[C++] An implementation of atoi

This is an implementation of atoi function I wrote. Seems it works.

/* Implementation of 
*  int atoi(char *)
*  Print negative sign if there is one.
*  If the return number is bigger than
*  2^31-1, return 2^31-1.
*/

#include

using namespace std;


int atoi(char *ptr)
{

    int i = 0;
    int flag = 1;
    int result = 0;
    // determine the first non white-space char
    while( ptr[i] == ' ')
       i++;
     
    if( ptr[i] == '+') {flag = 1; i++;}
    if( ptr[i] == '-') {flag = -1; i++;}
 
    // if the first non-space char is not a number,
    // return 0 
    if ( ptr[i] < 48 || ptr[i] > 57 ) return 0;
    
    // if the first non-space char is a number...
    result =static_cast( ptr[i]) - 48;
    i++;         
    while(ptr[i] >=48 && ptr[i] <=57 )  
    { 
        result = result*10 + (static_cast(ptr[i]) - 48);
     
        if (result < 0) return 2147483647;
        i++;
       
    }
      
   return flag*result;
}

int main()
{
    char a[10] = "  123a4";
    char b[20] = "   abc321 44";
    char c[20] = "   3 44 5";
    char d[20] = "   2147483648 ";
    char e[30] = "   -345 ";
    char f[30] ="   +345  ";
    char g[30] = "    --345  ";

    cout << atoi(a) << endl;
    cout << atoi(b) << endl;
    cout << atoi(c) << endl;
    cout << atoi(d) << endl;
    cout << atoi(e) << endl;
    cout << atoi(f) << endl;
    cout << atoi(g) << endl;
    return 0;
}