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

 

C++

Language

Standard Library Headers

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Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Library feature-test macros
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
integer_sequence (C++14)
Relational operators (deprecated in C++20)
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
Common vocabulary types
pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)
launder (C++17)

 

std::pair

Member functions

pair::pair

pair::operator=

pair::swap

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=

std::swap

get (C++11)

Deduction guides(C++17)

Helper classes

tuple_size (C++11)

tuple_element (C++11)

 

Defined in header <utility>
template< class T1, class T2 > bool operator==( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(1)	(until C++14)
template< class T1, class T2 > constexpr bool operator==( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(1)	(since C++14)
template< class T1, class T2 > bool operator!=( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(2)	(until C++14)
template< class T1, class T2 > constexpr bool operator!=( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(2)	(since C++14)
template< class T1, class T2 > bool operator<( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(3)	(until C++14)
template< class T1, class T2 > constexpr bool operator<( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(3)	(since C++14)
template< class T1, class T2 > bool operator<=( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(4)	(until C++14)
template< class T1, class T2 > constexpr bool operator<=( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(4)	(since C++14)
template< class T1, class T2 > bool operator>( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(5)	(until C++14)
template< class T1, class T2 > constexpr bool operator>( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(5)	(since C++14)
template< class T1, class T2 > bool operator>=( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(6)	(until C++14)
template< class T1, class T2 > constexpr bool operator>=( const pair<T1,T2>& lhs, const pair<T1,T2>& rhs );	(6)	(since C++14)

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

3-6) Compares lhs and rhs lexicographically, that is, compares the first elements and only if they are equivalent, compares the second elements.

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)

Example

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

Run this code

#include <iostream>
#include <utility>
#include <vector>
#include <algorithm>
#include <string>
 
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 << "," << p.second << ")\n";
    }
}

Output:

(1,foo)
(2,bar)
(2,baz)