DevTut

# 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
  • `operator<=` returns `!(rhs
  • `operator>` returns `rhs
  • `operator>=` returns `!(lhs<` 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)
    }
}

    ← std::atomics →