#include <cstdlib>
#include <iostream>
 
using namespace std;
 
int main()
{
  char a_char[10];
  cin >> a_char;
  cout << "As an integer: " << atoi(a_char);
}

char s[2];
s[0] = ch;
s[1] = '\0';

intVal = ch - '0';

char* number = new char[20];
cin>>number;
c.postfix(number); // use fuction postfix to convert

void Calculator:postfix(char* item){ // I don't know this is right??
int i1;
for(int i=0;i<20;i++){
if(item[i]='+'){
i1 = atoi(item[i-1]); //<<< this is error T_T
item[i]=item[i-2]+item[i-1];
top=top-2;
i=i-2;
stack[top] = item[i];
}
else{
stack[top] = item[i];
top++;
}
}
}

char* number = new char[20];
cin>>number;
c.postfix(number); // use fuction postfix to convert
 
and this is my postfix fuction
 
void Calculator : Postfix(char* item)  
{
 // give variables meaningful names
// instead of "item" , "expression" would have been a better choice
// the same applies to i1
  int i1; 
  for(int i=0;i<20;i++)
  {
    if(item[i]='+')    // this should be if (item[i] == '+')
   {
      i1 = atoi(item[i-1]);  // if using C++ y use old C functions
      // also i think atoi works with only null terminated strings

      item[i]=item[i-2]+item[i-1];
      top=top-2;
      i=i-2;
      stack[top] = item[i];
    }
    else 
   {
      stack[top] = item[i];
      top++;
    }
 }
}