Trait to check if some specialization of template class is base class of specific class

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囚心锁ツ 2020-12-10 14:37

There is std::is_base_of in modern STL. It allow us to determine whether the second parameter is derived from first parameter or if they are the same classes bo

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温柔的废话 (楼主)

2020-12-10 15:05

If you can assume that a derived type uses a public inheritance from B (and so the upcasting is possible), then you can use the following SFINAE:

namespace detail
{
    template 
    struct is_derived_from_B
    {
        using U = typename std::remove_cv<
                                  typename std::remove_reference::type
                                >::type;

        template 
        static auto test(B*)
            -> typename std::integral_constant>::value>;

        static std::false_type test(void*);

        using type = decltype(test(std::declval()));
    };
}

template 
using is_derived_from_B = typename detail::is_derived_from_B::type;

Tests:

static_assert(is_derived_from_B>::value, "!");
static_assert(!is_derived_from_B::value, "!");
static_assert(!is_derived_from_B>::value, "!");
static_assert(!is_derived_from_B>::value, "!");

DEMO 1

It can be generalized to accept any base class template:

namespace detail
{
    template