std::optional::operator=(3)

💅 std::optional::operator=(3) 💅

okay so bestie, welcome to the std::optional::operator=(3) page from the C++ Standard Libary manual. This is the assignment-operator moment for std::optional, and we are keeping every overload, trait, caveat, and footnote because the details are literally the whole vibe. ✨

🎀 NAME 🎀

std::optional::operator= — std::optional::operator= (the assignment operator for std::optional, no cap).

✨ Synopsis ✨

optional& operator=( std::nullopt_t ) noexcept;          (1) (since C++17)
                                                                     (constexpr since C++20)
constexpr optional& operator=( const optional& other );  (2) (since C++17)
constexpr optional& operator=( optional&& other )        (3) (since C++17)
noexcept(/* see below */);
template< class U = T >                                  (4) (since C++17)
optional& operator=( U&& value );
(constexpr since C++20)
template< class U >                                      (5) (since C++17)
optional& operator=( const optional<U>& other );
(constexpr since C++20)
template< class U >                                      (6) (since C++17)
optional& operator=( optional<U>&& other );
(constexpr since C++20)

Replaces the contents of *this with the contents of other, bestie. 💖

1. 1) — If *this contains a value before the call, the contained value is destroyed by calling its destructor as if by value().T::~T(). *this does not contain a value after this call.
2. 2,3) — Assigns the state of other.
   • If both *this and other do not contain a value, the function has no effect.
   • If *this contains a value, but other does not, then the contained value is destroyed by calling its destructor. *this does not contain a value after the call.
   • If other contains a value, then depending on whether *this contains a value, the contained value is either direct-initialized or assigned from *other (2) or std::move(*other) (3). Note that a moved-from optional still contains a value.
   • Overload (2) is deleted when either std::is_copy_constructible_v<T> or std::is_copy_assignable_v<T> is false. It is trivial if std::is_trivially_copy_constructible_v<T>, std::is_trivially_copy_assignable_v<T> and std::is_trivially_destructible_v<T> are all true.
   • Overload (3) does not participate in overload resolution when either std::is_move_constructible_v<T> or std::is_move_assignable_v<T> is false. It is trivial if std::is_trivially_move_constructible_v<T>, std::is_trivially_move_assignable_v<T> and std::is_trivially_destructible_v<T> are all true.
3. 4) — Perfect-forwarded assignment: depending on whether *this contains a value before the call, the contained value is either direct-initialized from std::forward<U>(value) or assigned from std::forward<U>(value). The function does not participate in overload resolution unless std::decay_t<U> (until C++20) std::remove_cvref_t<U> (since C++20) is not std::optional<T>, std::is_constructible_v<T, U> is true, std::is_assignable_v<T&, U> is true, and at least one of the following is true:
   • T is not a scalar type;
   • std::decay_t<U> is not T.
4. 5,6) — Assigns the state of other.
   • If both *this and other do not contain a value, the function has no effect.
   • If *this contains a value, but other does not, then the contained value is destroyed by calling its destructor. *this does not contain a value after the call.
   • If other contains a value, then depending on whether *this contains a value, the contained value is either direct-initialized or assigned from *other (5) or std::move(*other) (6). Note that a moved-from optional still contains a value.
   • These overloads do not participate in overload resolution unless the following conditions are met:
     • T is not constructible, convertible, or assignable from any expression of type (possibly const) std::optional<U>, i.e., the following 12 type traits are all false:
       • std::is_constructible_v<T, std::optional<U>&>
       • std::is_constructible_v<T, const std::optional<U>&>
       • std::is_constructible_v<T, std::optional<U>&&>
       • std::is_constructible_v<T, const std::optional<U>&&>
       • std::is_convertible_v<std::optional<U>&, T>
       • std::is_convertible_v<const std::optional<U>&, T>
       • std::is_convertible_v<std::optional<U>&&, T>
       • std::is_convertible_v<const std::optional<U>&&, T>
       • std::is_assignable_v<T&, std::optional<U>&>
       • std::is_assignable_v<T&, const std::optional<U>&>
       • std::is_assignable_v<T&, std::optional<U>&&>
       • std::is_assignable_v<T&, const std::optional<U>&&>
     • For overload (5), std::is_constructible_v<T, const U&> and std::is_assignable_v<T&, const U&> are both true.
     • For overload (6), std::is_constructible_v<T, U> and std::is_assignable_v<T&, U> are both true.

🌸 Parameters 🌸

• other — another optional object whose contained value to assign.
• value — value to assign to the contained value.

🫶 Return value 🫶

*this — that's the whole return value, bestie. 👑

🔥 Exceptions 🔥

2-6) Throws any exception thrown by the constructor or assignment operator of T. If an exception is thrown, the initialization state of *this (and of other in case of (2,3) and (5,6)) is unchanged, i.e. if the object contained a value, it still contains a value, and the other way round. The contents of value and the contained values of *this and other depend on the exception safety guarantees of the operation from which the exception originates (copy-constructor, move-assignment, etc.).

3) Has following noexcept specification:

noexcept(std::is_nothrow_move_assignable_v<T> &&
std::is_nothrow_move_constructible_v<T>)

🦋 Notes 🦋

An optional object op may be turned into an empty optional with both op = {}; and op = nullopt;. The first expression constructs an empty optional object with {} and assigns it to op.

⭐ Example ⭐

// Run this code

#include <iostream>
#include <optional>

int main()
{
    std::optional<const char*> s1 = "abc", s2; // constructor
    s2 = s1; // assignment
    s1 = "def"; // decaying assignment (U = char[4], T = const char*)
    std::cout << *s2 << ' ' << *s1 << '\n';
}

Output:

abc def

🧚 Defect reports 🧚

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

• P0602R4 — applied to C++17. Behavior as published: "copy/move assignment operator may not be required to propagate triviality even if underlying operations are trivial". Correct behavior: "required to propagate triviality".
• P2231R1 — applied to C++20. Behavior as published: "converting assignment operators were not constexpr while the required operations can be in C++20". Correct behavior: "made constexpr".

🌈 See also 🌈

• emplace — constructs the contained value in-place (public member function).

👑 Category 👑

• conditionally noexcept

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okay that's the whole page, bestie. Source/version + date from the footer: http://cppreference.com and 2024.06.10, for std::optional::operator=(3). go slay your assignments 💅✨