C++!!!对一个数字'312654'如何把各位数相加得解?

#include <iostream>
using namespace std;
void main()
{
int n,sum=0;
cout<<"求各位数字之和\n请输入一个数:";
cin>>n;
while(n>0)
sum+=n%10,n/=10;
cout<<"所求的整数数的各位数之和为"<<sum;
}

#include<iostream.h>

int main()
{
int value=0,flag=1,i=0;
char str[80];
cout<<"请输入一个数"<<endl;
cin>>str;
cout<<"各位数和:"<<endl;
while(flag==1)
{
if(str[i]>='0' && str[i]<='9') value+=str[i++]-'0';
else flag=0;
}
cout<<value<<'\n'<<endl;
return 0;
}

#include <iostream>
using namespace std;
void main()
{
int x=0,y=0,i=1;
cout<<"求各位数相加的和\n"<<"输入一个整数";
cin>>x;
do{
y=y+x%10;
x=x/10;
}while(x);
cout