std::variant::operator=(3)

💖 NAME 💖

Okay bestie, this is std::variant::operator= — the assignment operator for std::variant — from the std::variant::operator=(3) manual page in the C++ Standard Libary (yes, that’s literally the spelling on the page 💅). It’s giving “assign a new value to an existing variant object,” and it has three overloads: copy-assignment, move-assignment, and converting assignment. All since C++17, with the converting one getting its constexpr glow-up in C++20. Periodt. ✨

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✨ Synopsis ✨

constexpr variant& operator=( const variant& rhs );  (1) (since C++17)
constexpr variant& operator=( variant&& rhs )        (2) (since C++17)
noexcept(/* see below */);
template< class T >
variant& operator=( T&& t ) noexcept(/* see below */);  (3) (since C++17) (until C++20)
template< class T >
constexpr variant& operator=( T&& t ) noexcept(/* see below */);  (since C++20)

Assigns a new value to an existing variant object. Slay. 💅

1. Copy-assignment:
   • If both *this and rhs are valueless by exception, does nothing.
   • Otherwise, if rhs is valueless, but *this is not, destroys the value contained in *this and makes it valueless.
   • Otherwise, if rhs holds the same alternative as *this, assigns the value contained in rhs to the value contained in *this. If an exception is thrown, *this does not become valueless: the value depends on the exception safety guarantee of the alternative’s copy assignment.
   • Otherwise, if the alternative held by rhs is either nothrow copy constructible or not nothrow move constructible (as determined by std::is_nothrow_copy_constructible and std::is_nothrow_move_constructible, respectively), equivalent to this->emplace<rhs.index()>(*std::get_if<rhs.index()>(std::addressof(rhs))). *this may become valueless_by_exception if an exception is thrown on the copy-construction inside emplace.
   • Otherwise, equivalent to this->operator=(variant(rhs)).
   • This overload is defined as deleted unless std::is_copy_constructible_v<T_i> and std::is_copy_assignable_v<T_i> are both true for all T_i in Types.... This overload is trivial if std::is_trivially_copy_constructible_v<T_i>,std::is_trivially_copy_assignable_v<T_i> and std::is_trivially_destructible_v<T_i> are all true for all T_i in Types....
2. Move-assignment:
   • If both *this and rhs are valueless by exception, does nothing.
   • Otherwise, if rhs is valueless, but *this is not, destroys the value contained in *this and makes it valueless.
   • Otherwise, if rhs holds the same alternative as *this, assigns std::move(*std::get_if<j>(std::addressof(rhs))) to the value contained in *this, with j being index(). If an exception is thrown, *this does not become valueless: the value depends on the exception safety guarantee of the alternative’s move assignment.
   • Otherwise (if rhs and *this hold different alternatives), equivalent to this->emplace<rhs.index()>(std::move(*std::get_if<rhs.index()>(std::addressof(rhs)))). If an exception is thrown by T_i’s move constructor, *this becomes valueless_by_exception.
   • This overload participates in overload resolution only if std::is_move_constructible_v<T_i> and std::is_move_assignable_v<T_i> are both true for all T_i in Types.... This overload is trivial if std::is_trivially_move_constructible_v<T_i>, std::is_trivially_move_assignable_v<T_i>, and std::is_trivially_destructible_v<T_i> are all true for all T_i in Types....
3. Converting assignment:
   • Determines the alternative type T_j that would be selected by overload resolution for the expression F(std::forward<T>(t)) if there was an overload of imaginary function F(T_i) for every T_i from Types... in scope at the same time, except that:
     • An overload F(T_i) is only considered if the declaration T_i x[] = { std::forward<T>(t) }; is valid for some invented variable x;
     • If *this already holds a T_j, assigns std::forward<T>(t) to the value contained in *this. If an exception is thrown, *this does not become valueless: the value depends on the exception safety guarantee of the assignment called.
     • Otherwise, if std::is_nothrow_constructible_v<T_j, T> || !std::is_nothrow_move_constructible_v<T_j> is true, equivalent to this->emplace<j>(std::forward<T>(t)). *this may become valueless_by_exception if an exception is thrown on the initialization inside emplace.
     • Otherwise, equivalent to this->emplace<j>(T_j(std::forward<T>(t))).
   • This overload participates in overload resolution only if std::decay_t<T> (until C++20) std::remove_cvref_t<T> (since C++20) is not the same type as variant and std::is_assignable_v<T_j&, T> is true and std::is_constructible_v<T_j, T> is true and the expression F(std::forward<T>(t)) (with F being the above-mentioned set of imaginary functions) is well formed.

