std::apply(3)

✨ NAME ✨

std::apply — std::apply. That's literally the name, bestie. This is the std::apply(3) page from the C++ Standard Libary manual, and it's lowkey iconic 💅

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🛠️ Synopsis 🛠️

Defined in header <tuple> — okay so here's the timeline drama:

template< class F, class Tuple >
(since C++17)
constexpr decltype(auto) apply( F&& f, Tuple&& t );
(until C++23)
template< class F, tuple-like Tuple >
constexpr decltype(auto) apply( F&& f, Tuple&& t ) noexcept(/* see
(since C++23)
below */);

Invoke the Callable object f with the elements of t as arguments. That's the whole vibe ✨

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>>>{}); .

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💖 Parameters 💖

• f — Callable object to be invoked.
• t — tuple whose elements to be used as arguments to f.

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🎁 Return value 🎁

The value returned by f. That's it, that's the gift 🫶

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😭 Exceptions 😭

• (until C++23) — (none). No exceptions listed for that era.
• (since C++23) — here's the noexcept specification, bestie:

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

where Is... denotes the parameter pack:

• 0, 1, ..., std::tuple_size_v<std::remove_reference_t<Tuple>> - 1.

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📝 Notes 📝

• (until C++23) — 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.
• (since C++23) — Tuple is constrained to be tuple-like, i.e. each type therein is required to be a specialization of std::tuple or another type (such as std::array and std::pair) that models tuple-like.

Feature-test macro check:

• __cpp_lib_apply — value 201603L, standard (C++17), feature std::apply. Slay 💅

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🌸 Example 🌸

Okay this example is a whole journey. Here's the code exactly as the page has it:

// Run this code

#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]

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🦋 See also 🦋

• make_tuple — creates a tuple object of the type defined by the argument types (C++11) (function template).
• forward_as_tuple — creates a tuple of forwarding references (C++11) (function template).
• make_from_tuple — construct an object with a tuple of arguments (C++17) (function template).
• invoke — invokes any Callable object with given arguments (C++17) (function template).
• invoke_r — invokes any Callable object with given arguments and possibility to specify return type (since C++23) (C++23) (function template).

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🏷️ Category 🏷️

• conditionally noexcept

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That's the whole page, bestie — source http://cppreference.com, version/date 2024.06.10, page std::apply(3) 💖✨