C++实现大整数乘法

#include <iostream>
#include <vector>
#include <cstring>
#include <cstdio>
using namespace std;
typedef long long LL;
struct BigInteger{
  static const int BASE = 100000000;
  static const int WIDTH = 8;
  vector<int> s;
 
  BigInteger operator = (const string& str){
    s.clear();
    int x, len=(str.length()-1)/WIDTH+1;
    for(int i=0;i<len;i++){
      int r=str.length()-i*WIDTH;
      int l=max(0,r-WIDTH);
      sscanf(str.substr(l,r-l).c_str(),"%d",&x);
      s.push_back(x);
    }
    return *this;
  }
 
  BigInteger operator * (const BigInteger& b){
    BigInteger c;
    int lena=this->s.size(),lenb=b.s.size(),lenc=lena+lenb-1;
    LL *buf =new LL[lenc+1];
    for(int i=0;i<lenc+1;i++)buf[i]=0;
    for(int i=0;i<lena;i++)
      for(int j=0;j<lenb;j++){
        buf[i+j]+=(this->s[i])*((LL)b.s[j]);
        buf[i+j+1]+=buf[i+j]/BASE;
        buf[i+j]=buf[i+j]%BASE;
      }
    for(int i=0;i<lenc;i++)c.s.push_back(buf[i]);
    if(buf[lenc])c.s.push_back(buf[lenc]);
    return c;
  }
 
  BigInteger operator * (const int& x){
    char c[128];
    sprintf(c,"%d",x);
    string str(c);
    BigInteger res;
    res=str;
    return *this*res;
  }
};
 
ostream& operator<<(ostream& out,const BigInteger& b){
  int len=b.s.size();
  out<<b.s[len-1];
  for(int i=len-2;i>=0;i--){
    int buf=b.s[i],h=8;
    while(buf>0){buf/=10;h--;}
    for(int j=0;j<h;j++)out<<0;
    if(b.s[i])out<<b.s[i];
  }
  return out;
}
 
int main()
{
  int n;BigInteger b;
  b="1000000000000";
  cout<< b<<endl;
  cout<< (b*b)*4*b*b <<endl;
}