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std::apply

Defined in header `<tuple>`
template <class F, class Tuple> constexpr decltype(auto) apply(F&& f, Tuple&& t);		(since C++17)

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

#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

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