I have been running this and typing "12+" as the expression. 'm down to trying to add the top value and next value and it keeps giving me the result 'c' but I want the result to be 3. So is there any way to get my program to convert char 'c' to int '3' and char 'd' to int 4 and so on ?

//array based stack implementation
class Stack
{
private:
    int capacity;        //max size of stack
    int top;            //index for top element
    char *listArray;       //array holding stack elements

public:
    Stack (int size = 50){ //constructor
        capacity = size;
        top = 0;
        listArray = new char[size];
    }

    ~Stack() { delete [] listArray; } //destructor

    void push(char it) {    //Put "it" on stack
        listArray[top++] = it;
    }
    char pop() {   //pop top element
        return listArray [--top];
    }

    char& topValue() const { //return top element
        return listArray[top-1];
    }

    char& nextValue() const {//return second to top element
        return listArray[top-2];
    }

    int length() const { return top; } //return length

};

int main()
{
    string exp;
    char it = ' ';
    int count;
    int push_length;

    cout << "Enter an expression in postfix notation:\n";
    cin >> exp;
    cout << "The number of characters in your expression is " << exp.length() << ".\n";
    Stack STK;

    for(count= 0; count < exp.length() ;count++)
    {

        if (exp[count] == '+')
        {
          it = exp[count - 1];
          cout << it << "?\n";

              while (!isdigit(it))
        {
            cout << it << "!\n";
            it = exp[count--];
        }

        STK.push(it);
        cout << STK.topValue() << "\n";

        it = exp[count - 2];
        cout << it << "\n";

        if (isdigit(it))
        {
            STK.push(it);

        }
        cout << STK.topValue() << "\n";
        cout << STK.nextValue() << "\n";
        it = STK.topValue() + STK.nextValue();
        cout << it << "\n";

        STK.pop();
        STK.pop();
        STK.push(it);
        cout << STK.topValue() << "\n";

        }

    }
    cout << "The number of characters pushed into the stack is " << STK.length() << ".\n";
    push_length = STK.length();
    return(0);
}

You can simply use something like STK.push(it-'0') instead of STK.push(it).