operator==,!=,<,<=,>,>=,<=> (std::pair)

From 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 )

7) synth-three-way ( lhs. first , rhs. first ) if it is not equal to 0 , otherwise synth-three-way ( lhs. second , rhs. second ) .

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.

Run this code

#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 `pair` s of the same type	accept `pair` s of different types

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Member functions
pair::pair
pair::operator=
pair::swap (C++11)
Non-member functions
make_pair
operator== operator!= operator< operator<= operator> operator>= operator<=> (until C++20) (until C++20) (until C++20) (until C++20) (until C++20) (C++20)
swap (std::pair) (C++11)
get (std::pair) (C++11)
Helper classes
tuple_size <std::pair> (C++11)
tuple_element <std::pair> (C++11)
basic_common_reference <std::pair> (C++23)
common_type <std::pair> (C++23)
formatter <std::pair> (C++23)
piecewise_construct_t (C++11)
Deduction guides (C++17)