std::pair::operator=(3)

💖 NAME 💖

okay so we're in the C++ Standard Libary manual, page std::pair::operator=(3), and the NAME line is literally std::pair::operator= - std::pair::operator=. iconic, no notes 💅

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✨ Synopsis ✨

bestie, there are ten whole overloads. ten. hydrate before reading:

pair& operator=( const pair& other );                              (1) (until C++20)
constexpr pair& operator=( const pair& other );                    (1) (since C++20)
constexpr const pair& operator=( const pair& other ) const;        (2) (since C++23)

template< class U1, class U2 >
pair& operator=( const pair<U1, U2>& other );                      (3) (until C++20)
template< class U1, class U2 >
constexpr pair& operator=( const pair<U1, U2>& other );            (3) (since C++20)
template< class U1, class U2 >
constexpr const pair& operator=( const pair<U1, U2>& other ) const; (4) (since C++23)

pair& operator=( pair&& other ) noexcept(/* see below */);         (5) (since C++11) (until C++20)
constexpr pair& operator=( pair&& other ) noexcept(/* see below */); (5) (since C++20)
constexpr const pair& operator=( pair&& other ) const;             (6) (since C++23)

template< class U1, class U2 >
pair& operator=( pair<U1, U2>&& p );                               (7) (since C++11) (until C++20)
template< class U1, class U2 >
constexpr pair& operator=( pair<U1, U2>&& p );                     (7) (since C++20)
template< class U1, class U2 >
constexpr const pair& operator=( pair<U1, U2>&& p ) const;         (8) (since C++23)

template< pair-like P >
constexpr pair& operator=( P&& u );                                (9) (since C++23)
template< pair-like P >
constexpr const pair& operator=( P&& u ) const;                    (10) (since C++23)

Replaces the contents of the pair. that's the whole vibe of this page, fr fr.

1. Copy assignment operator. Replaces the contents with a copy of the contents of other.

The assignment operator is implicitly declared. Using this assignment operator makes the program ill-formed if either T1 or T2 is a const-qualified type, or a reference type, or a class type with an inaccessible copy assignment operator, or an array type of such class. (until C++11)

This overload is defined as deleted if either std::is_copy_assignable<T1>::value or std::is_copy_assignable<T2>::value is false. (since C++11)

2. Copy assignment operator for const-qualified operand.

This overload participates in overload resolution only if std::is_copy_assignable_v<const T1> and std::is_copy_assignable_v<const T2> are both true.

3. Assigns other.first to first and other.second to second.

This overload participates in overload resolution only if std::is_assignable<T1&, const U1&>::value and std::is_assignable<T2&, const U2&>::value are both true. (since C++11)

4. Assigns other.first to first and other.second to second.

This overload participates in overload resolution only if std::is_assignable_v<const T1&, const U1&> and std::is_assignable_v<const T2&, const U2&> are both true.

5. Move assignment operator. Replaces the contents with those of other using move semantics.

This overload participates in overload resolution only if std::is_move_assignable<T1>::value and std::is_move_assignable<T2>::value are both true.

6. Move assignment operator for const-qualified operand.

This overload participates in overload resolution only if std::is_assignable_v<const T1&, T1> and std::is_assignable_v<const T2&, T2> are both true.

7. Assigns std::forward<U1>(p.first) to first and std::forward<U2>(p.second) to second.

This overload participates in overload resolution only if std::is_assignable<T1&, U1>::value and std::is_assignable<T2&, U2>::value are both true.

8. Assigns std::forward<U1>(p.first) to first and std::forward<U2>(p.second) to second.

This overload participates in overload resolution only if std::is_assignable_v<const T1&, U1> and std::is_assignable_v<const T2&, U2> are both true.

9. Assigns std::get<0>(std::forward<P>(u)) to first and std::get<1>(std::forward<P>(u)) to second.

This overload participates in overload resolution only if

• std::same_as<std::remove_cvref_t<P>, std::pair> is false,
• std::remove_cvref_t<P> is not a specialization of std::ranges::subrange,
• std::is_assignable_v<T1&, decltype(std::get<0>(std::forward<P>(p)))> is true, and
• std::is_assignable_v<T1&, decltype(std::get<1>(std::forward<P>(p)))> is true.

