# 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. This metafunction is a convenient way to leverage SFINAE prior to C++20's concepts, in particular for conditionally removing functions from the candidate set based on whether an expression is valid in the unevaluated context (such as operand to `decltype` expression), allowing to exist separate function overloads or specializations based on supported operations.

### 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 = void>
    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 = void>
    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 the resolution of CWG issue 1558 (a C++11 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;

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_void_t` | 201411L | (C++17) | `std::void_t`

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <map>
    #include <type_traits>
    #include <vector>

    // Variable template that checks if a type has begin() and end() member functions
    template<typename, typename = void>
    constexpr bool is_iterable{};

    template<typename T>
    constexpr bool is_iterable<
        T,
        std::void_t<decltype(std::declval<T>().begin()),
                    decltype(std::declval<T>().end())
        >
    > = true;

    // An iterator trait those value_type is the value_type of the iterated container,
    // supports even back_insert_iterator (where 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> {};

    class A {};

    #define SHOW(...) std::cout << std::setw(34) << #__VA_ARGS__ \
                                << " == " << __VA_ARGS__ << '\n'

    int main()
    {
        std::cout << std::boolalpha << std::left;
        SHOW(is_iterable<std::vector<double>>);
        SHOW(is_iterable<std::map<int, double>>);
        SHOW(is_iterable<double>);
        SHOW(is_iterable<A>);

        using container_t = std::vector<int>;
        container_t v;

        static_assert(std::is_same_v<
            container_t::value_type,
            iterator_trait<decltype(std::begin(v))>::value_type
        >);

        static_assert(std::is_same_v<
            container_t::value_type,
            iterator_trait<decltype(std::back_inserter(v))>::value_type
        >);
    }

Output:

    is_iterable<std::vector<double>>   == true
    is_iterable<std::map<int, double>> == true
    is_iterable<double>                == false
    is_iterable<A>                     == false

### See also

enable_if (C++11) | conditionally removes a function overload or template specialization from overload resolution (class template)

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