std::void_t (3) Linux Manual Page

std::void_t – std::void_t

Synopsis

Defined in header<type_traits>

    template <class...>
    (since C++ 17)

        using void_t = void;

Utility metafunction that maps a sequence of any types to the type void

Notes

This metafunction is used in template metaprogramming to detect ill-formed types in SFINAE context:

template <typename... Ts>
struct make_void {
    typedef void type;
};

template <typename... Ts>
using void_t = typename make_void<Ts...>::type;

It can also be used to detect validity of an expression:

// primary template handles types that do not support pre-increment:

template <class, class = std::void_t<>>

struct has_pre_increment_member : std::false_type {
};

// specialization recognizes types that do support pre-increment:

template <class T>

struct has_pre_increment_member<T,
                                std::void_t<decltype(++std::declval<T &>())>> : std::true_type {
};

Until CWG_1558 (a C++14 defect), unused parameters in alias_templates were not guaranteed to ensure SFINAE and could be ignored, so earlier compilers require a more complex definition of void_t, such as

template <typename... Ts>
struct make_void {
    typedef void type;
};

template <typename... Ts>
using void_t = typename make_void<Ts...>::type;

Examples

// Run this code

#include <iostream>

#include <type_traits>

#include <vector>

#include <map>
class A
{
};
template <typename T, typename = void>

struct is_iterable : std::false_type {
};

template <typename T>

struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin()),
                                  decltype(std::declval<T>().end())>>
    : std::true_type {
};

  // An iterator trait which value_type is always the value_type of the
  // iterated container, even with back_insert_iterator which value_type is void

template <typename T, typename = void>

struct iterator_trait

    : std::iterator_traits<T> {
};

template <typename T>

struct iterator_trait<T, std::void_t<typename T::container_type>>

    : std::iterator_traits<typename T::container_type::iterator> {
};
int main()

{
    std::cout << std::boolalpha;
    std::cout << is_iterable<std::vector<double>>::value << '\n';
    std::cout << is_iterable<std::map<int, double>>::value << '\n';
    std::cout << is_iterable<double>::value << '\n';
    std::cout << is_iterable<A>::value << '\n';

    std::vector<int> v;
    std::cout << std::is_same<iterator_trait<decltype(std::back_inserter(v))>::value_type, iterator_trait<decltype(v.cbegin())>::value_type>::value << '\n';
}

Output:

  true
  true
  false
  false
  true

See also

enable_if hides a function overload or template specialization based on compile-time boolean
          (class template)
(C++11)