In C++, the Standard Template Library (STL) provides a collection of template classes and functions, which include commonly used data structures like stacks and queues. These structures are built on top of other STL containers, offering both convenience and efficiency.

Stack in C++

A stack is a Last-In-First-Out (LIFO) data structure, where the last element added is the first one to be removed. The stack adapts an underlying container to provide specific push, pop, and top operations. In the STL, a default deque (double-ended queue) is used to implement a stack, but any underlying sequential container can be used (like vector or list).

Key Operations:

• push(element): Adds an element to the top of the stack.
• pop(): Removes the element at the top of the stack.
• top(): Returns a reference to the top element of the stack.
• empty(): Checks if the stack is empty.
• size(): Returns the number of elements in the stack.

Example:

#include <iostream>  
#include <stack>  
int main() {  
    std::stack<int> s;  
    s.push(1);  
    s.push(2);  
    s.push(3);  
    std::cout << "Top element: " << s.top() << std::endl; // Output: 3  
    s.pop();  
    std::cout << "After popping, top element: " << s.top() << std::endl; // Output: 2  
    return 0;  
}  

Queue in C++

A queue is a First-In-First-Out (FIFO) data structure, where elements are added at the back and removed from the front. It uses a deque by default, but other containers like list can also be used.

Key Operations:

• push(element): Adds an element to the back of the queue.
• pop(): Removes the element from the front of the queue.
• front(): Returns a reference to the front element.
• back(): Returns a reference to the last element.
• empty(): Checks if the queue is empty.
• size(): Returns the number of elements in the queue.

Example:

#include <iostream>  
#include <queue>  
int main() {  
    std::queue<int> q;  
    q.push(1);  
    q.push(2);  
    q.push(3);  
    std::cout << "Front element: " << q.front() << std::endl; // Output: 1  
    std::cout << "Back element: " << q.back() << std::endl; // Output: 3  
    q.pop();  
    std::cout << "After popping, front element: " << q.front() << std::endl; // Output: 2  
    return 0;  
}  

Key Insights:

1. Underlying Containers: Both stack and queue can be implemented with different underlying containers, influencing performance characteristics. Deques are default due to their efficient front and back operations.

2. Efficiency: Operations such as push/pop in stack and push/pop in queue are generally O(1) due to their direct manipulation of the container ends.

3. Use Cases:

   • Use stack when you need to reverse items order (e.g., backtracking algorithms).
   • Use queue for managing tasks in order (e.g., breadth-first search in graphs).

Understanding these structures' operations and efficiency allows C++ programmers to choose the proper data structure for their specific problem, ensuring optimal use of resources for both time and space complexity.