NAME¶

std::apply - std::apply

Synopsis¶

Defined in header <tuple>
template< class F, class Tuple > (since C++17)
constexpr decltype(auto) apply( F&& f, Tuple&& t ); (until C++23)
template< class F, tuple-like Tuple >
constexpr decltype(auto) apply( F&& f, Tuple&& t ) noexcept(/* see (since C++23)
below */);

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))...)) (since
C++23)
)

where Is... denotes the parameter pack:

* 0, 1, ..., std::tuple_size_v<std::remove_reference_t<Tuple>> - 1.

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 (until C++23)
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 (since C++23)
std::array and std::pair) that models tuple-like.

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 creates a tuple object of the type defined by the argument types
(C++11) (function template)
forward_as_tuple creates a tuple of forwarding references
(C++11) (function template)
make_from_tuple construct an object with a tuple of arguments
(C++17) (function template)
invoke invokes any Callable object with given arguments
invoke_r and possibility to specify return type
(C++17) (since C++23)
(C++23) (function template)

Category:¶

* conditionally noexcept