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

From cppreference.com
< cpp‎ | utility‎ | pair
(fix template args)
m (<cstddef> for std::size_t, "" -> '', fmt)
 
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{{cpp/utility/pair/title | operator{{=}}}}
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{{cpp/utility/pair/title|operator{{=}}}}
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{{cpp/utility/pair/sidebar}}
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{{cpp/utility/pair/navbar}}
−
{{ddcl list begin}}
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{{dcl begin}}
−
{{ddcl list item | num=1 | 1=
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{{dcl rev multi|num=1|dcl1=
−
pair& operator=(const pair& p);
+
pair& operator=( const pair& other );
 +
|since2=c++20|dcl2=
 +
constexpr pair& operator=( const pair& other );
 
}}
 
}}
−
{{ddcl list item | num=2 | 1=
+
{{dcl|num=2
−
template<class T1, class T2>
+
|since=c++23|1=
−
pair& operator=(const pair<T1,T2>& p);
+
constexpr const pair& operator=( const pair& other ) const;
 
}}
 
}}
−
{{ddcl list item | num=3 | notes={{mark c++11 version}} | 1=
+
{{dcl rev multi|num=3
−
pair& operator=(pair&& p);
+
|dcl1=
 +
template< class U1, class U2 >
 +
pair& operator=( const pair<U1, U2>& other );
 +
|since2=c++20|dcl2=
 +
template< class U1, class U2 >
 +
constexpr pair& operator=( const pair<U1, U2>& other );
 
}}
 
}}
−
{{ddcl list item | num=4 | notes={{mark c++11 version}} | 1=
+
{{dcl|num=4
−
template<class T1, class T2>
+
|since=c++23|1=
−
pair& operator=(pair<T1,T2>&& p);
+
template< class U1, class U2 >
 +
constexpr const pair& operator=( const pair<U1, U2>& other ) const;
 
}}
 
}}
−
{{ddcl list end}}
+
{{dcl rev multi|num=5
 +
|since1=c++11|dcl1=
 +
pair& operator=( pair&& other ) noexcept(/* see below */);
 +
|since2=c++20|dcl2=
 +
constexpr pair& operator=( pair&& other ) noexcept(/* see below */);
 +
}}
 +
{{dcl|num=6
 +
|since=c++23|1=
 +
constexpr const pair& operator=( pair&& other ) const;
 +
}}
 +
{{dcl rev multi|num=7|since1=c++11|dcl1=
 +
template< class U1, class U2 >
 +
pair& operator=( pair<U1, U2>&& p );
 +
|since2=c++20|dcl2=
 +
template< class U1, class U2 >
 +
constexpr pair& operator=( pair<U1, U2>&& p );
 +
}}
 +
{{dcl|num=8
 +
|since=c++23|1=
 +
template< class U1, class U2 >
 +
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}}
  
 
Replaces the contents of the pair.
 
Replaces the contents of the pair.
  
−
1) Assigns {{tt|p.first}} to {{tt|first}} and {{tt|p.second}} to {{tt|second}}
+
@1@ Copy assignment operator. Replaces the contents with a copy of the contents of {{c|other}}.
 +
{{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.
 +
|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}}.
 +
}}
  
−
2) Assigns {{tt|p.first}} to {{tt|first}} and {{tt|p.second}} to {{tt|second}}
+
@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}}}}.
  
−
3) Assigns {{cpp|std::forward<T1>(p.first)}} to {{tt|first}} and {{cpp|std::forward<T2>(p.second)}} to {{tt|second}}.  
+
@3@ Assigns {{c|other.first}} to {{tt|first}} and {{c|other.second}} to {{tt|second}}.
 +
{{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}}}}.
 +
}}
  
−
{{todo}}
+
@4@ Assigns {{c|other.first}} to {{tt|first}} and {{c|other.second}} to {{tt|second}}.
−
<!-- how do I do that
+
@@ {{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}}}}.
−
  noexcept(
+
−
    is_nothrow_move_assignable<T1>::value &&
+
−
    is_nothrow_move_assignable<T2>::value
+
−
    )
+
−
thing? I can't seem to find a mention of noexcept using site search. -->
+
  
−
4) Assigns {{cpp|std::forward<T1>(p.first)}} to {{tt|first}} and {{cpp|std::forward<T2>(p.second)}} to {{tt|second}}.
+
@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}}}}.
 +
 
 +
@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}}}}.
 +
 
 +
@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}}}}.
 +
 
 +
@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}}}}.
 +
 
 +
@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===
−
{{param list begin}}
+
{{par begin}}
−
{{param list item | p | pair of values to replace the contents of this pair }}
+
{{par|other|pair of values to replace the contents of this pair}}
−
{{param list 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===
−
{{cpp|*this}}
+
{{c|*this}}
 +
 
 +
===Exceptions===
 +
@1-4@ {{cpp/impldef exception item}}
 +
@5@{{noexcept|
 +
    std::is_nothrow_move_assignable<T1>::value &&
 +
    std::is_nothrow_move_assignable<T2>::value
 +
}}
 +
@6-10@ {{cpp/impldef exception item}}
  
 
===Example===
 
===Example===
−
{{example cpp
+
{{example
−
| code=
+
|code=
−
  | output=
+
#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=
 +
<nowiki>
 +
(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, {}}
 +
</nowiki>
 
}}
 
}}
 +
 +
===Defect reports===
 +
{{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 end}}
  
 
===See also===
 
===See also===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/utility/tuple/dsc operator{{=}}}}
 +
{{dsc end}}
  
−
{{dcl list begin}}
+
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
−
{{dcl list end}}
+

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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