std::apply

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

make_from_tuple (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Type operations

declval (C++11)

as_const (C++17)

 

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

What 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::decay_t<Tuple>>>{}); }

Example

Run this code

int add(int first, int second)
{
    return first + second;    
}
 
template<typename T>
T add_generic(T first, T second)
{
    return first + second;    
}
 
int main()
{
    std::cout << std::apply(add, std::make_tuple(1,2)) << '\n';
 
    // template argument deduction/substitution fails
    // std::cout << std::apply(add_generic, std::make_tuple(2.0f,3.0f)) << '\n'; 
}

Output:

3

See also

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)