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Subject: Re: [boost] BOOST_STATIC_CONSTANT
From: Vladimir Batov (Vladimir.Batov_at_[hidden])
Date: 2017-08-17 00:45:46


On 2017-08-17 10:19, Gavin Lambert via Boost wrote:
> On 17/08/2017 11:22, Vladimir Batov wrote:
>> There is something seemingly basic that puzzles me greatly. I am
>> hoping people might clarify something I totally fail to understand.
>>
>> I've been using BOOST_STATIC_CONSTANT for ages (with GCC). Now I have
>> something seemingly non-controversial:
>>
>> template<typename user_type>
>> struct impl_ptr
>> {
>> using yes_type = boost::type_traits::yes_type;
>> using no_type = boost::type_traits::no_type;
>> using ptr_type = typename std::remove_const<user_type>::type*;
>>
>> template<typename Y>
>> static yes_type test (Y*, typename Y::impl_ptr_type* =nullptr);
>> static no_type test (...);
>>
>> BOOST_STATIC_CONSTANT(bool, value = (1 ==
>> sizeof(test(ptr_type(nullptr)))));
>> };
>>
>> When compiled with gcc-.5.4.0, both lines below pass:
>>
>> BOOST_TEST(true == boost::impl_ptr<Shared>::value);
>> BOOST_TEST(true == boost::impl_ptr<Shared const>::value);
>>
>> The only difference between them is "const". However, when I compile
>> with clang-4.0, the second (const) line passes. However, the first one
>> fails! I refuse to believe it's a clang glitch. It must be me doing
>> something really stupid. Help?!
>
> I can't answer this specific question, but since you're targeting
> C++11 anyway, shouldn't you be using std:: type traits?
>
> So, something like:
>
> template <typename T>
> class has_impl_ptr_type
> {
> template <typename T1>
> static typename T1::impl_ptr_type test(int);
> template <typename>
> static void test(...);
> public:
> enum { value = !std::is_void<decltype(test<T>(0))>::value };
> };

Thanks, Gavin. Is it how it is done these days? Nice. Incorporated. Many
thanks... clang still fails... now I am starting to look suspiciously in
clang's direction.


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