std::pair

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.

#include <iostream>
#include <utility>

int main()
{
  std::pair<int,int> p = std::make_pair(1,2); //Creating the pair
  std::cout << p.first << " " << p.second << std::endl; //Accessing the elements
	// We can also create a pair and assign the elements later
	std::pair<int,int> p1;
	p1.first = 3;
	p1.second = 4;
	std::cout << p1.first << " " << p1.second << std::endl;
	
	//We can also create a pair using a constructor
	std::pair<int,int> p2 = std::pair<int,int>(5, 6);
	std::cout << p2.first << " " << p2.second << std::endl;
	
	return 0;
	}

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)
    }
}

Found a mistake? Have a question or improvement idea? Let me know.

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