CODEWITHSUNDEEP
c++templatesc++17
user3721426
user3721426

Reputation: 273

Why can I not deduce if the static member function exists

I have the following code:

#include <utility>

template<class T,class E = void>
struct func_impl;

template<class T,class E = void>
constexpr inline bool has_func = false;

template<class T>
constexpr inline bool has_func<T,decltype(func_impl<T>::apply(std::declval<T>()))> = true;

template<>
struct func_impl<int>
{
   static int apply(int i);
};

static_assert(has_func<int>);

The static_assert fails, where I expected it to succeed. What did I do wrong?

Upvotes: 4

Views: 167

Answers (1)

songyuanyao
songyuanyao

Reputation: 172994

The problem is the default value of the 2nd template parameter E comes from the primary template, it's void and doesn't match the template parameter specialized in the specialization; which is specialized as decltype(func_impl<T>::apply(std::declval<T>())) (i.e. int in this case). Then the primary template but not specializatioin will be selected.

You could use std::void_t.

template<class T>
constexpr inline bool has_func<T, std::void_t<decltype(func_impl<T>::apply(std::declval<T>()))>> = true;
//                                ^^^^^^^^^^^                                                 ^

LIVE

Upvotes: 6

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