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

From cppreference.com
< cpp‎ | utility‎ | pair
m (→‎See also: ~)
m (<cstddef> for std::size_t, "" -> '', fmt)
 
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−
{{cpp/utility/pair/title | operator{{=}}}}
+
{{cpp/utility/pair/title|operator{{=}}}}
 
{{cpp/utility/pair/navbar}}
 
{{cpp/utility/pair/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
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}}
 
}}
 
{{dcl rev multi|num=3
 
{{dcl rev multi|num=3
−
|since1=c++11|dcl1=
+
|dcl1=
 
template< class U1, class U2 >
 
template< class U1, class U2 >
 
pair& operator=( const pair<U1, U2>& other );
 
pair& operator=( const pair<U1, U2>& other );
Line 36: Line 36:
 
{{dcl rev multi|num=7|since1=c++11|dcl1=
 
{{dcl rev multi|num=7|since1=c++11|dcl1=
 
template< class U1, class U2 >
 
template< class U1, class U2 >
−
pair& operator=( pair<U1, U2>&& other );
+
pair& operator=( pair<U1, U2>&& p );
 
|since2=c++20|dcl2=
 
|since2=c++20|dcl2=
 
template< class U1, class U2 >
 
template< class U1, class U2 >
−
constexpr pair& operator=( pair<U1, U2>&& other );
+
constexpr pair& operator=( pair<U1, U2>&& p );
 
}}
 
}}
 
{{dcl|num=8
 
{{dcl|num=8
 
|since=c++23|1=
 
|since=c++23|1=
 
template< class U1, class U2 >
 
template< class U1, class U2 >
−
constexpr const pair& operator=( pair<U1, U2>&& other ) const;
+
constexpr const pair& operator=( pair<U1, U2>&& p ) const;
 +
}}
 +
{{dcl|num=9|since=c++23|1=
 +
template< pair-like P >
 +
constexpr pair& operator=( P&& u );
 +
}}
 +
{{dcl|num=10|since=c++23|1=
 +
template< pair-like P >
 +
constexpr const pair& operator=( P&& u ) const;
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
Line 50: Line 58:
 
Replaces the contents of the pair.
 
Replaces the contents of the pair.
  
−
@1@ Copy assignment operator. Replaces the contents with a copy of the contents of other.
+
@1@ Copy assignment operator. Replaces the contents with a copy of the contents of {{c|other}}.
 
{{rrev multi|until1=c++11|rev1=
 
{{rrev multi|until1=c++11|rev1=
−
* The assignment operator is implicitly declared. Using this assignment operator makes the program ill-formed if either {{tt|T1}} or {{tt|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.
+
The assignment operator is implicitly declared. Using this assignment operator makes the program ill-formed if either {{tt|T1}} or {{tt|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.
 
|rev2=
 
|rev2=
−
* This overload is defined as deleted if either {{c|std::is_copy_assignable<T1>::value}} or {{c|std::is_copy_assignable<T2>::value}} is {{c|false}}.
+
This overload is defined as deleted if either {{c|std::is_copy_assignable<T1>::value}} or {{c|std::is_copy_assignable<T2>::value}} is {{c|false}}.
 
}}
 
}}
  
 
@2@ Copy assignment operator for const-qualified operand.
 
@2@ Copy assignment operator for const-qualified operand.
−
* {{cpp/enable_if|{{c|std::is_copy_assignable_v<const T1>}} and {{c|std::is_copy_assignable_v<const T2>}} are both {{c|true}}}}.
+
@@ {{cpp/enable if|{{c|std::is_copy_assignable_v<const T1>}} and {{c|std::is_copy_assignable_v<const T2>}} are both {{c|true}}}}.
  
−
@3@ Assigns {{tt|other.first}} to {{tt|first}} and {{tt|other.second}} to {{tt|second}}.
+
@3@ Assigns {{c|other.first}} to {{tt|first}} and {{c|other.second}} to {{tt|second}}.
−
* {{cpp/enable_if|{{c|std::is_assignable<T1&, const U1&>::value}} and {{c|std::is_assignable<T2&, const U2&>::value}} are both {{c|true}}}}.
+
{{rrev|since=c++11|
 +
{{cpp/enable if|{{c|std::is_assignable<T1&, const U1&>::value}} and {{c|std::is_assignable<T2&, const U2&>::value}} are both {{c|true}}}}.
 +
}}
  
−
@4@ Assigns {{tt|other.first}} to {{tt|first}} and {{tt|other.second}} to {{tt|second}}.
+
@4@ Assigns {{c|other.first}} to {{tt|first}} and {{c|other.second}} to {{tt|second}}.
−
* {{cpp/enable_if|{{c|std::is_assignable_v<const T1&, const U1&>}} and {{c|std::is_assignable_v<const T2&, const U2&>}} are both {{c|true}}}}.
+
@@ {{cpp/enable if|{{c|std::is_assignable_v<const T1&, const U1&>}} and {{c|std::is_assignable_v<const T2&, const U2&>}} are both {{c|true}}}}.
  
