Learn to use Pair in C++ STL

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The pair in the C++ STL is a container defined under the <utility> library used to store a pair of two data elements or objects. The two member elements maybe of the same or different types

Definition in <utility>

std::pair is defined as follows under <utility> header file:

template <class T1, class T2> struct pair;

Declaration

Syntax

pair<data_type1, data_type2> Pair_name;

A pair with name a storing two integers and a pair with name b storing one integer and one character are declared as follows:

pair<int, int> a;
pair<int, char> b;    

Properties

Some of the properties of pair include:

• The first element of the pair is referred to as "first" and the second element of the pair is referred to as "second".
• Pair can be treated as a valid data type and direct assignment and accession operations can be performed.
• To access an element of a pair, we use the name of the pair followed by a dot and the position of the element.

Functions

The following functions are applicable for pairs.

• Operators; (=, ==, !=, >=, <=): These operators can be used as in their normal definition with other pairs.

• make_pair: It creates a value pair without explicitly specifying the data types.
  Syntax:

Pair_name = make_pair (value1,value2);

• swap : This function swaps the contents of one pair object with the contents of another pair object. The pairs must be of same type.
  Syntax: pair1.swap(pair2) ;

Illustration of Pair

#include <iostream> 
#include<utility> 
  
using namespace std; 
  
int main() 
{ 
    pair<char, int>a = make_pair('A', 1); 
    pair<char, int>b = make_pair('B', 2); 
  
    cout << "Before swapping: " << endl; 
    cout << "Contents of a = " << a.first << " " << a.second; 
    cout << "Contents of b= " << b.first << " " << b.second; 
    a.swap(b); 
  
    cout << "After swapping: " << endl; 
    cout << "Contents of a = " << a.first << " " << a.second ; 
    cout << "Contents of b = " << b.first << " " << b.second ; 
  
    return 0; 
} 

Output:

Before swapping:
Contents of a = (A, 1)
Contents of b = (B, 2)

After swapping:
Contents of a = (B, 2)
Contents of b = (A, 1)

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