Chapter 54: std::pair

Section 54.1: Compare operators

Parameters of these operators are lhs and rhs

operator== tests if both elements on lhs and rhs pair are equal. The return value is true if both lhs.first

== rhs.first AND lhs.second == rhs.second, otherwise false

std::pair<int, int> p1 = std::make_pair(1, 2); std::pair<int, int> p2 = std::make_pair(2, 2);

if (p1 == p2)

std::cout << "equals";

else

std::cout << "not equal"//statement will show this, because they are not identical

operator!= tests if any elements on lhs and rhs pair are not equal. The return value is true if either

lhs.first != rhs.first OR lhs.second != rhs.second, otherwise return false.

operator< tests 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.

operator<= returns !(rhs<lhs)

operator> returns rhs<lhs

operator>= returns !(lhs<rhs)

Another example with containers of pairs. It uses operator< because it needs to sort container.

#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(const auto& p: v) {

std::cout << "(" << p.first << "," << p.second << ") "; //output: (1,foo) (2,bar) (2,baz)

}

}

Section 54.2: Creating a Pair and accessing the elements

Pair allows us to treat two objects as one object. Pairs can be easily constructed with the help of template function std::make_pair.

Alternative way is to create pair and assign its elements (first and second) later.