std::apply (3) - Linux Manuals

std::apply: std::apply

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);

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