Re: [boost] BOOST_STATIC_CONSTANT

Subject: Re: [boost] BOOST_STATIC_CONSTANT
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2017-08-17 10:50:42

On 08/17/17 08:33, Gavin Lambert via Boost wrote:
> On 17/08/2017 16:35, dodheim wrote:
>> Boost - Dev mailing list wrote
>>> On 17/08/2017 13:27, Glen Fernandes wrote:
>>>> This is to detect a member 'impl_ptr_type' type? Just:
>>>>
>>>>       template<class, class = void>
>>>>       struct has_impl_ptr_type
>>>>           : boost::false_type { };
>>>>       template
>>> <class T>
>>>>       struct has_impl_ptr_type&lt;T, boost::void_t&lt;typename
>>>> T::impl_ptr_type&gt; >
>>>>           : boost::true_type { };
>>>
>>> That *is* a nicer way to do it.  Requires a minimum of Boost 1.64 or
>>> C++17, though.  (Or VS2015, apparently.)
>>
>> This only requires C++11; it doesn't work on earlier versions of MSVC
>> because MSVC was (and still is) lagging behind on support for 
>> expression-SFINAE.
> 
> std::void_t does not exist until C++17.  However the VS2015 STL adds it 
> anyway.
> 
> boost::void_t does not exist until 1.64.  However it only works on 
> compilers that support expression-SFINAE (which is most C++11 compilers 
> except VS, until VS2015).

Boost.Core has had enable_if_has_type for this purpose for a long time.

By itself, neither void_t nor enable_if_has_type requires expression 
SFINAE. It will only require one in some uses, typically involving 
decltype, when you test if an expression is valid (i.e. has a type). It 
works with as far as C++03 with regard to testing for nested types.