Vector push_back() in C++ STL

The push_back() function in C++ STL is used to insert a new element into a vector while automatically managing its storage requirements.

• Inserts an element at the end of the vector.
• Automatically reallocates memory when additional capacity is required.

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

int main() {
    vector<int> v = {1, 4, 6};

    // Add an element at the end of the vector
    v.push_back(9);

    for (int i : v)
        cout << i << " ";
    return 0;
}

1 4 6 9 

Explanation: The element 9 is inserted at the end of the vector, increasing its size from 3 to 4.

Syntax

The vector push_back() is a member method of the std::vector class defined inside the <vector> header file.

v.push_back(val);

Parameters: val - rlement to be added to the vector at the end.
Return Value: this function does not return any value.

Working of vector::push_back()

The push_back() function appends the new element to the end of the vector. Its behavior depends on the current capacity of the vector:

• If sufficient capacity is available, the element is inserted directly at the end.
• If the vector has reached its capacity, it allocates a larger memory block.
• The existing elements are moved or copied, and the new element is inserted.

Examples of vector::push_back()

The following examples demonstrate different use cases of the push_back() function.

Example 1: Initializing an Empty Vector

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

int main() {
    vector<int> v;

    // Add elements to the vector
    v.push_back(3);
    v.push_back(7);
    v.push_back(9);

    for (int i : v)
        cout << i << " ";
    return 0;
}

3 7 9 

Explanation: Elements are inserted one by one into an initially empty vector.

Example 2: Adding Elements to a Vector of Strings

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

int main() {
    vector<string> v = {"Hello", "Geeks"};

    // Add string to the vector
    v.push_back("Welcome");

    for (string s: v) 
        cout << s << " ";
    return 0;
}

Hello Geeks Welcome 

Explanation: The string "Welcome" is appended to the end of the vector.

Example 3: Adding Elements Inside a Loop

#include <bits/stdc++.h> 
using namespace std; 
int main() { 
    vector<int> v; 
    
    for (int i = 1; i <= 5; i++) 
        v.push_back(i * 10); 
        
    for (int x : v) 
        cout << x << " "; 
    
    return 0; 
    
}

10 20 30 40 50 

Explanation: push_back() is commonly used to dynamically build a vector during program execution.

Key Features of vector::push_back()

The following points highlight the important characteristics and behavior of the push_back() function:

• push_back() always inserts the new element at the end of the vector.
• The size of the vector increases by one after each successful insertion.
• The vector automatically reallocates memory when its current capacity is exhausted.
• It can be used with vectors storing any data type, including user-defined classes and objects.