.h>
#define stc_assert_expr(CexprB) sizeof(struct{static_assert(CexprB,#CexprB);})
//^not sure if this is legal C but it compiles with gcc and clang
//(I'm using the bitfield version anyway, which is definitely legal C)

Dumbified example usage:

#include <assert.h>

#define stc_assert_expr(CexprB) sizeof(struct{int _:(CexprB)?1:-1;})
int main(int argc, char**argv)
{
#define minus1(X) (0*stc_assert_expr((X)>0)+(X)-1) \
    /*an integer constant expression with an assertionin it*/

    char ar[minus1(3)];
    switch(argc){
        case minus1(2): ;
    }
}

Suppose I wanted to make these macros usable in C++ also.

The above example doesn't work because C++ won't accept struct definitions inside sizeof. Is there a C++ construct I can replace my stc_assert_expr(CexprB) with that would keep the static_assert semantics?

Answer 1

You can use an array expression instead of a struct definition - negative array sizes are illegal in both languages:

#define stc_assert_expr(condition) sizeof(char[(condition) ? 1 : -1])