std::invoke,std::invoke_r(3)

🌸 NAME 🌸

std::invoke, std::invoke_r — okay so these are the ✨ universal callable-glue✨ functions. Manual name is C++ Standard Libary (yes bestie, that's how it's spelled on the tin, no cap, we're leaving it 💅).

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🎀 Synopsis 🎀

Defined in header <functional>

template< class F, class... Args >

std::invoke_result_t<F, Args...>                         (1) (since C++17)
                                                              (constexpr since C++20)
    invoke( F&& f, Args&&... args ) noexcept(/* see below */);
template< class R, class F, class... Args >

constexpr R                                              (2) (since C++23)

    invoke_r( F&& f, Args&&... args ) noexcept(/* see below */);

1. Calls the Callable object f with the parameters args as if by INVOKE(std::forward<F>(f), std::forward<Args>(args)...). This overload only shows up in overload resolution if std::is_invocable_v<F, Args...> is true — like, it literally won't even be in the room otherwise 😤
2. Calls the Callable object f with the parameters args as if by INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...). This overload only participates in overload resolution if std::is_invocable_r_v<R, F, Args...> is true. Same energy, different gatekeeping 🚪✨

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🦋 Parameters 🦋

• f — the Callable object getting invoked, bestie. it's the star of the show.
• args — the arguments you're passing to f. feed the icon what it needs 🍓

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💖 Return value 💖

1. The value returned by f. simple. clean. slay.
2. The value returned by f, implicitly converted to R, if R is not (possibly cv-qualified) void. None otherwise — so if it's void, you get literally nothing back and that's fine 💀

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🔥 Exceptions 🔥

1. noexcept specification: noexcept(std::is_nothrow_invocable_v<F, Args...>)
2. noexcept specification: noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>)

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👑 Possible implementation 👑

invoke (1)

namespace detail
{
    template<class>
    constexpr bool is_reference_wrapper_v = false;
    template<class U>
    constexpr bool is_reference_wrapper_v<std::reference_wrapper<U>> = true;

    template<class T>
    using remove_cvref_t = std::remove_cv_t<std::remove_reference_t<T>>;

    template<class C, class Pointed, class Object, class... Args>
    constexpr decltype(auto) invoke_memptr(Pointed C::* member, Object&& object,
                                           Args&&... args)
    {
        using object_t = remove_cvref_t<Object>;
        constexpr bool is_member_function = std::is_function_v<Pointed>;
        constexpr bool is_wrapped = is_reference_wrapper_v<object_t>;
        constexpr bool is_derived_object = std::is_same_v<C, object_t>
                                        || std::is_base_of_v<C, object_t>;

        if constexpr (is_member_function)
        {
            if constexpr (is_derived_object)
                return (std::forward<Object>(object) .* member)
                           (std::forward<Args>(args)...);
            else if constexpr (is_wrapped)
                return (object.get() .* member)(std::forward<Args>(args)...);
            else
                return ((*std::forward<Object>(object)) .* member)
                           (std::forward<Args>(args)...);
        }
        else
        {
            static_assert(std::is_object_v<Pointed> && sizeof...(args) == 0);
            if constexpr (is_derived_object)
                return std::forward<Object>(object) .* member;
            else if constexpr (is_wrapped)
                return object.get() .* member;
            else
                return (*std::forward<Object>(object)) .* member;
        }
    }
} // namespace detail

template<class F, class... Args>
constexpr std::invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
    noexcept(std::is_nothrow_invocable_v<F, Args...>)
{
    if constexpr (std::is_member_pointer_v<detail::remove_cvref_t<F>>)
        return detail::invoke_memptr(f, std::forward<Args>(args)...);
    else
        return std::forward<F>(f)(std::forward<Args>(args)...);
}

invoke_r (2)

template<class R, class F, class... Args>
    requires std::is_invocable_r_v<R, F, Args...>
constexpr R invoke_r(F&& f, Args&&... args)
    noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>)
{
    if constexpr (std::is_void_v<R>)
        std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
    else
        return std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
}

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✨ Notes ✨

Feature-test macro vibes, iykyk 🧚:

• __cpp_lib_invoke — value 201411L, std (C++17), feature std::invoke, (1)
• __cpp_lib_invoke_r — value 202106L, std (C++23), feature std::invoke_r, (2)

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🌈 Example 🌈

// Run this code

#include <functional>
#include <iostream>
#include <type_traits>

struct Foo
{
    Foo(int num) : num_(num) {}
    void print_add(int i) const { std::cout << num_ + i << '\n'; }
    int num_;
};

void print_num(int i)
{
    std::cout << i << '\n';
}

struct PrintNum
{
    void operator()(int i) const
    {
        std::cout << i << '\n';
    }
};

int main()
{
    std::cout << "invoke a free function: ";
    std::invoke(print_num, -9);

    std::cout << "invoke a lambda: ";
    std::invoke([](){ print_num(42); });

    std::cout << "invoke a member function: ";
    const Foo foo(314159);
    std::invoke(&Foo::print_add, foo, 1);

    std::cout << "invoke (i.e., access) a data member num_: "
              << std::invoke(&Foo::num_, foo) << '\n';

    std::cout << "invoke a function object: ";
    std::invoke(PrintNum(), 18);

#if defined(__cpp_lib_invoke_r)
    auto add = [](int x, int y){ return x + y; };
    std::cout << "invoke a lambda converting result to float: ";
    auto ret = std::invoke_r<float>(add, 11, 22);
    static_assert(std::is_same<decltype(ret), float>());
    std::cout << std::fixed << ret << "\ninvoke print_num: ";
    std::invoke_r<void>(print_num, 44);
#endif
}

💜 Possible output 💜

invoke a free function: -9
invoke a lambda: 42
invoke a member function: 314160
invoke (i.e., access) a data member num_: 314159
invoke a function object: 18
invoke a lambda converting result to float: 33.000000
invoke print_num: 44

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🫶 See also 🫶

• mem_fn — creates a function object out of a pointer to a member (function template) (C++11)
• result_of — deduces the result type of invoking a callable object with a set of arguments (class template) (C++11) (removed in C++20)
• invoke_result — same deduction mission, class template energy (C++17)
• is_invocable, is_invocable_r, is_nothrow_invocable, is_nothrow_invocable_r — checks if a type can be invoked (as if by std::invoke) with the given argument types (class template) (C++17)
• apply — calls a function with a tuple of arguments (function template) (C++17)

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🍓 Category 🍓

• conditionally noexcept (the whole personality tbh)

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aaaand that's the whole page bestie 💅✨ sourced straight from http://cppreference.com, version dated 2024.06.10 — std::invoke,std::invoke_r(3), no cap, go invoke something iconic 🎀🦋💖