Question

I have a struct call ‘A’, which has an attribute ‘i’, like this:

typedef struct a {
   a() { i = 0;}
   int i;
} A;

And I would like to maintain a stack of A in my Main class:

class Main {
   public:  
      void save();
      void doSomethingToModifyCurrentA();
      void restore();

   private:
     A currentA;
     stack<A> aStack;
  }

I would like to write the function save() which save the current values of A (e.g. i) to the stack and I can go on doSomethingToModifyCurrentA() to change currentA. And then later on, I can restort A by calling restore().

My question are

• How can I allocate memory for a copy of A to the ‘stack’?
• How can I pop out the copy of A and free the memory and restore the value of ‘currentA’?

Answer 1

You don’t need to do anything to reserve memory on the stack. The underlying container (deque by default) manages memory for you.

The three important methods are…

mystack.push (myvalue);
mystack.top ();
mystack.pop ();

The pop doesn’t read the top value – just discards it. The top method returns a reference to the current top value, so you can write…

??? = mystack.top ();
mystack.top () = ???;

To read or overwrite the top value.

These methods translate to the following calls in the underlying deque…

mydeque.push_back (myvalue);
mydeque.back ();
mydeque.pop_back ();

Personally, I usually just use the deque directly – though strictly, the stack is better for readability and maintainability as it better expresses the intent and prevents you from doing some things incompatible with that intent.