🧱 C++ Stacks – Implement LIFO Data Structures with std::stack

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🧲 Introduction – Why Use std::stack in C++

A C++ stack is a Last-In, First-Out (LIFO) data structure. In C++, the Standard Template Library provides std::stack, a container adapter that allows data to be stored and accessed in LIFO order. It is ideal for managing function calls, parsing expressions, undo mechanisms, and more.

🎯 In this guide, you’ll learn:

• What std::stack is and how it works
• Stack operations: push(), pop(), top()
• Internal container customization
• Use cases and best practices

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📘 What Is a C++ Stack?

A std::stack is a container adapter, meaning it wraps an existing container (default is deque) and provides a restricted interface that supports LIFO operations only.

Include it with:

#include <stack>

Declare a stack:

stack<int> s;

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💻 Code Examples – With Output

✅ Basic Stack Operations

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

int main() {
    stack<int> s;
    s.push(10);
    s.push(20);
    s.push(30);
    cout << "Top: " << s.top() << endl;
    s.pop();
    cout << "New Top: " << s.top() << endl;
    return 0;
}

🟢 Output:

Top: 30  
New Top: 20

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🧩 Stack Member Functions

Function	Description
push(val)	Pushes val to top of stack
pop()	Removes top element
top()	Accesses top element
empty()	Returns true if stack is empty
size()	Returns number of elements

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🔧 Customizing Underlying Container

By default, std::stack uses std::deque, but it can be changed to std::vector or another sequence container.

stack<int, vector<int>> s2;  // Using vector instead of deque

Note: Not all STL containers are supported (e.g., list may not be fully compatible).

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📊 Stack Use Cases

🧠 Expression Evaluation: Postfix, infix, and prefix conversions
🔙 Undo Mechanisms: Applications like editors and calculators
🧾 Syntax Parsing: Balancing brackets, checking expressions
🕳 Depth-First Search (DFS): Graph traversal
📦 Call Stack Simulation: Manage recursive calls manually

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💡 Best Practices & Tips

📘 Use empty() before calling top() or pop() to avoid exceptions
💡 Use std::vector for performance in memory-constrained systems
⚠️ Don’t iterate through a stack—use a temporary copy if needed
📦 Avoid unnecessary copies—pass stack& to functions when needed

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📌 Summary – Recap & Next Steps

🔍 Key Takeaways:

• std::stack is a LIFO data structure with restricted access
• Supports push/pop/top/empty/size operations only
• Ideal for undo functionality, expression parsing, and control flow simulation

⚙️ Real-World Relevance:
Used in compilers, interpreters, games, parsers, and data-processing tools where LIFO behavior is required.

✅ Next Steps:
Explore C++ Queues, a FIFO (First-In, First-Out) structure for order-preserving tasks.

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❓FAQ – C++ Stacks

❓ Is std::stack a container?
No, it is a container adapter built over another container like deque or vector.

❓ How can I view all elements of a stack?
Use a copy of the stack and pop elements one by one while printing.

❓ Can I iterate through a stack directly?
No. The STL does not support iteration for std::stack.

❓ Is top() safe if the stack is empty?
No. Always check empty() before accessing top().

❓ When should I use vector as the underlying container?
When you need better cache locality or have size constraints.

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