Re: [boost] BOOST_STATIC_CONSTANT

Subject: Re: [boost] BOOST_STATIC_CONSTANT
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-08-17 00:19:49

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 };
};