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std::apply

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

#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]