std::apply (3) - Linux Man Pages

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

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)