std::make_from_tuple(3)

💖 NAME 💖

Okay bestie, welcome to the std::make_from_tuple(3) page from the C++ Standard Libary 🫶. The name is literally:

std::make_from_tuple - std::make_from_tuple

Yeah, it's giving "the function is the name and the name is the function" ✨.

---

🌸 Synopsis 🌸

First, the vibe check: it lives in header <tuple>.

Defined in header <tuple>
template< class T, class Tuple >           (since C++17)
constexpr T make_from_tuple( Tuple&& t );  (until C++23)
template< class T, tuple-like Tuple >      (since C++23)
constexpr T make_from_tuple( Tuple&& t );

Construct an object of type T, using the elements of the tuple t as the arguments to the constructor. Slay.

Given the exposition-only function /*make-from-tuple-impl*/ defined as follows:

template<class T, tuple-like Tuple, std::size_t... I> // no constraint on Tuple before C++23
constexpr T /*make-from-tuple-impl*/(Tuple&& t, std::index_sequence<I...>)
{
    return T(std::get<I>(std::forward<Tuple>(t))...);
}

The effect is equivalent to:

return /*make-from-tuple-impl*/<T>(
    std::forward<Tuple>(t),
    std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>>{}
);.

If either of these is true, the program is ill-formed 🚨:

• std::tuple_size_v<std::remove_reference_t<Tuple>> is 1 and (since C++23) std::reference_constructs_from_temporary_v<T, decltype(std::get<0>(std::declval<Tuple>()))> is true, or
• std::is_constructible_v<T, decltype(std::get<I>(std::declval<Tuple>()))...> is false.

---

🎀 Parameters 🎀

• t — tuple whose elements to be used as arguments to the constructor of T.

---

🦋 Return value 🦋

The constructed T object or reference. That's it, bestie.

---

🌈 Notes 🌈

• Tuple need not be std::tuple, and instead may be anything that supports std::get and std::tuple_size; in particular, std::array and (until C++23) std::pair may be used.
• 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 (since C++23) std::array and std::pair) that models tuple-like.
• Due to guaranteed copy elision, T need not be movable. Iconic.

Feature-test macro vibes:

Feature-test macro         Value    Std      Feature
__cpp_lib_make_from_tuple  201606L  (C++17)  std::make_from_tuple

---

🍓 Example 🍓

// Run this code

#include <iostream>
#include <tuple>

struct Foo
{
    Foo(int first, float second, int third)
    {
        std::cout << first << ", " << second << ", " << third << '\n';
    }
};

int main()
{
    auto tuple = std::make_tuple(42, 3.14f, 0);
    std::make_from_tuple<Foo>(std::move(tuple));
}

---

⭐ Output ⭐

42, 3.14, 0

---

🩹 Defect reports 🩹

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• DR: LWG 3528
  • Applied to: C++17
  • Behavior as published: cast containing reinterpret_cast etc. was prohibited
  • Correct behavior: allowed in the case of 1-tuple

---

👀 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)
• apply — calls a function with a tuple of arguments (C++17) (function template)

---

Source/version check: http://cppreference.com, dated 2024.06.10, manual std::make_from_tuple(3) — bye bestie, go make some tuples! 💅💖