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