vector::push_back in C++: Adding Elements to a Vector

Hey there! In this article, we’re going to examine a widely-used C++ function for adding elements to a vector - push_back. We’ll start with a brief overview, then discuss how you can use it. Through examples, we’ll observe its operation, and in the end, touch upon some of its more intriguing aspects.

How to add an element to a vector in C++

Vectors in C++ are dynamic arrays, meaning you can add or remove elements from them. To add elements, we use the push_back function:

void push_back (const value_type& val);

• This function takes an element of the vector’s type.
• It appends this element to the end of the vector.

The cool part? If the vector runs out of space, it automatically manages memory to accommodate the new element. Handy, isn’t it?

Let’s look at a simple program to demonstrate this behavior:

#include <iostream>
#include <vector>
using namespace std;

int main() {
  vector<int> numbers;
  cout << "Enter some numbers (0 to stop):" << endl;
  
  int input;
  do {
    cin >> input;
    numbers.push_back(input);
  } while(input != 0);

  cout << "You've added " << numbers.size() << " numbers to the vector." << endl;
  return 0;
}

This program lets you input numbers and adds them to the vector using push_back. The input ends when the user enters 0.

What’s happening “behind the scenes”?

When you’re adding elements, occasionally the vector needs more memory than initially allocated. In such cases, it reserves more space, copies over the old elements, and inserts the new one. While this process might sound complicated, in practice, it’s pretty efficient. However, when resizing occurs, any references to the vector’s elements could become invalid.

But… is it always efficient?

In most cases, yes! The push_back function is designed to be efficient. Even if it occasionally needs to allocate more memory, on average, it operates rather swiftly. This is what developers call “amortized constant time”.

However, if you know in advance the number of elements you’ll be adding, it’s a good practice to pre-allocate the required space using the reserve function. This way, the vector won’t need to resize multiple times, making the process even more efficient.

So, what about errors?

Of course, computers have their limits. If you run out of memory when trying to add an element, an error might occur. But here’s the good news! Vectors handle such errors gracefully. If an error happens and memory can’t be allocated, the vector remains unchanged. You won’t lose a thing!

In conclusion…

The push_back function is an essential tool in a C++ programmer’s toolkit when working with vectors. It’s simple, efficient, and pretty safe. Whether you’re a newbie or an experienced developer, understanding the intricacies of this function will definitely come in handy. So, the next time you want to add an element to a vector, just use push_back and carry on!

Exercises

1. Adding elements to a vector using push_back:
   Write a C++ program that prompts the user to input strings and adds them to a vector. The program should display the entered strings in reverse order once the input ends. Input concludes when the user types “stop”.
2. Dealing with vector’s memory:
   Using the capacity and reserve functions, craft a program that adds numbers from 1 to 100 into a vector. As you add numbers, the program should display the current vector’s capacity (capacity). Observe how the vector’s capacity changes as elements are added.
3. References to vector elements after push_back:
   Create a vector with five integers. Store a reference to the third element. Then, add another 10 elements to the vector using push_back. Check if the previously stored reference becomes invalid by trying to access the element through that reference.