c++ referencing class functions error: identifier is undefined - c++

Question

I am trying to call a function for my stack class. If I have all of the functions within the main file the project works, however, when called from the class it says the the Error: identifier "function name" is undefined. I think it is a simple syntax error, but i can't find it.
main.cpp
#include<iostream>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include "stack.h"
#define MAX 10
#define EMPTY -1
struct stack
{
char data[MAX];
int top;
};
int mystack::isempty(struct stack *s)
{
return (s->top == EMPTY) ? 1 : 0;
}
void mystack::emptystack(struct stack* s)
{
s->top=EMPTY;
}
void mystack::push(struct stack* s,int item)
{
if(s->top == (MAX-1))
{
printf("\nSTACK FULL");
}
else
{
++s->top;
s->data[s->top]=item;
}
}
char mystack::pop(struct stack* s)
{
char ret=(char)EMPTY;
if(!isempty(s))
{
ret= s->data[s->top];
--s->top;
}
return ret;
}
void mystack::display(struct stack s)
{
while(s.top != EMPTY)
{
printf("\n%d",s.data[s.top]);
s.top--;
}
}
int isoperator(char e)
{
if(e == '+' || e == '-' || e == '*' || e == '/' || e == '%' || e == '^')
return 1;
else
return 0;
}
int priority(char e)
{
int pri = 0;
if(e =='%' || e == '^')
pri = 3;
else
{
if (e == '*' || e == '/' || e =='%')
pri = 2;
else
{
if(e == '+' || e == '-')
pri = 1;
}
}
return pri;
}
void infix2postfix(char* infix, char * postfix, int insertspace)
{
char *i,*p;
struct stack X;
char n1;
emptystack(&X); // any time a class like this is called it says Error: identifier "emptystack"
// is undefined
i = &infix[0];
p = &postfix[0];
while(*i)
{
while(*i == ' ' || *i == '\t')
{
i++;
}
if( isdigit(*i) || isalpha(*i) )
{
while( isdigit(*i) || isalpha(*i))
{
*p = *i;
p++;
i++;
}
if(insertspace)
{
*p = ' ';
p++;
}
}
if( *i == '(' )
{
push(&X,*i);
i++;
}
if( *i == ')')
{
n1 = pop(&X);
while( n1 != '(' )
{
*p = n1;
p++;
if(insertspace)
{
*p = ' ';
p++;
}
n1 = pop(&X);
}
i++;
}
if( isoperator(*i) )
{
if(mystack::isempty(&X))
push(&X,*i);
else
{
n1 = pop(&X);
while(priority(n1) >= priority(*i))
{
*p = n1;
p++;
if(insertspace)
{
*p = ' ';
p++;
}
n1 = pop(&X);
}
push(&X,n1);
push(&X,*i);
}
i++;
}
}
while(!isempty(&X))
{
n1 = pop(&X);
*p = n1;
p++;
if(insertspace)
{
*p = ' ';
p++;
}
}
*p = '\0';
}
int main()
{
char in[50],post[50];
strcpy(&post[0],"");
printf("Enter Infix Expression : ");
fflush(stdin);
gets(in);
infix2postfix(&in[0],&post[0],1);
printf("Postfix Expression is : %s\n",&post[0]);
return 0;
}
stack.h
#include <iostream>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
using namespace std;
class mystack
{
public:
int isempty(struct stack *s);
void emptystack(struct stack* s);
void push(struct stack* s,int item);
char pop(struct stack* s);
void display(struct stack s);
};
I am using visual studio if that helps.
EDIT: added comment for clarity.
Thanks,
Ryan

Answer 1

At a cursory glance, this function:
void emptystack(struct stack* s)
{
s->top=EMPTY;
}
Is missing the scope operator (::), so you probably intended to write it as:
void mystack::emptystack(struct stack* s)
{
s->top=EMPTY;
}
I'm not sure if that's your problem though, since "I'm trying to call a function" is a bit vague. You might want to narrow down precisely where the error is occurring, then edit your question with additional information.
Edit: In looking at your implementation a bit more, I'm not sure why you created the mystack class at all. It looks like you just want to define a bunch of functions that operate on your stack struct, which doesn't require a class definition. If you want to do it this way for some unusual reason, then you'll have to instantiate a mystack object before you can call its member functions. Something of the nature:
mystack * myStackObj = new mystack();
myStackObj->emptystack(&X);
Though I'm not sure why you would want to do this. The other alternative is to roll your stack struct into the class instead, either by making the whole struct a member of the class or by simply adding data and top to the class. Then if you instantiated a mystack object it would have the data of the stack and could call methods on its own data. I'd also suggest looking at a tutorial/documentation/book related to C++ classes and their usage. Here's one, but there are undoubtedly plenty of others.