evaluation of infix to postfix.cpp

#include <iostream>
#include<string>
using namespace std;
class stack
{
private:
int len=100;
char arr[100];
public:
int top;
stack()
{
top=-1;
}
void push(char);
char pop();
void display();
char top_ele();
};
int postfix_eval(string post);
int main()
{
string exp;
cout<<"Enter the Expression "<<endl;
getline(cin,exp);
int required = postfix_eval(exp);
cout<<required<<endl;
}
void stack :: push(char x)
{
int check = top+1;
try
{
if(check>=len-1)
{
throw top;
}
else
{
top++;
arr[top]=x;
}
}
catch(int)
{
cout<<"Stack Overflow"<<endl;
}
}
char stack :: pop()
{
int check = top-1;
try
{
if(check<-1)
{
throw top;
}
else
{
top--;
return arr[top+1];
}
}
catch(int)
{
cout<<"Stack Underflow"<<endl;
}
}
void stack :: display()
{
cout<<"Element at "<<top<<" position is "<<arr[top]<<endl;
}
char stack :: top_ele()
{
return arr[top];
}
int postfix_eval(string post)
{
post.push_back(')');
int i=0;
stack numb;
while(post.at(i) != ')')
{
if((post.at(i) >= '0'&& post.at(i)<= '9'))
{
numb.push(post.at(i));
}
else if((post.at(i) == '+')||(post.at(i) == '-')||
(post.at(i) == '/')||(post.at(i) == '*'))
{ int res;
int a,b;
numb.push(post.at(i));
switch(numb.pop())
{
case '+':
a = numb.pop()-'0';
b = numb.pop()-'0';
res = a+b;
break;
case '-':
a = numb.pop()-'0';
b = numb.pop()-'0';
res = a - b;
break;
case '/':
a = numb.pop()-'0';
b = numb.pop()-'0';
res = a/b;
break;
case '*':
a = numb.pop()-'0';
b = numb.pop()-'0';
res = a*b;
break;
case'^':
a = numb.pop()-'0';
b = numb.pop()-'0';
res = a^b;
}
numb.push(res);
}
i++;
}
int value = numb.pop();
return value;
}