编译错误求助

#include<bits/stdc++.h>
using namespace std;
struct bigint{
    int len,a[110];
    int &operator[](int i)
    {
        return a[i];
    }
    bigint(int x=0){
        memset(a,0,sizeof(a));
        for(int len=1;x;len++)
            a[len]=x%10,x/=10;
        len--;
    }
    void flatten(int l)
    {
        len=l;
        for(int i=1;i<=len;i++)
            a[i+1]+=a[i]/10,a[i]=a[i]%10;
        while(a[len]) len--;
    }
    void print()
    {
        for(int i=max(len,1);i>=1;i--)
            cout<<a[i];
    }
    bigint operator+(bigint a,bigint b)
    {
        bigint c;
        c.len=max(a.len,b.len);
        for(int i=1;i<=c.len;i++)
            c[i]+=a[i]+b[i];
        c.flatten(c.len+1);
        return c;
    }
    bigint operator*(bigint a,int b)
    {
        bigint c;
        int len=a.len;
        for(int i=1;i<=len;i++)
            c[i]+=a[i]*b;
        c.flatten(2*len);
        return c;
    }
};
int main()
{
    bigint ans(0),fac(0);
    int n;
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        fac=fac*i;
        ans=ans+fac;
    }
    ans.print();
    return 0;
}

注：信息是
[Error] 'bigint bigint::operator+(bigint, bigint)' must take either zero or one argument

[Error] 'bigint bigint::operator*(bigint, int)' must take either zero or one argument