A Beginner’s Guide to C++ pair: Make, Access, Swap, Compare

The pair class in the C++ Standard Template Library (STL) is a simple yet powerful utility that lets you group two values into a single object. This is extremely useful when dealing with related data like coordinates, key-value pairs, etc.

In this post, we’ll go over:

• Basic usage of pair
• Practical code example
• All built-in functionalities available for pair
• Array of pairs
• Swapping and comparing pairs

What is a pair?

A pair<T1, T2> is a templated class that stores two heterogeneous or homogeneous values. It's commonly used when:

• You want to return two values from a function
• Store key-value mappings
• Store data points with two attributes (like a coordinate)

📌 The Code Example

Let’s start with a simple code that introduces pair, arrays, input/output, and swapping:

cpp


Edit#include <bits/stdc++.h>
using namespace std;

int main() {
    cout << "hello" << endl;
    pair<int, string> p;
    p = make_pair(2, "abc");
    cout << p.first << " " << p.second << endl;

    int a[] = {1, 2, 3};
    int b[] = {4, 5, 6};
    pair<int, int> parr[3];

    for (int i = 0; i < 3; i++) {
        parr[i] = {a[i], b[i]};
    }

    swap(parr[0], parr[1]);
    swap(parr[0], parr[2]);

    for (int i = 0; i < 3; i++) {
        cout << parr[i].first << " " << parr[i].second << endl;
    }

    return 0;
}

🔍 

✅ Step 1: Using pair<int, string>

pair<int, string> p;
p = make_pair(2, "abc");
cout << p.first << " " << p.second << endl;

Here we create a pair of an int and a string. The make_pair function is used to assign values. The output will be:

2 abc

✅ Step 2: Pairing Two Arrays

int a[] = {1, 2, 3};
int b[] = {4, 5, 6};
pair<int, int> parr[3];

We define two integer arrays a and b, and then we pair elements from a and b together.

for (int i = 0; i < 3; i++) {
    parr[i] = {a[i], b[i]};
}

After this loop:

• parr[0] = {1, 4}
• parr[1] = {2, 5}
• parr[2] = {3, 6}

✅ Step 3: Swapping Pairs

swap(parr[0], parr[1]);  // parr[0] <-> parr[1]
swap(parr[0], parr[2]);  // new parr[0] <-> parr[2]

Let’s visualize it:

• After first swap:
   parr[0] = {2, 5}, parr[1] = {1, 4}, parr[2] = {3, 6}
• After second swap:
   parr[0] = {3, 6}, parr[2] = {2, 5}

Final state:

parr[0] = {3, 6}
parr[1] = {1, 4}
parr[2] = {2, 5}

✅ Step 4: Output Final Pairs

3 6
1 4
2 5

Edit

for (int i = 0; i < 3; i++) {
    cout << parr[i].first << " " << parr[i].second << endl;
}

Expected output:

3 6
1 4
2 5

✅ Declaring and Initializing Pairs

🔹 Using make_pair()

pair<int, string> p = make_pair(2, "abc");

🔹 Using Direct Initialization (C++11 onwards)

pair<int, string> p = {2, "abc"};

📋 Built-in Functionalities of pair

Let’s go through all useful operations and functions associated with pair:

1. ✅ .first and .second

• Access the first and second values.

cout << p.first << " " << p.second;

2. ✅ make_pair()

• Creates a pair without explicitly writing types.

auto p = make_pair(10, "hello");

3. ✅ Comparison Operators

• pair supports <, <=, >, >=, ==, !=
• Comparison is lexicographic: first compares first, then second.

pair<int, int> a = {1, 5}, b = {1, 7};
if (a < b) cout << "a is less";

4. ✅ swap(pair1, pair2)

• Swaps the contents of two pairs.

swap(p1, p2);

5. ✅ Copy Assignment and Copy Constructor

pair<int, string> p1 = {1, "one"};
pair<int, string> p2 = p1; // Copy

6. ✅ Nested Pairs

Yes, pairs inside pairs!

pair<int, pair<int, int>> nested = {1, {2, 3}};
cout << nested.first;          // 1
cout << nested.second.first;   // 2

🧪 Arrays and Vectors of Pairs

✅ Array of Pairs

pair<int, int> arr[3];
arr[0] = {1, 4}; arr[1] = {2, 5}; arr[2] = {3, 6};

✅ Vector of Pairs

vector<pair<int, string>> v;
v.push_back({1, "one"});
v.emplace_back(2, "two");

🎯 Real-World Use Cases

• Graph Representation: Edges as pair<int, int>
• Sorting with Criteria: Sort a list of students with {marks, roll}
• Dictionaries and Maps: STL map<int, string> internally uses pair<int, string>