operator==,!=,<,<=,>,>=,<=>(std::pair)
Defined in header <utility> |
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| (1) | ||
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(until C++14) | |
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(since C++14) | |
| (2) | ||
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(until C++14) | |
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(since C++14) (until C++20) |
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| (3) | ||
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(until C++14) | |
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(since C++14) (until C++20) |
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| (4) | ||
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(until C++14) | |
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(since C++14) (until C++20) |
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| (5) | ||
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(until C++14) | |
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(since C++14) (until C++20) |
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| (6) | ||
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(until C++14) | |
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(since C++14) (until C++20) |
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(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.
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.
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 |
(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).
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 3865 | C++98 | comparison operators only accepted pairs of the same type |
accept pairs of different types |
See also
|
(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
|
lexicographically compares the values in the tuple (function template) |
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