std::apply (3) Linux Manual Page

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);

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.

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.

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…>)
  {
      return std::invoke(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))…);
  }
  } // namespace detail

  template <class F, class Tuple>
  constexpr decltype(auto) apply(F&& f, Tuple&& t)
  {
      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; };
int main()

{
    // OK
    std::cout << std::apply(add, std::make_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::make_pair(2.0f, 3.0f)) << '\n';
}

Output:

  3
  5

See also

                 creates a tuple object of the type defined by the argument types
make_tuple (function template)
                 creates a tuple of rvalue references
forward_as_tuple (function template)
make_from_tuple Construct an object with a tuple of arguments
                 (function template)
(C++17)
invoke invokes any Callable object with given arguments
                 (function template)
(C++17)