[cfe-dev] static_assert without C++11 and with a message?

[Howard Hinnant]

On Jun 19, 2012, at 3:22 AM, Stephen Kelly wrote:

> 
> Hi there,
> 
> I was looking into libc++, and noticed the implementation of static_assert 
> for pre-c++11 compilers.
> 
> Inside of the _LIBCPP_HAS_NO_STATIC_ASSERT ifdef, there is a typo (__t 
> should be __m I presume?),

Nope.

> but even when that is fixed, I can't see how the 
> static assert macro could work.
> 
> It would expand to 
> 
> typedef __static_assert_check<unsigned> "some message";
> 
> in both the true and false case. For the false case it fails to compile the 
> __static_assert_check<unsigned> already, but for the true case it also fails 
> to compile. How should that work?

For me, preprocessing this in C++03 mode:

#include <__config>

int main()
{
    const int i = 0;
    static_assert(i == 0, "");
}

Yields:

typedef __char16_t char16_t;
typedef __char32_t char32_t;

namespace std {
  inline namespace __1 {
  }
}

template <bool> struct __static_assert_test;
template <> struct __static_assert_test<true> {};
template <unsigned> struct __static_assert_check {};

int main()
{
    const int i = 0;
    typedef __static_assert_check<sizeof(__static_assert_test<(i == 0)>)> __t6;
}

Since i == 0 is true, __static_assert_test<true> is instantiated, which has a complete type.  So __t6 is a typedef for:

    __static_assert_check<sizeof(__static_assert_test<true>)>

If i != 0, an attempt would be made to form a typedef to:

    __static_assert_check<sizeof(__static_assert_test<false>)>

but __static_assert_test<false> is an incomplete type, and thus a compile time error will be emitted.

Is this not the behavior you're seeing?

Howard