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 #include #include #include #include

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)
 }
```
}