std::void_t(3)

🎀 NAME 🎀

<pink>okay so</pink> bestie, this is std::void_t — std::void_t. That’s the name, no cap. You’re on the std::void_t(3) manual page from the C++ Standard Library, and it’s honestly a lil template-metaprogramming icon 💅.

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✨ 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. It’s a convenient way to leverage SFINAE prior to C++20’s concepts, especially for conditionally removing functions from the candidate set based on whether an expression is valid in the unevaluated context (such as an operand to a decltype expression). That lets separate function overloads or specializations exist based on supported operations. slay ✨

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🧠 Notes 🧠

This metafunction is used in template metaprogramming to detect ill-formed types in a 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 🔬

• __cpp_lib_void_t — value: 201411L; std: (C++17); feature: std::void_t

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🌈 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 = false;

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

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👀 See also 👀

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

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mwah 💋 — source: http://cppreference.com, version/date: 2024.06.10, manual page: std::void_t(3). periodt 🫶