std::apply

 

C++

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Feature test macros(C++20)

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation(C++20) bitset hash (C++11) Relational operators(deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

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,class Tuple, std::size_t... I>
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 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	Feature
__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(){// OKstd::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';    // OKstd::cout<< std::apply(add_lambda, std::pair(2.0f, 3.0f))<<'\n';   // advanced examplestd::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)