An abstract-data-type is a mathematical construct which is a set of values along with operations defined for those values.
The list is an abstract-data-type, we can change how it internally behaves and it doesn't affects the applications using that data type because the interface is exactly the same.

Stack

Method Name	Description
push(x)	Insert x as the top element of the stack
pop()	Remove the top element of the stack and return it.
top()	Return the top element without removing it from the stack.

Assume the stack is implemented using an array.

Worst Case (start of the array)

push(): the whole array will need to be pushed towards right direction.
pop(): the whole array will need to be pushed towards left direction.

Best Case (end of the array)

pop() or push() won't cause any performance issues because there are no shifts involved, since the elements are added or removed from end (right side).

Pop()

int pop() {
    //return the current element and then decrement current.
    return array_name[current--];
}

Push()

void push(int value) {
    //current is incremented, then the new value is placed there
    array_name[++current] = value;
}

top()

int top() {
    return array_name[current];
}

isEmpty()

int isEmpty() {
    //when last element is poped, current goes from 0 to -1
    return (current == -1);
}

isFull()

int isFull() {
    return (current == size - 1);
}

Class Implementation

class Stack {
    public:
        Stack() { size = 10; current = -1;}    //constructor
        // The pop function   
        int pop(){ return A[current--];}           
        // The push function
        void push(int x){A[++current] = x;}
        // The top function
        int top(){ return A[current];}
        // Will return true when stack is empty
        int isEmpty(){return ( current == -1 );}
        // Will return true when stack is full
        int isFull(){ return ( current == size-1);}
     private:
       // The data element
       int object;
       // Index of the array
       int current;
       // max size of the array
       int size;
       // Array of 10 elements   
       int A[10];
};