I swear, you guys are making this way too hard.
Code:
int main()
{
char *strBignum = "790278573\0";
int i,sum=0;
for(i=0; i<strlen(strBignum); i++) {
sum += (int)(strBignum[i]-'0');
}
cout << "The sum is: " << sum <<endl;
}
The only trick is the ascii math. Just subtract the ascii value of '0' from the char, and cast it to an int.