I have a problem with my program

[joeyma1996]

Hello, this is my first post. I am making a calculator using stacks. curpos is the the variable of the top of the stack that is initially set to -1. Every time I call the push function, curpos will increment by 1. Then the function will store in the number the user entered. The pop function will decrease by 1 and "pop" the number of curpos out. These 2 functions work fine. The part where I don't get is my add function. The add function is from the RPN class, which is an inherited class of the Stack class. When I call my add function, it will go to the "else" part and displays an error. It goes to else because apparently my curpos is -1(I checked with cout << curpos). However, i am sure that i set my curpos to 1 after my program prompted the user to enter 2 numbers. Since I do not get why my curpos is set back to -1 when my push and pop functions work perfectly, is there something wrong with my inherited class?

#include <iostream>
#include <stdlib.h>
#include <string.h>
using namespace std;
#define SIZE 20

void displayerror();

class Stack { //Declare a class
protected:
int curpos;
float value;
float stack[SIZE];
public:
Stack() {
curpos = -1;
}
void push(float);
float pop();
int size();
void show();
};

class RPN: public Stack {
public:
RPN(){};
~RPN(){};
float add();
float subtract();
float multiply();
float divide();
private:
};

void Stack::push(float num) { //Push function
if (curpos >= SIZE)
cout << "stack overflow" << endl;
else {
curpos++;
stack[curpos] = num;
value = stack[curpos];
//cout << value << endl;
}
}

float Stack::pop() { //Pop function
if (curpos != -1) {
curpos--;
return value;
}
else {
cout << "Error Empty Stack" << endl;
return -1;
}
//cout << size() << endl;
}

int Stack::size() { //Returns the postion
return curpos;
}

void Stack::show() { //Shows stack size and the corresponding character
cout << value << endl;
}

float RPN::add() {
//cout << size() << endl;
if(size() > 0) {
push(pop() + pop());
//return stack[curpos];
}
else {
displayerror();
}
}

float RPN::subtract() {
if(size() > 0) {
push(0 - pop() + pop());
}
else {
displayerror();
}
}

float RPN::multiply() {
if(size() > 0) {
push(pop() * pop());
}
else {
displayerror();
}
}

float RPN::divide() {
if(size() > 0) {
push((1 / pop()) * pop());
}
else {
displayerror();
}
}

int main() {
Stack stk;
RPN calc;
char op[5];
char input[50];
float num1, num2;

while(true) { //Loop perpetuates until user chooses to quit
cout << "Type in anykey except q to continue" << endl;
cin >> input;
if (input[0] == 'q')
break;
cout << "Type in the number, the operator, and the number" << endl;
cin >> num1 >> op >> num2;
cin.ignore();
stk.push(num1);
stk.push(num2);

switch(op[0]) {
case '+': calc.add();
break;
case '-': calc.subtract();
break;
case '*': calc.multiply();
break;
case '/': calc.divide();
break;
default: cout << "Unacceptable input" << endl;
break;
}
stk.show();
}
}

void displayerror() { //Displays error message
cout << "ERROR: Not enough items " << endl;
}

[JLBorges]

#include <iostream>
#include <stdlib.h>
#include <string.h>
using namespace std;
#define SIZE 20

void displayerror();

class Stack   //Declare a class
{
protected:
    int curpos;
    // float value; // **** we don't really need this; we can use stack[curpos]

    float stack[SIZE];
public:
    Stack()
    {
        curpos = -1;
    }
    void push(float);
    float pop();
    int size() const ; // **** made const-correct
    void show() const ; // ****
};

class RPN: public Stack
{
public:
    RPN() {};
    ~RPN() {};
    void /*float*/ add();
    void /*float*/ subtract();
    void /*float*/ multiply();
    void /*float*/ divide();
private:
};

void Stack::push(float num)   //Push function
{
    if (curpos >= SIZE)
        cout << "stack overflow" << endl;
    else
    {
        curpos++;
        stack[curpos] = num;
        // value = stack[curpos];
//cout << value << endl;
    }
}

float Stack::pop()   //Pop function
{
    if (curpos != -1)
    {
        const float value = stack[curpos] ; // ****
        curpos--;
        return value;
    }
    else
    {
        cout << "Error Empty Stack" << endl;
        return -1;
    }
//cout << size() << endl;
}

int Stack::size() const // ****  //Returns the postion
{
    return curpos;
}

void Stack::show()  const // **** //Shows stack size and the corresponding character
{
    cout << stack[curpos] /*value*/ << endl;
}

void /*float*/ RPN::add()
{
//cout << size() << endl;
    if(size() > 0)
    {
        push(pop() + pop());
//return stack[curpos];
    }
    else
    {
        displayerror();
    }
}

void /*float*/ RPN::subtract()
{
    if(size() > 0)
    {
        // push(0 - pop() + pop()); // **** the pops are are indeterminably-sequenced
                         // see: http://en.cppreference.com/w/cpp/language/eval_order

        const float a = pop() ;
        const float b = pop() ;
        push(b-a) ;
    }
    else
    {
        displayerror();
    }
}

void /*float*/ RPN::multiply()
{
    if(size() > 0)
    {
        push(pop() * pop());
    }
    else
    {
        displayerror();
    }
}

void /*float*/ RPN::divide()
{
    if(size() > 0)
    {
        // push((1 / pop()) * pop());  // **** the pops are are indeterminably-sequenced
                         // see: http://en.cppreference.com/w/cpp/language/eval_order

        const float a = pop() ;
        const float b = pop() ;
        push(b/a) ;
    }
    else
    {
        displayerror();
    }
}

int main()
{
    // Stack stk; // **** we want to use the RPN object
    RPN calc;

    // char op[5]; // ****
    // char input[50]; // ****

    char op ; // **** we want to read just one character fpr op
    char input ; // **** we want to read just one character fpr input
    float num1, num2;

    while(true)   //Loop perpetuates until user chooses to quit
    {
        cout << "Type in anykey except q to continue" << endl;
        cin >> input;
        if( input == 'q' ) // **** // if (input[0] == 'q')
            break;
        cout << "Type in the number, the operator, and the number" << endl;
        cin >> num1 >> op >> num2;
        cin.ignore();

        calc.push(num1) ; // **** // stk.push(num1);
        calc.push(num2) ; // **** // stk.push(num2);

        switch(op) // switch(op[0])
        {
        case '+':
            calc.add();
            break;
        case '-':
            calc.subtract();
            break;
        case '*':
            calc.multiply();
            break;
        case '/':
            calc.divide();
            break;
        default:
            cout << "Unacceptable input" << endl;

            calc.pop() ; // ***** remove the incorrect value from the stack
            calc.pop() ; // ***** remove the incorrect value from the stack

            break;
        }
        calc.show() ; // **** // stk.show();
    }
}

void displayerror()   //Displays error message
{
    cout << "ERROR: Not enough items " << endl;
}

[joeyma1996]

Thank you so much!