std:: apply

Invoke the Callable object f with the elements of t as arguments.

Given the exposition-only function apply-impl defined as follows:

template < class F, class Tuple, std:: size_t ... I >
constexpr decltype ( auto )
apply-impl ( F && f, Tuple && t, std:: index_sequence < I... > ) // exposition only
{
return INVOKE ( std:: forward < F > ( f ) , std :: get < I > ( std:: forward < Tuple > ( t ) ) ... ) ;
}

The effect is equivalent to:

return apply-impl ( std:: forward < F > ( f ) , std:: forward < Tuple > ( t ) ,
std:: make_index_sequence <
std:: tuple_size_v < std:: decay_t < Tuple >>> { } ) ; .

Parameters

Return value

The value returned by f .

Exceptions

Notes

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';
}

3
5
[25, Hello, 9.31, c]

See also