Question

I want to compile a time error checking as mentioned below . But I am not able to find out how to use it inside main()?

#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
int main(){
BUILD_BUG_ON_NULL(12);
}

Below is the mentioned error

1--error C2332: 'struct' : missing tag name
2--error C2143: syntax error : missing ')' before '{'
3--error C2027: use of undefined type 'main::<unnamed-tag>'
4--error C2143: syntax error : missing ';' before '{'
5--error C2059: syntax error : ')'

Can anyone please let me know what I am doing wrong?

Answer 1

First the macros BUILD_BUG_ON_ZERO and BUILD_BUG_ON_NULL trigger a compilation error if their argument is different than 0.

If the macro argument is 0, they will not trigger any compilation error but yield a 0 for BUILD_BUG_ON_ZERO and a (void *) 0 for BUILD_BUG_ON_NULL

These macros comes from the Linux kernel which is written in C and they are only working for C programs.

In C++ these macros are not working. The reason is in C++ you cannot declare a structure in a sizeof expression.

You don’t mention in your question if you are compiling your program in C or in C++, but I strongly suspect you are compiling it in C++. So don’t use these macros in C++.