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Showing content from https://en.cppreference.com/w/cpp/algorithm/../utility/apply.html below:

std::apply - cppreference.com

template< class F, class Tuple >
constexpr decltype(auto) apply( F&& f, Tuple&& t );

template< class F, tuple-like Tuple >
constexpr decltype(auto) apply( F&& f, Tuple&& t ) noexcept(/* see below */);

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>>>{}); .

The value returned by f.

#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:

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