Postfix Evaluation

#define max_items 50
typedef int itemtype;

#include<iostream>
#include<string>
#include<math.h>
#include<fstream>

using namespace std;


class posteval
{
private:
string input;
itemtype result;
int top;
itemtype *items;
int cheak;
int counter;

public:
posteval();
void getexp();
void display();
bool isfull();
bool isempty();
void push(itemtype x);
itemtype pop();
void eval();
void cheakexp();

};


posteval::posteval()
{

top=-1;
items=new itemtype[max_items];
input=" ";
result=0;
cheak=0;
counter=0;

}

void posteval::getexp()
{

ifstream myfile("test.txt");
myfile>>input;

}

void posteval::display()
{
cout<<"\nThe resultant of the given postfix expression is : "<<result<<endl;

}

bool posteval::isfull()
{
if (top==max_items)
{
return 1;
}
else
{
return 0;
}
}

bool posteval::isempty()
{
if (top==-1)
{
return 1;
}
else
{
return 0;
}
}

void posteval::push(itemtype x)
{
if(isfull())
{
cout<<"\nStack Overflow";
exit(1);
}
else
{
top++;
items[top]=x;
}
}


itemtype posteval::pop()
{
itemtype z;

if(isempty())
{
cout<<"\nStack Underflow";
exit(1);
}
else
{
z=items[top];
top--;
return z;
}
}

void posteval::eval()
{

char exp;
itemtype operand1;
itemtype operand2;

exp=input[cheak];

while(cheak<input.length())
{
if(exp=='0'||exp=='1'||exp=='2'||exp=='3'||exp=='4'||exp=='5'||exp=='6'||exp=='7'||exp=='8'||exp=='9')
{
int x;
x=exp-48;
push(x);
}
else
{
operand2=pop();
operand1=pop();



if(exp=='+')
{
result=operand1+operand2;
}

else if(exp=='-')
{
result=operand1-operand2;
}

else if(exp=='*')
{
result=operand1*operand2;
}

else if(exp=='/')
{
result=operand1/operand2;
}

else if(exp=='$')
{
result=pow(operand1,operand2);
}

push(result);

}
cheak++;
exp=input[cheak];
}

if(!isempty())
{
result=pop();
}

}

void posteval::cheakexp()
{

for (int i=0;i<input.length();i++)
{
if(input[i]=='0'||input[i]=='1'||input[i]=='2'||input[i]=='3'||input[i]=='4'||input[i]=='5'||input[i]=='6'||input[i]=='7'||input[i]=='8'||input[i]=='9')
{
counter++;
}

else
{
counter-=1;
}

}


if(counter==1)
{

posteval::eval();
posteval::display();
}

else
{
cout<<"\nExpression is invalid\n";
}

}

void main()
{
posteval ob;
ob.getexp();
ob.cheakexp();

}