C++ Stacks

In C++, a stack is a useful data structure that stores elements in a last-in-first-out (LIFO) order. This means the last element added to the stack is the first one to be removed.

Stacks are implemented as a container adaptor, a class that uses another container class as its underlying container. The underlying container class can be a vector, deque, or list.

Creating a Stack

To create a stack in C++, the <stack> header and the std namespace need to be included:

#include <stack>
using namespace std;

stack<dataType> stackName;

In the syntax:

• dataType: The data type of the elements to be stored in the stack.
• stackName: The name of the stack.

For example:

#include <stack>
using namespace std;

// Create a stack of integers
stack<int> myStack;

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Adding Elements to a Stack

The .push() method can be used to append an element to the top of a stack:

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

int main() {
  // Create a stack of integers
  stack<int> myStack;

  // Add elements to the stack
  myStack.push(10);
  myStack.push(20);
  myStack.push(30);

  // Print the elements of the stack
  while(!myStack.empty()) {
    cout << myStack.top() << "\n";

    myStack.pop();
  }

  return 0;
}

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Here is the output:

30
20
10

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Accessing Elements in a Stack

The .top() method can be used to access the top element of a stack:

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

int main() {
  // Create a stack of integers
  stack<int> myStack;

  // Add elements to the stack
  myStack.push(10);
  myStack.push(20);
  myStack.push(30);

  // Print the top element of the stack
  cout << myStack.top() << "\n";

  return 0;
}

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Here is the output:

30

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Removing Elements from a Stack

The .pop() method can be used to delete or remove an element from the top of a stack:

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

int main() {
  // Create a stack of integers
  stack<int> myStack;

  // Add elements to the stack
  myStack.push(10);
  myStack.push(20);
  myStack.push(30);

  // Remove the top element of the stack
  myStack.pop();

  // Print the elements of the stack
  while(!myStack.empty()) {
    cout << myStack.top() << "\n";

    myStack.pop();
  }

  return 0;
}

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Here is the output:

20
10

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Getting the Size of a Stack

The .size() method can be used to get the size of a stack:

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

int main() {
  // Create a stack of integers
  stack<int> myStack;

  // Add elements to the stack
  myStack.push(10);
  myStack.push(20);
  myStack.push(30);

  // Get the size of the stack
  cout << myStack.size() << "\n";

  return 0;
}

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Here is the output:

3

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Checking if a Stack is Empty

The .empty() method is used for checking if a stack is empty. If it is, the method returns 1. Otherwise, it returns 0:

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Frequently Asked Questions

1. What is the time complexity of stack operations in C++?

All major stack operations—.push(), .top(), .pop(), .size(), and .empty()—have constant time complexity O(1).

2. Is std::stack thread-safe?

No, std::stack is not thread-safe. You must use synchronization (like mutex) in multithreaded applications.

3. When should I use a stack in C++?

Stacks are ideal for:

• Reversing data
• Undo features
• Expression evaluation
• Depth-first search (DFS)
• Function call tracking (Recursion)