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

[Howard Hinnant]

On Jun 19, 2012, at 6:30 PM, Richard Smith wrote:

> On Tue, Jun 19, 2012 at 9:06 AM, Howard Hinnant <hhinnant at apple.com> wrote:
>> 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;
> 
> This isn't quite right: the first argument to static_assert is
> contextually converted to bool. So a more correct emulation would
> produce:
> 
>    typedef __static_assert_check<sizeof(__static_assert_test<bool(i ==
> 0)>)> __t6;
> 
> Testcase:
> 
>  int n;
>  static_assert(&n, "");

Right.  But your fix doesn't seem to be working:

test.cpp:4:2: error: non-type template argument of type 'bool' is not an integral constant expression
 static_assert(&n, "");
 ^~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode45-DP1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/__config:378:63: note: expanded from
      macro 'static_assert'
    typedef __static_assert_check<sizeof(__static_assert_test<(bool(__b))>)> \
                                                              ^~~~~~~~~~~
1 error generated.

Howard