C++ program to convert an Expression from Infix form to Postfix form

#include <iostream.h>
#include <string.h>
#include <ctype.h>
const int MAX = 50 ;
class infix
{
private :
char target[MAX], stack[MAX] ;
char *s, *t ;
int top ;
public :
infix( ) ;
void setexpr ( char *str ) ;
void push ( char c ) ;
char pop( ) ;
void convert( ) ;
int priority ( char c ) ;
void show( ) ;
} ;
infix :: infix( )
{
top = -1 ;
strcpy ( target, "" ) ;
strcpy ( stack, "" ) ;
t = target ;
s = ""  ;
}
void infix :: setexpr ( char *str )
{
s = str ;
}
void infix :: push ( char c )
{
if ( top == MAX )
cout << "\nStack is full\n" ;
else
{
top++ ;
stack[top] = c ;
}
}
char infix :: pop( )
{
if ( top == -1 )
{
cout << "\nStack is empty\n" ;
return -1 ;
}
else
{
char item = stack[top] ;
top-- ;
return item ;
}
}
void infix :: convert( )
{
while ( *s )
{
if ( *s == ' ' || *s == '\t' )
{
s++ ;
continue ;
}
if ( isdigit ( *s ) || isalpha ( *s ) )
{
while ( isdigit ( *s ) || isalpha ( *s ) )
{
*t = *s ;
s++ ;
t++ ;
}
}
if ( *s == '(' )
{
push ( *s ) ;
s++ ;
}
char opr ;
if ( *s == '*' || *s == '+' || *s == '/' || *s == '%' || *s == '-' || *s == '$' )
{
if ( top != -1 )
{
opr = pop( ) ;
while ( priority ( opr ) >= priority ( *s ) )
{
*t = opr ;
t++ ;
opr = pop( ) ;
}
push ( opr ) ;
push ( *s ) ;
}
else
push ( *s ) ;
s++ ;
}
if ( *s == ')' )
{
opr = pop( ) ;
while ( ( opr ) != '(' )
{
*t = opr ;
t++ ;
opr =  pop( ) ;
}
s++ ;
}
}
while ( top != -1 )
{
char opr = pop( ) ;
*t = opr ;
t++ ;
}
*t = '\0' ;
}
int infix :: priority ( char c )
{
if ( c == '$' )
return 3 ;
if ( c == '*' || c == '/' || c == '%' )
return 2 ;
else
{
if ( c == '+' || c == '-' )
return 1 ;
else
return 0 ;
}
}
void infix :: show( )
{
cout << target ;
}
void main( )
{
char expr[MAX] ;
infix q ;
 
cout << "\nEnter an expression in infix form: " ;
cin.getline ( expr, MAX ) ;
 
q.setexpr ( expr ) ;
q.convert( ) ;
 
cout << "\nThe postfix expression is: " ;
q.show( ) ;
}