std::refere...m_temporary(3)

💅 NAME 💅

std::reference_converts_from_temporary — literally the trait that tells you if a reference is about to get stuck holding a temporary 🫶

ok so real talk, this page lives in the C++ Standard Library manual, section 3, and the whole title got squished down to std::refere...m_temporary(3) in the header because apparently nobody had room. bestie. anyway 💀

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✨ Synopsis ✨

Defined in header <type_traits>
template< class T, class U >               (since C++23)
struct reference_converts_from_temporary;

okay so here's the lore 🎀 — let V be std::remove_cv_t<U> if U is a scalar type or cv void, or U otherwise. Then: if T is a reference type, and you take a hypothetic expression e such that decltype(e) is V, and the variable definition T ref = e; is well-formed and binds a temporary object to ref, then it provides the member constant value equal to true. Otherwise, value is false. no cap, that's the whole personality 🌸

⚠️ Okay so this part is important and I need you to actually read it: if T is an lvalue reference type to a const- but not volatile-qualified object type, or an rvalue reference type, then both std::remove_reference_t<T> and std::remove_reference_t<U> shall be complete types, cv void, or an arrays of unknown bound; otherwise the behavior is undefined. 💀

⚠️ And also: if an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined. like, undefined undefined. 🙈

⚠️ And one more, because the standard loves a third warning: if the program adds specializations for std::reference_converts_from_temporary or std::reference_converts_from_temporary_v, the behavior is undefined. please do not. 🫶

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👑 Member constants 👑

• value — true if T is a reference type, a U value can be bound to T in copy-initialization, and a temporary object would be bound to the reference, false otherwise. it's a public static member constant ✨

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🧚 Member functions 🧚

• operator bool — converts the object to bool, returns value (public member function)
• operator() — returns value (C++14) (public member function)

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🦋 Member types 🦋

• value_type → bool
• type → std::integral_constant<bool, value>

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🌈 Notes 🌈

std::reference_converts_from_temporary can be used for rejecting some cases that always produce dangling references. main character energy honestly — it's out here saving your code from itself 🔥

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🍓 Example 🍓

// Run this code

#include <type_traits>

int main() {}

static_assert(std::reference_converts_from_temporary_v<int&&, int> == true);
static_assert(std::reference_converts_from_temporary_v<const int&, int> == true);
static_assert(std::reference_converts_from_temporary_v<int&&, int&&> == false);
static_assert(std::reference_converts_from_temporary_v<const int&, int&&> == false);
static_assert(std::reference_converts_from_temporary_v<int&&, long&&> == true);
static_assert(std::reference_converts_from_temporary_v<int&&, long> == true);

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💖 See also 💖

• is_convertible (C++11) / is_nothrow_convertible (C++20) — checks if a type can be converted to the other type (class template)
• invoke / invoke_r (C++17) (C++23) — invokes any Callable object with given arguments and possibility to specify return type (function template)
• bind (C++11) — binds one or more arguments to a function object (function template)
• visit (C++17) — calls the provided functor with the arguments held by one or more variants (function template)
• constructor — constructs a new std::function instance (public member function of std::function<R(Args...)>)
• constructor (C++23) — constructs a new std::move_only_function object (public member function of std::move_only_function)
• constructor — constructs the task object (public member function of std::packaged_task<R(Args...)>)

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okay that's the whole page, you ate that 💅✨ this lil beauty came from http://cppreference.com, version 2024.06.10, and yeah the footer still says std::refere...m_temporary(3) because we're keeping it iconic 😭🫶 bye bestie, go bind some references responsibly 🌸