Error checking through Macro

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

EDIT: the question was originally tagged as C++, but now as just C.

I'm not going to chase further fundamental changes of the question.

Original answer for the C++ tagged question:

---

This source code:

#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(0);
}

compiled with g++ 4.7.1., produced

foo.cpp: In function 'int main()':
foo.cpp:4:1: error: types may not be defined in 'sizeof' expressions
foo.cpp:4:21: warning: statement has no effect [-Wunused-value]

Which says directly what’s wrong.

So it is a good idea to compile with different compilers.

---

Possibly you’re looking for compile time assertions.

With C++11 you can use static_assert, e.g. via a macro such as

#define STATIC_ASSERT( e ) static_assert( e, #e )

In C++03 you could implement it as a valid/invalid typedef, because a typedef can be repeated in the same translation unit, and can be used in a class definition:

#define STATIC_ASSERT( e ) typedef char staticAssert_shouldBeTrue[e? 1 : -1]

One problem with that was that g++ had/has a compiler bug, where a repeated typedef is not always accepted as it should be, necessitating generation of a locally unique name for each one, e.g. by using __LINE__.

But you can always just use the definition in the Boost library, since Boost supports most extant compilers with special-casing for each one as necessary.

Answer 2

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++.

Answer 3

Compiling with gcc -std=c99 -pedantic-errors, I get

screwed.c: In function ‘main’:
screwed.c:5:1: error: negative width in bit-field ‘<anonymous>’
screwed.c:5:1: error: struct has no named members [-pedantic]

and those are the errors the compilation should give when the code is compiled as C. The width of a bit-field must be non-negative (positive if it has a name), and a struct must have at least one named member (two, if the last one is a flexible array member). structs without tags are allowed.

You either compiled the code not as C, or your compiler is non-conforming.

When compiling as C++, the additional error

error: types may not be defined in ‘sizeof’ expressions

is generated (but the one about the struct without named members disappears).

In C++, you may not define type in a sizeof expression, and your compiler chose a less clear way of telling you.