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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../utility/pair/operator_cmp.html below:

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

Tests if both elements of

and

are equal, that is, compares

with

and

with

.

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.

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

Compares

and

lexicographically by

, 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

,

, or

do not meet the

requirements.

Compares

and

lexicographically by

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

is the return type of

.

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

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)

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

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<.

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"}

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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