How do I reserve capacity and shrink-to-fit with std::stack?

In C++, the std::stack is an adapter container that provides a LIFO (last-in, first-out) data structure. However, unlike other standard containers, std::stack does not provide direct methods to manage its internal capacity (reserve or shrink-to-fit). To achieve similar functionality, you can utilize the underlying container that the std::stack is based on, typically std::deque or std::vector. Here’s how you can reserve capacity and shrink-to-fit.

Example

#include <iostream>
#include <stack>
#include <vector>

int main() {
    // Creating a stack using vector
    std::stack> myStack;

    // Reserving capacity using the underlying vector
    myStack.push(1);
    myStack.push(2);
    myStack.push(3);

    // Shrinking to fit
    std::vector &vec = const_cast<std::vector>(*myStack._Get_container());
    vec.reserve(5); // Reserve capacity for 5 elements
    
    // Displaying the size
        
    std::cout << "Stack size: " << myStack.size() << std::endl;

    return 0;
}