# std::apply

From cppreference.com
< cpp‎ | utility

Defined in header `<tuple>` |  |
template< class F, class Tuple > constexpr decltype(auto) apply( F&& f, Tuple&& t ); |  | (since C++17) (until C++23)
template< class F, tuple-like Tuple > constexpr decltype(auto) apply( F&& f, Tuple&& t ) noexcept(/* see below */); |  | (since C++23)

Invoke the Callable object f with the elements of t as arguments.

Given the exposition-only function `apply-impl` defined as follows: template<class F,`` `tuple-like``` Tuple, std::size_t... I> // no constraint on Tuple before C++23
constexpr decltype(auto)
`` `apply-impl`(F&& f, Tuple&& t, std::index_sequence<I...>) // exposition only
`{`
`` return`` `INVOKE`(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...);
`}`

The effect is equivalent to return`` `apply-impl`(std::forward<F>(f), std::forward<Tuple>(t),
`` std::make_index_sequence<
`` std::tuple_size_v<std::decay_t<Tuple>>>{}); .

### Parameters

f | - | Callable object to be invoked
t | - | tuple whose elements to be used as arguments to f

### Return value

The value returned by f.

### Exceptions

(none) | (until C++23)
`noexcept` specification: noexcept( noexcept(std::invoke(std::forward<F>(f), std::get<Is>(std::forward<Tuple>(t))...)) ) where `Is...` denotes the parameter pack: 0, 1, ..., std::tuple_size_v<std::remove_reference_t<Tuple>> - 1. | (since C++23)

### Notes

`Tuple` need not be std::tuple, and instead may be anything that supports `std::get` and `std::tuple_size`; in particular, std::array and std::pair may be used. | (until C++23)
`Tuple` is constrained to be tuple-like, i.e. each type therein is required to be a specialization of std::tuple or another type (such as std::array and std::pair) that models `tuple-like`. | (since C++23)

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_apply` | 201603L | (C++17) | `std::apply`

### Example

Run this code

    #include <iostream>
    #include <tuple>
    #include <utility>

    int add(int first, int second) { return first + second; }

    template<typename T>
    T add_generic(T first, T second) { return first + second; }

    auto add_lambda = [](auto first, auto second) { return first + second; };

    template<typename... Ts>
    std::ostream& operator<<(std::ostream& os, std::tuple<Ts...> const& theTuple)
    {
        std::apply
        (
            [&os](Ts const&... tupleArgs)
            {
                os << '[';
                std::size_t n{0};
                ((os << tupleArgs << (++n != sizeof...(Ts) ? ", " : "")), ...);
                os << ']';
            }, theTuple
        );
        return os;
    }

    int main()
    {
        // OK
        std::cout << std::apply(add, std::pair(1, 2)) << '\n';

        // Error: can't deduce the function type
        // std::cout << std::apply(add_generic, std::make_pair(2.0f, 3.0f)) << '\n';

        // OK
        std::cout << std::apply(add_lambda, std::pair(2.0f, 3.0f)) << '\n';

        // advanced example
        std::tuple myTuple{25, "Hello", 9.31f, 'c'};
        std::cout << myTuple << '\n';
    }

Output:

    3
    5
    [25, Hello, 9.31, c]

### See also

make_tuple (C++11) | creates a `tuple` object of the type defined by the argument types (function template)
forward_as_tuple (C++11) | creates a `tuple` of forwarding references (function template)
make_from_tuple (C++17) | construct an object with a tuple of arguments (function template)
invoke invoke_r (C++17)(C++23) | invokes any Callable object with given arguments and possibility to specify return type (since C++23) (function template)

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