Also, the lil’ overload-resolution vibe-check examples:

std::variant<std::string> v1;
v1 = "abc"; // OK
std::variant<std::string, std::string> v2;
v2 = "abc"; // Error
std::variant <std::string, bool> v3;
v3 = "abc"; // OK, chooses string; bool is not a candidate
std::variant<float, long, double> v4; //holds float
v4 = 0; // OK, holds long; float and double are not candidates

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🎀 Parameters 🎀

• rhs — another variant
• t — a value convertible to one of the variant’s alternatives

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🫶 Return value 🫶

*this — it returns itself, main-character energy only. 👑

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😭 Exceptions 😭

1. May throw any exception thrown by assignment and copy/move initialization of any alternative.
2. noexcept specification:

noexcept(((std::is_nothrow_move_constructible_v<Types> &&
           std::is_nothrow_move_assignable_v<Types>) && ...))

3. noexcept specification:

noexcept(std::is_nothrow_assignable_v<T_j&, T> &&
std::is_nothrow_constructible_v<T_j, T>)

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🌈 Example 🌈

// Run this code

#include <iomanip>
#include <iostream>
#include <string>
#include <type_traits>
#include <variant>

std::ostream& operator<<(std::ostream& os, std::variant<int, std::string> const& va)
{
    os << ": { ";

    std::visit([&](auto&& arg)
    {
        using T = std::decay_t<decltype(arg)>;
        if constexpr (std::is_same_v<T, int>)
            os << arg;
        else if constexpr (std::is_same_v<T, std::string>)
            os << std::quoted(arg);
    }, va);

    return os << " };\n";
}

int main()
{
    std::variant<int, std::string> a{2017}, b{"CppCon"};
    std::cout << "a" << a << "b" << b << '\n';

    std::cout << "(1) operator=( const variant& rhs )\n";
    a = b;
    std::cout << "a" << a << "b" << b << '\n';

    std::cout << "(2) operator=( variant&& rhs )\n";
    a = std::move(b);
    std::cout << "a" << a << "b" << b << '\n';

    std::cout << "(3) operator=( T&& t ), where T is int\n";
    a = 2019;
    std::cout << "a" << a << '\n';

    std::cout << "(3) operator=( T&& t ), where T is std::string\n";
    std::string s{"CppNow"};
    std::cout << "s: " << std::quoted(s) << '\n';
    a = std::move(s);
    std::cout << "a" << a << "s: " << std::quoted(s) << '\n';
}

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🌸 Possible output 🌸

a: { 2017 };
b: { "CppCon" };

(1) operator=( const variant& rhs )
a: { "CppCon" };
b: { "CppCon" };

(2) operator=( variant&& rhs )
a: { "CppCon" };
b: { "" };

(3) operator=( T&& t ), where T is int
a: { 2019 };

(3) operator=( T&& t ), where T is std::string
s: "CppNow"
a: { "CppNow" };
s: ""

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🦋 Defect reports 🦋

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. History really said “let me fix her.” 💀

• LWG 3024 (C++17) — behavior as published: copy assignment operator doesn't participate in overload resolution if any member type is not copyable. Correct behavior: defined as deleted instead.
• P0602R4 (C++17) — behavior as published: copy/move assignment may not be trivial even if underlying operations are trivial. Correct behavior: required to propagate triviality.
• P0608R3 (C++17) — behavior as published: converting assignment blindly assembles an overload set, leading to unintended conversions. Correct behavior: narrowing and boolean conversions not considered.
• P2231R1 (C++20) — behavior as published: converting assignment was not constexpr while the required operations can be in C++20. Correct behavior: made constexpr.
• LWG 3585 (C++17) — behavior as published: converting assignment was sometimes unexpectedly ill-formed because there was no available move assignment. Correct behavior: made well-formed.

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👑 See also 👑

• emplace — constructs a value in the variant, in place (public member function). Obsessed. 🫶

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⭐ Category ⭐

• Category: conditionally noexcept

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Source/version + date vibes, bestie: http://cppreference.com · 2024.06.10. Now go forth and assign with main-character energy 💅✨🦋