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operator==,!=,<,<=,>,>=,<=>(std::pair) - cppreference.com

Defined in header <utility>

(1)

template< class T1, class T2, class U1, class U2 > bool operator==( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (until C++14)

template< class T1, class T2, class U1, class U2 > constexpr bool operator==( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (since C++14)
(2)

template< class T1, class T2, class U1, class U2 > bool operator!=( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (until C++14)

template< class T1, class T2, class U1, class U2 > constexpr bool operator!=( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (since C++14)
(until C++20)
(3)

template< class T1, class T2, class U1, class U2 > bool operator<( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (until C++14)

template< class T1, class T2, class U1, class U2 > constexpr bool operator<( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (since C++14)
(until C++20)
(4)

template< class T1, class T2, class U1, class U2 > bool operator<=( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (until C++14)

template< class T1, class T2, class U1, class U2 > constexpr bool operator<=( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (since C++14)
(until C++20)
(5)

template< class T1, class T2, class U1, class U2 > bool operator>( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (until C++14)

template< class T1, class T2, class U1, class U2 > constexpr bool operator>( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (since C++14)
(until C++20)
(6)

template< class T1, class T2, class U1, class U2 > bool operator>=( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (until C++14)

template< class T1, class T2, class U1, class U2 > constexpr bool operator>=( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (since C++14)
(until C++20)

template< class T1, class T2, class U1, class U2 > constexpr std::common_comparison_category_t<synth-three-way-result<T1, U1>, synth-three-way-result<T2, U2>> operator<=>( const std::pair<T1, T2>& lhs, const std::pair<U1, U2>& rhs );

 (7) (since C++20)

1,2) Tests if both elements of lhs and rhs are equal, that is, compares lhs.first with rhs.first and lhs.second with rhs.second.

The behavior is undefined if the type and value category of either lhs.first == rhs.first or lhs.second == rhs.second do not meet the BooleanTestable requirements.

(until C++26)

This overload participates in overload resolution only if both decltype(lhs.first == rhs.first) and decltype(lhs.second == rhs.second) model boolean-testable.

(since C++26)

3-6) Compares lhs and rhs lexicographically by operator<, that is, compares the first elements and only if they are equivalent, compares the second elements. The behavior is undefined if the type and value category of any of lhs.first < rhs.first, rhs.first < lhs.first, or lhs.second < rhs.second do not meet the BooleanTestable requirements.

7) Compares lhs and rhs lexicographically by

synth-three-way

, that is, compares the first elements and only if they are equivalent, compares the second elements.

synth-three-way-result

is the return type of synth-three-way.

The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively.

(since C++20)

Parameters

lhs, rhs - pairs to compare

Return value

1) true if both lhs.first == rhs.first and lhs.second == rhs.second, otherwise false.

2) !(lhs == rhs)

3) If lhs.first < rhs.first, returns true. Otherwise, if rhs.first < lhs.first, returns false. Otherwise, if lhs.second < rhs.second, returns true. Otherwise, returns false.

4) !(rhs < lhs)

5) rhs < lhs

6) !(lhs < rhs)

Notes

The relational operators are defined in terms of each element's operator<.

(until C++20)

The relational operators are defined in terms of synth-three-way, which uses operator<=> if possible, or operator< otherwise.

Notably, if an element type does not itself provide operator<=>, but is implicitly convertible to a three-way comparable type, that conversion will be used instead of operator<.

(since C++20)
Feature-test macro Value Std Feature
__cpp_lib_constrained_equality 202403L (C++26) Constrained operator== for std::pair

Example

Because operator< is defined for pairs, containers of pairs can be sorted.

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <string>
#include <utility>
#include <vector>

int main()
{
    std::vector<std::pair<int, std::string>> v = {{2, "baz"}, {2, "bar"}, {1, "foo"}};
    std::sort(v.begin(), v.end());
    
    for (auto p : v)
        std::cout << '{' << p.first << ", " << std::quoted(p.second) << "}\n";
}

Output: 

{1, "foo"}
{2, "bar"}
{2, "baz"}

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 296 C++98 the descriptions of operators other than == and < were missing added
LWG 2114
(P2167R3) 		C++98 type preconditions for boolean operations were missing added
LWG 3865 C++98 comparison operators only accepted pairs of the same type accept pairs of different types

See also