Simple atoi()

int my_atoi(const char*  src)
{
    int result = 0;
    int digit = 1;
    const int DIGIT_DIFFERENCE = 10;
    const char ZERO_SYMBOL = '0';
    for(int i = (strlen(str)-1); i >= 0; i--)
    {
        result = result + (digit * int(str[i]-ZERO_SYMBOL));
        digit = digit * DIGIT_DIFFERENCE;
    }
    return result;
}