你这是编译错误。从哪里copy过来的吧

#include<iostream>

const int maxsize=100;
template <class DateType>
class Seqlist
{
    public:
        Seqlist(DateType a[],int b);
        Seqlist<int> add(Seqlist<int> a,Seqlist<int> b);
    private:
        DateType date[maxsize];
        int length;
};

template <class DateType>
Seqlist<DateType>::Seqlist(DateType a[],int b)
{
    if(b>maxsize)throw "参数非法";
    else
        length=b;
    for(int i=0;i<length;i++)
        date[i]=a[i];
}

template <class DateType>
Seqlist<int> Seqlist<DateType>::add(Seqlist<int> a,Seqlist<int> b)
{
    int flag=0;int d[3]={0,0,0};int x=3;
    Seqlist<int> c(d,x);
    int i=0;
    while(i<a.length&&i<b.length)
    {
        c.date[i]=(a.date[i]+b.date[i]+flag)%10;
        flag=(a.date[i]+b.date[i]+flag)/10;
        i++;
    }
    while(i<a.length)
    {
        c.date[i]=(a.date[i]+flag)%10;
        flag=(a.date[i]+flag)/10;
        i++;
    }
    while(i<b.length)
    {
        c.date[i]=(b.date[i]+flag)%10;
        flag=(b.date[i]+flag)/10;
        i++;
    }
    if(flag==1)c.date[i]=1;
    if(a.length<b.length)c.length=b.length+flag;
    else c.length=a.length+flag;
    return c;
}
int main()
{
    int a[3]={1,2,3};
    int b[3]={1,2,3};
    int e=3;
    Seqlist<int> a1(a,e),a2(b,e);
    Seqlist<int> c=a1.add(a1,a2);
    return 0;
}

报错的话加个断点，一步步跑一下就知道了，也没多少代码