std::apply

Since C++23

Header: <tuple>

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

# Declarations

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)

# Parameters

# Return value

The value returned by f.

# 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.

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.

# 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';
}

# See also