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Difference between revisions of "cpp/utility/pair/operator="

From cppreference.com
< cpp‎ | utility‎ | pair
m (→‎Example: " " → ' ')
m (<cstddef> for std::size_t, "" -> '', fmt)
 
Line 126: Line 126:
 
{{example
 
{{example
 
|code=
 
|code=
 +
#include <cstddef>
 
#include <iomanip>
 
#include <iomanip>
 
#include <iostream>
 
#include <iostream>
Line 131: Line 132:
 
#include <vector>
 
#include <vector>
  
−
template <class Os, class T>
+
template<class Os, class T>
 
Os& operator<<(Os& os, const std::vector<T>& v)
 
Os& operator<<(Os& os, const std::vector<T>& v)
 
{
 
{
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}
 
}
  
−
template <class Os, class U1, class U2>
+
template<class Os, class U1, class U2>
 
Os& operator<<(Os& os, const std::pair<U1, U2>& pair)
 
Os& operator<<(Os& os, const std::pair<U1, U2>& pair)
 
{
 
{
−
     return os << "{" << pair.first << ", " << pair.second << "}";
+
     return os << '{' << pair.first << ", " << pair.second << '}';
 
}
 
}
  
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     std::pair<int, std::vector<int>> p{1, {2}<!---->}, q{2, {5, 6}<!---->};
 
     std::pair<int, std::vector<int>> p{1, {2}<!---->}, q{2, {5, 6}<!---->};
 
      
 
      
−
     p = q; // (1) operator=( const pair& other );
+
     p = q; // (1) operator=(const pair& other);
 
     std::cout << std::setw(23) << std::left
 
     std::cout << std::setw(23) << std::left
 
               << "(1) p = q;"
 
               << "(1) p = q;"
Line 156: Line 157:
 
      
 
      
 
     std::pair<short, std::vector<int>> r{4, {7, 8, 9}<!---->};
 
     std::pair<short, std::vector<int>> r{4, {7, 8, 9}<!---->};
−
     p = r; // (3) operator=( const pair<U1, U2>& other );
+
     p = r; // (3) operator=(const pair<U1, U2>& other);
 
     std::cout << std::setw(23)
 
     std::cout << std::setw(23)
 
               << "(3) p = r;"
 
               << "(3) p = r;"
Line 162: Line 163:
 
      
 
      
 
     p = std::pair<int, std::vector<int>>{3, {4}<!---->};
 
     p = std::pair<int, std::vector<int>>{3, {4}<!---->};
−
     p = std::move(q); // (5) operator=( pair&& other );
+
     p = std::move(q); // (5) operator=(pair&& other);
 
     std::cout << std::setw(23)
 
     std::cout << std::setw(23)
 
               << "(5) p = std::move(q);"
 
               << "(5) p = std::move(q);"
Line 168: Line 169:
 
      
 
      
 
     p = std::pair<int, std::vector<int>>{5, {6}<!---->};
 
     p = std::pair<int, std::vector<int>>{5, {6}<!---->};
−
     p = std::move(r); // (7) operator=( pair<U1, U2>&& other );
+
     p = std::move(r); // (7) operator=(pair<U1, U2>&& other);
 
     std::cout << std::setw(23)
 
     std::cout << std::setw(23)
 
               << "(7) p = std::move(r);"
 
               << "(7) p = std::move(r);"

Latest revision as of 21:28, 25 October 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::pair
 
(1)
pair& operator=( const pair& other );
 (until C++20)
constexpr pair& operator=( const pair& other );
 (since C++20)
constexpr const pair& operator=( const pair& other ) const;
 (2) (since C++23)
(3)
template< class U1, class U2 >
pair& operator=( const pair<U1, U2>& other );
 (until C++20)
template< class U1, class U2 >
constexpr pair& operator=( const pair<U1, U2>& other );
 (since C++20)
template< class U1, class U2 >
constexpr const pair& operator=( const pair<U1, U2>& other ) const;
 (4) (since C++23)
(5)
pair& operator=( pair&& other ) noexcept(/* see below */);
 (since C++11)
(until C++20)
constexpr pair& operator=( pair&& other ) noexcept(/* see below */);
 (since C++20)
constexpr const pair& operator=( pair&& other ) const;
 (6) (since C++23)
(7)
template< class U1, class U2 >
pair& operator=( pair<U1, U2>&& p );
 (since C++11)
(until C++20)
template< class U1, class U2 >
constexpr pair& operator=( pair<U1, U2>&& p );
 (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.

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
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

Contents

[edit] 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
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)

[edit] Return value

*this

[edit] 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.

[edit] 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, {}}

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 885 C++98 missing heterogeneous copy assignment added (as overload (3))
LWG 2729 C++11 pair::operator= was unconstrained and might
result in unnecessary undefined behavior 		constrained

[edit] See also

 assigns the contents of one tuple to another
(public member function of std::tuple<Types...>) [edit]
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