BOOST_STATIC_ASSERT works as follows. There is class STATIC_ASSERTION_FAILURE which is defined as:

namespace boost{

template <bool> struct STATIC_ASSERTION_FAILURE;

template <> struct STATIC_ASSERTION_FAILURE<true>{};

}

The key feature is that the error message triggered by the undefined expression sizeof(STATIC_ASSERTION_FAILURE<0>), tends to be consistent across a wide variety of compilers. The rest of the machinery of BOOST_STATIC_ASSERT is just a way to feed the sizeof expression into a typedef. The use of a macro here is somewhat ugly; however boost members have spent considerable effort trying to invent a static assert that avoided macros, all to no avail. The general conclusion was that the good of a static assert working at namespace, function, and class scope outweighed the ugliness of a macro.