10. Assigns std::get<0>(std::forward<P>(u)) to first and std::get<1>(std::forward<P>(u)) to second.

This overload participates in overload resolution only if

• std::same_as<std::remove_cvref_t<P>, std::pair> is false,
• std::remove_cvref_t<P> is not a specialization of std::ranges::subrange,
• std::is_assignable_v<const T1&, decltype(std::get<0>(std::forward<P>(p)))> is true, and
• std::is_assignable_v<const T1&, decltype(std::get<1>(std::forward<P>(p)))> is true.

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🎀 Parameters 🎀

• other — pair of values to replace the contents of this pair
• p — pair of values of possibly different types to replace the contents of this pair
• u — pair-like object of values to replace the contents of this pair

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🦋 Type requirements 🦋

• T1 must meet the requirements of CopyAssignable from U1. (until C++11)
• T2 must meet the requirements of CopyAssignable from U2. (until C++11)

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🌸 Return value 🌸

*this

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🔥 Exceptions 🔥

• 1-4) May throw implementation-defined exceptions.
• 5) noexcept specification:

noexcept(

    std::is_nothrow_move_assignable<T1>::value &&
    std::is_nothrow_move_assignable<T2>::value

)

• 6-10) May throw implementation-defined exceptions.

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💜 Example 💜

// Run this code

#include <cstddef>
#include <iomanip>
#include <iostream>
#include <utility>
#include <vector>

template<class Os, class T>
Os& operator<<(Os& os, const std::vector<T>& v)
{
    os << '{';
    for (std::size_t t = 0; t != v.size(); ++t)
        os << v[t] << (t + 1 < v.size() ? ", " : "");
    return os << '}';
}

template<class Os, class U1, class U2>
Os& operator<<(Os& os, const std::pair<U1, U2>& pair)
{
    return os << '{' << pair.first << ", " << pair.second << '}';
}

int main()
{
    std::pair<int, std::vector<int>> p{1, {2}}, q{2, {5, 6}};

    p = q; // (1) operator=(const pair& other);
    std::cout << std::setw(23) << std::left
              << "(1) p = q;"
              << "p: " << p << "     q: " << q << '\n';

    std::pair<short, std::vector<int>> r{4, {7, 8, 9}};
    p = r; // (3) operator=(const pair<U1, U2>& other);
    std::cout << std::setw(23)
              << "(3) p = r;"
              << "p: " << p << "  r: " << r << '\n';

    p = std::pair<int, std::vector<int>>{3, {4}};
    p = std::move(q); // (5) operator=(pair&& other);
    std::cout << std::setw(23)
              << "(5) p = std::move(q);"
              << "p: " << p << "     q: " << q << '\n';

    p = std::pair<int, std::vector<int>>{5, {6}};
    p = std::move(r); // (7) operator=(pair<U1, U2>&& other);
    std::cout << std::setw(23)
              << "(7) p = std::move(r);"
              << "p: " << p << "  r: " << r << '\n';
}

Output:

(1) p = q;             p: {2, {5, 6}}     q: {2, {5, 6}}
(3) p = r;             p: {4, {7, 8, 9}}  r: {4, {7, 8, 9}}
(5) p = std::move(q);  p: {2, {5, 6}}     q: {2, {}}
(7) p = std::move(r);  p: {4, {7, 8, 9}}  r: {4, {}}

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🧚 Defect reports 🧚

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

• LWG 885 — applied to C++98 — behavior as published: missing heterogeneous copy assignment; correct behavior: added (as overload (3)).
• LWG 2729 — applied to C++11 — behavior as published: pair::operator= was unconstrained and might result in unnecessary undefined behavior; correct behavior: constrained.

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👑 See also 👑

• operator= — assigns the contents of one tuple to another (public member function of std::tuple<Types...>)

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🏷️ Category 🏷️

• conditionally noexcept

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made with 💖 from http://cppreference.com, 2024.06.10, page std::pair::operator=(3) — that's the footer bestie, we're done 💅✨