Re: [boost] [result_of] BOOST_RESULT_OF_USE_DECLTYPE in a C++03 mode with libc++

Subject: Re: [boost] [result_of] BOOST_RESULT_OF_USE_DECLTYPE in a C++03 mode with libc++
From: Eric Niebler (eric_at_[hidden])
Date: 2012-10-08 13:55:17

On 10/8/2012 9:59 AM, Michel Morin wrote:
> Eric Niebler wrote:
>> On 10/8/2012 6:08 AM, Michel Morin wrote:
>>> This is just a small note related to boost::result_of.
>>>
>>> When BOOST_RESULT_OF_USE_DECLTYPE is defined,
>>> boost::result_of tries to use decltype-based implementation
>>> (regardless of the compiler's decltype support).
>>>
>>> Surprisingly, in a C++03 mode, libc++ defines `decltype`
>>> using `__typeof__`:
>>>
>>>   Bug 12488 (WONTFIX):
>>>   decltype can be used in C++03 with no errors, no warnings
>>>   http://llvm.org/bugs/show_bug.cgi?id=12488
>>>
>>> So, when used with libc++, boost::result_of can be used with
>>> BOOST_RESULT_OF_USE_DECLTYPE even in a C++03 mode.
>>
>> Does __typeof__ have EXACTLY the same semantics as C++11 decltype?
> 
> No, it doesn't.
> I don't like the idea of defining `decltype` macro in libc++:
> 
>   #ifdef _LIBCPP_HAS_NO_DECLTYPE
>   #define decltype(x) __typeof__(x)
>   #endif
> 
> I created the bug report to libc++, but it got WONTFIX.

Oh, I get it now. That's not good. I've added my $0.02 to the bug
report. Thanks for making me aware of this.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com