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( ) ;
}