Stacks (std::stack)

Stacks (std::stack)

Picture a stack of plates: you place plates on top, and the top plate comes off first. That rule is LIFO - Last In, First Out.

Create a stack

#include <stack>

std::stack<int> plates;

Push, peek, pop

plates.push(1);        // 1 goes on the top
plates.push(2);        // top is now 2
std::cout << plates.top();  // reads 2 without removing
plates.pop();          // removes the top (2)

• push(x) puts x on the top.
• top() reads the top element and leaves it in place.
• pop() removes the top element and does not return it.

Because pop returns nothing, read top() first if you need the value.

Empty and size

if (plates.empty()) {
    // no elements left
}
std::cout << plates.size();   // the count on the stack

Empty the stack safely

while (!plates.empty()) {
    std::cout << plates.top() << "\n";
    plates.pop();
}

Checks the top for printing, then pops, until the stack is empty.

TL;DR

• A stack follows LIFO (last in, first out).
• push(x) adds, pop() removes the top, top() reads it.
• pop() returns nothing, so read top() first when you need the value.
• empty() and size() report what is left.
• Pair top() with pop() inside a loop to drain a stack.