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

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

From cppreference.com
< cpp‎ | utility‎ | pair

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.

### [edit] Parameters

lhs, rhs | - | pairs to compare

### [edit] Return value

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

2) true if either `lhs.first != rhs.first` or `lhs.second != rhs.second`, otherwise false

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)`

### [edit] 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)

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