Thread: storing abstract data on a stack?

I'm trying to write a calculator that gets its input by parsing a single line expression that includes numbers and their operators, like 1+2-3

The number and operator will both be stored in a class derived from the abstract class Token, one called Number and the other Operator. How do I make a stack hold a Token when Token is abstract? I can declare a Token stack[SIZE]; but what will the push and pop functions look like?

Code:

const int SIZE=10;
class stack
{
  int head;
  Token stk[SIZE];

   public:
    //init etc
    void push(Token* var) //doesn't work!
    {stk[++head] = *var;}
    Token* pop()
    {return stk[head--];}
};

I'm storing them as floating point numbers. Here's a bit of the code:

Code:

class Token
{
  public:
    virtual float getNumber()=0;
    virtual char getOperator()=0;
};

class Operator:public Token
{
    char op;
  public:
    Operator(char ch) {op=ch;}
    char getOperator();
    float getNumber();
};

class Number:public Token
{
    float fnum;
  public:
    Number(float fn) {fnum=fn;}
    float getNumber();
    char getOperator();
};