It's a bit OT, but for those of you who know C++ well, it's worth to think of a way to write a program that does something different in C89 and C++, work reliably and portably in both, but not in some obvious way like checking for a macro that only one of them defines.

I know of four ways but all of them use sizeof. The first uses the fact that apostrophe char-literals are of a different type in C and C++. The second uses that the visibility of a type declared inside a structure is different. The third uses the fact that struct labels are optional in C++ but not in C. The fourth is the only one that's my own invention:

typedef double number; struct hypergeom { number a, b, c; }; 
struct { struct hypergeom operator, (number); } function;
int main(void) {
        if(sizeof(number) == sizeof(function = function, (number)4))
                printf("C89\n");
        else
                printf("C++\n");
        return 0;
}
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I'd like to see a way to do this without sizeof. I couldn't figure out any even with the help of this list: http://david.tribble.com/text/cdiffs.htm. If you know any, please tell me.