# std::void_t

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class... > using void_t = void; |  | (since C++17)

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:

    // primary template handles types that have no nested ::type member:
    template< class, class = std::void_t<> >
    struct has_type_member : std::false_type { };

    // specialization recognizes types that do have a nested ::type member:
    template< class T >
    struct has_type_member<T, std::void_t<typename T::type>> : std::true_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 (C++11) | hides a function overload or template specialization based on compile-time boolean (class template)

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