Stack In C++ STL: Usage & Examples

You might have thought about how the back button on your browser works or how your text editor remembers what you did last. This functionality is because of something called a stack. When we learn about data structures and algorithms, understanding the stack in C++ STL is important.
It explains how to do operations and shows you how to use it in a project. You will learn how to use the C++ STL stack in your next coding project.

Table of Contents

What is a Stack in C++ STL?

The stack in C++ STL provides a collection of ready-to-use data structures and algorithms that simplify programming. Unlike general containers, a stack only lets you add and remove items from the top.

A stack is a container adapter that uses another container, like a deque or list, to store elements and enforces LIFO access. The main rule of a stack is that it works with the Last-In-First-Out rule. Think about a pile of plates you see in a cafeteria.

When you add a plate, you put it on top of the other plates. When you need a plate, you take the one that is on top. The first plate you place at the bottom of the pile remains until all the other plates above it are removed.

Stack in C++ STL Operations

When working with a stack in C++, you don’t interact with every element at once. Instead, you focus on the “top” of the stack. Here are the core functions you will use:

push(): Adds an element to the top.
pop(): Removes the element from the top.
top(): Returns a reference to the top element without removing it.
empty(): Checks if the stack is vacant.
size(): Returns the total number of elements currently stored.
emplace(): Similar to push, but constructs the element in place for better performance.
swap(): Exchanges the contents of two stacks.

How Does Stack in C++ STL Push/Pop Work?

The backbone of any stack in C++ STL program is the push and pop logic. These two functions define the flow of data.

The Push Operation

When you use s.push(value), the stack grows. The new element becomes the new “top”. If you push the numbers 10, 20, and 30 in that order, 30 will be at the top.

The Pop Operation

The s.pop() function removes the element at the top. It is important to note that in C++, pop() is a “void” function—it removes the item but does not return it. If you want to see what you are removing, you must call top() first.

Stack in C++ STL Functions Table

Function	Description	Time Complexity
push(g)	Adds element ‘g’ at the top	O(1)
pop()	Deletes the top-most element	O(1)
top()	Returns a reference to the top element	O(1)
empty()	Returns true if the stack is empty	O(1)
size()	Returns the number of elements	O(1)

Stack in C++ STL Example

To use a stack, you must include the <stack> header. Let’s look at a basic stack in C++ STL example that demonstrates how to initialise and manipulate data.

C++

#include <iostream>
#include <stack>
int main() {
// Creating a stack of integers
std::stack<int> myStack;
// stack in c++ stl push pop actions
myStack.push(100);
myStack.push(200);
myStack.push(300);
std::cout << “The top element is: “ << myStack.top() << std::endl;
myStack.pop();
std::cout << “After popping, the top element is: “ << myStack.top() << std::endl;
return 0;
}

In this stack in C++ STL program, the output would show 300 as the first top element. After the pop operation, 200 becomes the new top.

How is Stack implemented in C++ STL?

The C++ STL does the work for us, but understanding how the stack works in the C++ STL helps us write better code. The stack in the C++ STL uses a kind of list called std::deque to store things. We can use a std::vector or a std::list if we need to for the stack. The stack is flexible, so we can choose what works best.

For instance, if you want a stack backed by a vector, the declaration looks like this:

std::stack<int, std::vector<int>> vStack;

This flexibility is why the stack in C++ STL usage is so prevalent in competitive programming. It allows for O(1) time complexity for all primary operations, making it incredibly fast.

Applications of Stack in C++ STL

Why should you use a stack instead of a simple array? Here are the most common use cases:

Expression Evaluation: Used by compilers to evaluate prefix, postfix, and infix expressions.
Backtracking: Essential for algorithms that explore multiple paths, such as solving a maze or the N-Queens problem.
Function Calls: The computer’s memory uses a “call stack” to keep track of active subroutines.
String Reversal: Since the last character pushed is the first one popped, stacks are perfect for reversing data.

Also Read:

Tips for Using Stack in C++ STL

Listed below are some of the tips that can be useful:

Check Before Popping: Always use the empty() function before calling top() or pop(). Accessing the top of an empty stack will cause a segmentation fault or undefined behaviour.
Memory Efficiency: If you know the size of your data won’t change much, using a vector-backed stack might be slightly more memory-efficient than the default deque.

Emplace over Push: When dealing with complex objects or structures, emplace() is often faster than push() because it avoids creating temporary copies of the object.

FAQs

1. What is the main difference between a stack and a queue in C++?

The main difference is the order in which you access things. A stack in C++ works in a way that it does things in a last-in-first-out order. This means that a stack in C++ and a queue in C++ are really different when it comes to the order of access.

2. Can we iterate through a stack in C++?

The STL stack doesn't show iterators because it limits access to elements to keep the LIFO (Last-In-First-Out) structure. You can only access the top element of the stack. To see elements on the stack, you have to remove the top ones first.

3. What happens if I call pop() on an empty stack?

When you use C++ STL functions like pop or top on a container, it can cause a lot of problems. Calling pop() or top() on an empty stack doesn't work and can cause the program to crash.

4. Is the stack in STL faster than a manual array implementation?

The stack in C++ STL implementation is excellent. It usually works just as well as something you would write yourself. This is because the C++ STL stack is safer, more reliable, and easier to maintain compared to manual implementations.

5. How do I clear all elements from a stack?

So you want to know how to clear a stack in C++. The thing is, there is no clear "function" for a stack in C++. You must use !empty() to check if the stack is empty, then pop() to remove everything. Another way to do it is to swap the stack with another stack.