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std::apply

From cppreference.com
< cpp‎ | utility
 
 
Utilities library
General utilities
Date and time
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Formatting library (C++20)
(C++11)
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)   
(C++20)
Swap and type operations
(C++14)
(C++11)
(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
apply
(C++17)
(C++23)
Elementary string conversions
(C++17)
(C++17)
 
Defined in header <tuple>
template< class F, class Tuple >
constexpr decltype(auto) apply( F&& f, Tuple&& t );
(since C++17)
(until C++23)
template< class F, class Tuple >
constexpr decltype(auto) apply( F&& f, Tuple&& t ) noexcept(/* see below */);
(since C++23)

Invoke the Callable object f with a tuple of arguments.

Contents

[edit] Parameters

f - Callable object to be invoked
t - tuple whose elements to be used as arguments to f

[edit] Return value

The value returned by f.

[edit] Exceptions

(none)

(until C++23)
noexcept specification:  
noexcept(

    noexcept(std::invoke(std::forward<F>(f), std::get<Is>(std::forward<Tuple>(t))...))

)

where Is... denotes the parameter pack:

(since C++23)

[edit] 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.

Feature-test macro Value Std Comment
__cpp_lib_apply 201603L (C++17) std::apply

[edit] 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...>)
{
    // This implementation is valid since C++20 (via P1065R2)
    // In C++17, a constexpr counterpart of std::invoke is actually needed here
    return std::invoke(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...);
}
 
template <class F, class Tuple, class Idx>
inline constexpr bool apply_is_noexcept = false;
 
template <class F, class Tuple, std::size_t... I>
inline constexpr bool apply_is_noexcept<F, Tuple, std::index_sequence<I...>> = 
  noexcept(std::invoke(std::declval<F>(), std::get<I>(std::declval<Tuple>())...));
 
}  // namespace detail
 
template <class F, class Tuple>
constexpr decltype(auto) apply(F&& f, Tuple&& t)
  noexcept( // since C++23
    detail::apply_is_noexcept<F, Tuple, 
      std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>>>
  )
{
    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>>>{});
}

[edit] 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; };
 
template<typename... Ts>
std::ostream& operator<<(std::ostream& os, std::tuple<Ts...> const& theTuple)
{
    std::apply
    (
        [&os](Ts const&... tupleArgs)
        {
            os << '[';
            std::size_t n{0};
            ((os << tupleArgs << (++n != sizeof...(Ts) ? ", " : "")), ...);
            os << ']';
        }, theTuple
    );
    return os;
}
 
int main()
{
    // OK
    std::cout << std::apply(add, std::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::pair(2.0f, 3.0f)) << '\n'; 
 
    // advanced example
    std::tuple myTuple{25, "Hello", 9.31f, 'c'};
    std::cout << myTuple << '\n';
}

Output:

3
5
[25, Hello, 9.31, c]

[edit] See also

creates a tuple object of the type defined by the argument types
(function template) [edit]
creates a tuple of forwarding references
(function template) [edit]
Construct an object with a tuple of arguments
(function template) [edit]
(C++17)(C++23)
invokes any Callable object with given arguments and possibility to specify return type (since C++23)
(function template) [edit]