std::apply

 

C++

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Feature test macros (C++20)

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General utilities Date and time Function objects Formatting library (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) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

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 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	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()
{
    // 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)