−
@5@ Move assignment operator. Replaces the contents with those of {{tt|other}} using move semantics.
+
@5@ Move assignment operator. Replaces the contents with those of {{c|other}} using move semantics.
−
* {{cpp/enable_if|{{c|std::is_move_assignable<T1>::value}} and {{c|std::is_move_assignable<T2>::value}} are both {{c|true}}}}.
+
@@ {{cpp/enable if|{{c|std::is_move_assignable<T1>::value}} and {{c|std::is_move_assignable<T2>::value}} are both {{c|true}}}}.
  
 
@6@ Move assignment operator for const-qualified operand.
 
@6@ Move assignment operator for const-qualified operand.
−
* {{cpp/enable_if|{{c|std::is_assignable_v<const T1&, T1>}} and {{c|std::is_assignable_v<const T2&, T2>}} are both {{c|true}}}}.
+
@@ {{cpp/enable if|{{c|std::is_assignable_v<const T1&, T1>}} and {{c|std::is_assignable_v<const T2&, T2>}} are both {{c|true}}}}.
  
 
@7@ Assigns {{c|std::forward<U1>(p.first)}} to {{tt|first}} and {{c|std::forward<U2>(p.second)}} to {{tt|second}}.
 
@7@ Assigns {{c|std::forward<U1>(p.first)}} to {{tt|first}} and {{c|std::forward<U2>(p.second)}} to {{tt|second}}.
−
* {{cpp/enable_if|{{c|std::is_assignable<T1&, U1>::value}} and {{c|std::is_assignable<T2&, U2>::value}} are both {{c|true}}}}.
+
@@ {{cpp/enable if|{{c|std::is_assignable<T1&, U1>::value}} and {{c|std::is_assignable<T2&, U2>::value}} are both {{c|true}}}}.
  
 
@8@ Assigns {{c|std::forward<U1>(p.first)}} to {{tt|first}} and {{c|std::forward<U2>(p.second)}} to {{tt|second}}.
 
@8@ Assigns {{c|std::forward<U1>(p.first)}} to {{tt|first}} and {{c|std::forward<U2>(p.second)}} to {{tt|second}}.
−
* {{cpp/enable_if|{{c|std::is_assignable_v<const T1&, U1>}} and {{c|std::is_assignable_v<const T2&, U2>}} are both {{c|true}}}}.
+
@@ {{cpp/enable if|{{c|std::is_assignable_v<const T1&, U1>}} and {{c|std::is_assignable_v<const T2&, U2>}} are both {{c|true}}}}.
 +
 
 +
@9@ Assigns {{c|std::get<0>(std::forward<P>(u))}} to {{tt|first}} and {{c|std::get<1>(std::forward<P>(u))}} to {{tt|second}}.
 +
@@ {{cpp/enable if|
 +
* {{c|std::same_as<std::remove_cvref_t<P>, std::pair>}} is {{c|false}},
 +
* {{c/core|std::remove_cvref_t<P>}} is not a specialization of {{lc|std::ranges::subrange}},
 +
* {{c|std::is_assignable_v<T1&, decltype(std::get<0>(std::forward<P>(p)))>}} is {{c|true}}, and
 +
* {{c|std::is_assignable_v<T1&, decltype(std::get<1>(std::forward<P>(p)))>}} is {{c|true}}.}}
 +
 
 +
@10@ Assigns {{c|std::get<0>(std::forward<P>(u))}} to {{tt|first}} and {{c|std::get<1>(std::forward<P>(u))}} to {{tt|second}}.
 +
@@ {{cpp/enable if|
 +
* {{c|std::same_as<std::remove_cvref_t<P>, std::pair>}} is {{c|false}},
 +
* {{c/core|std::remove_cvref_t<P>}} is not a specialization of {{lc|std::ranges::subrange}},
 +
* {{c|std::is_assignable_v<const T1&, decltype(std::get<0>(std::forward<P>(p)))>}} is {{c|true}}, and
 +
* {{c|std::is_assignable_v<const T1&, decltype(std::get<1>(std::forward<P>(p)))>}} is {{c|true}}.}}
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | other | pair of values to replace the contents of this pair }}
+
{{par|other|pair of values to replace the contents of this pair}}
−
{{par end}}  
+
{{par|p|pair of values of possibly different types to replace the contents of this pair}}
 +
{{par|u|{{lti|cpp/utility/tuple/tuple-like|pair-like}} object of values to replace the contents of this pair}}
 +
{{par hreq}}
 +
{{par req|{{tt|T1}} must meet the requirements of {{named req|CopyAssignable}} from {{tt|U1}}. {{mark until c++11}}}}
 +
{{par req|{{tt|T2}} must meet the requirements of {{named req|CopyAssignable}} from {{tt|U2}}. {{mark until c++11}}}}
 +
{{par end}}
  
