c++ 输入十进制数 求输出各个为的数的和

这里取了一下巧！没有奖励，那就只给代码了：
#include <iostream>
#include <string>

using namespace std;

int main(int argc, char* argv[])
{
char buffer[200];
string exp;
int sum =0;
cout << "Please input tens digit:";
cin.getline(buffer,200);
exp = buffer;
//to deal with error!
for(string::iterator it = exp.begin(); it != exp.end(); ){
if(*it == ' '){
exp.erase(it);
}
else if(*it < '0'|| *it >'9'){
cout <<"error!"<<endl;
return 0;
}
else it++;

}
//to sum
for(string::size_type i=0; i<exp.size(); i++){
sum += exp[i] - '0';
}

cout<<"The sum of bits is:" << sum <<endl;
return 0;
}

#include <iostream>
using namespace std;
void main()
{
int n,i,sum