std::apply

 

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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, class Tuple > constexpr decltype(auto) apply( F&& f, Tuple&& t ) noexcept(/* see below */);		(since C++23)

Invoke the Callable object f with a tuple of arguments.

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

The 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.

Feature-test macro:

__cpp_lib_apply

Possible implementation

namespace detail { template <class F, class Tuple, std::size_t... I> constexpr decltype(auto) apply_impl(F&& f, Tuple&& t, std::index_sequence<I...>) { // This implementation is valid since C++20 (via P1065R2) // In C++17, a constexpr counterpart of std::invoke is actually needed here return std::invoke(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...); }   template <class F, class Tuple, class Idx> inline constexpr bool apply_is_noexcept = false;   template <class F, class Tuple, std::size_t... I> inline constexpr bool apply_is_noexcept<F, Tuple, std::index_sequence<I...>> = noexcept(std::invoke(std::declval<F>(), std::get<I>(std::declval<Tuple>())...));   } // namespace detail   template <class F, class Tuple> constexpr decltype(auto) apply(F&& f, Tuple&& t) noexcept( // since C++23 detail::apply_is_noexcept<F, Tuple, std::make_index_sequence<std::remove_reference_t<Tuple>>> ) { return detail::apply_impl( std::forward<F>(f), std::forward<Tuple>(t), std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>>{}); }

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)
invokeinvoke_r (C++17)(C++23)	invokes any Callable object with given arguments and possibility to specify return type (since C++23) (function template)