 
===Return value===
 
===Return value===
Line 92: Line 121:
 
     std::is_nothrow_move_assignable<T2>::value
 
     std::is_nothrow_move_assignable<T2>::value
 
}}
 
}}
−
@6-8@ {{cpp/impldef exception item}}
+
@6-10@ {{cpp/impldef exception item}}
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
| code=
+
|code=
 +
#include <cstddef>
 
#include <iomanip>
 
#include <iomanip>
 
#include <iostream>
 
#include <iostream>
Line 102: 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)
−
     os << "{";
+
{
 +
     os << '{';
 
     for (std::size_t t = 0; t != v.size(); ++t)
 
     for (std::size_t t = 0; t != v.size(); ++t)
−
         os << v[t] << (t+1 < v.size() ? "," : "");
+
         os << v[t] << (t + 1 < v.size() ? ", " : "");
−
     return os << "}";
+
     return os << '}';
 
}
 
}
  
−
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 << '}';
 
}
 
}
  
 
int main()
 
int main()
 
{
 
{
−
     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;" << "p" << p << "   q" << q << '\n';
+
               << "(1) p = q;"
−
 
+
              << "p: " << p << "     q: " << q << '\n';
−
     std::pair<short, std::vector<int>> r{ 4, {7,8,9} };
+
   
−
     p = r; // (2) operator=( const pair<U1,U2>& other );
+
     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)
 
     std::cout << std::setw(23)
−
               << "(2) p = r;" << "p" << p << " r" << r << '\n';
+
               << "(3) p = r;"
−
 
+
              << "p: " << p << " r: " << r << '\n';
−
     p = std::pair<int, std::vector<int>>{ 3, {4} };
+
   
−
     p = std::move(q); // (3) operator=( pair&& other );
+
     p = std::pair<int, std::vector<int>>{3, {4}<!---->};
 +
     p = std::move(q); // (5) operator=(pair&& other);
 
     std::cout << std::setw(23)
 
     std::cout << std::setw(23)
−
               << "(3) p = std::move(q);" << "p" << p << "   q" << q << '\n';
+
               << "(5) p = std::move(q);"
−
 
+
              << "p: " << p << "     q: " << q << '\n';
−
     p = std::pair<int, std::vector<int>>{ 5, {6} };
+
   
−
     p = std::move(r); // (4) operator=( pair<U1,U2>&& other );
+
     p = std::pair<int, std::vector<int>>{5, {6}<!---->};
 +
     p = std::move(r); // (7) operator=(pair<U1, U2>&& other);
 
     std::cout << std::setw(23)
 
     std::cout << std::setw(23)
−
               << "(4) p = std::move(r);" << "p" << p << " r" << r << '\n';
+
               << "(7) p = std::move(r);"
 +
              << "p: " << p << " r: " << r << '\n';
 
}
 
}
−
| output=
+
|output=
−
(1) p = q;            p:{ 2, {5,6} }   q:{ 2, {5,6} }  
+
<nowiki>
−
(2) p = r;            p:{ 4, {7,8,9} }  r:{ 4, {7,8,9} }  
+
(1) p = q;            p: {2, {5, 6}}     q: {2, {5, 6}}
−
(3) p = std::move(q);  p:{ 2, {5,6} }   q:{ 2, {} }  
+
(3) p = r;            p: {4, {7, 8, 9}}  r: {4, {7, 8, 9}}
−
(4) p = std::move(r);  p:{ 4, {7,8,9} }  r:{ 4, {} }
+
(5) p = std::move(q);  p: {2, {5, 6}}     q: {2, {}}
 +
(7) p = std::move(r);  p: {4, {7, 8, 9}}  r: {4, {}}
 +
</nowiki>
 
}}
 
}}
  
 
===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=885|std=C++98|before=missing heterogeneous copy assignment|after=added (as overload {{v|3}})}}
 
{{dr list item|wg=lwg|dr=2729|std=C++11|before={{tt|1=pair::operator=}} was unconstrained and might<br>result in unnecessary undefined behavior|after=constrained}}
 
{{dr list item|wg=lwg|dr=2729|std=C++11|before={{tt|1=pair::operator=}} was unconstrained and might<br>result in unnecessary undefined behavior|after=constrained}}
 
{{dr list end}}
 
{{dr list end}}
Line 152: Line 191:
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/utility/tuple/dsc operator{{=}}}}
+
{{dsc inc|cpp/utility/tuple/dsc operator{{=}}}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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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