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;}