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

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

where Is... denotes the parameter pack:

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

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 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)