[Error] ld returned 1 exit st

class stack
{
    protected:
        int *data;
        int pos;
    public:
        stack(int i=3)
        {
            cout<<"gou zhao"<<endl;
            data=new int [i];
            pos=i;
        }
        stack(stack &t)
        {
            cout<<"kao bei"<<endl;
            for(int i=0;i<t.pos;i++)
              {
              data[i]=t.data[i];
              }
            pos=t.pos;
        }
        ~stack()
        {
            cout<<"xi gou"<<endl;
            delete []data;
        }
        void input()
        {
            cout<<"pleacs input"<<endl;
            for(int i=0;i<pos;i++)
                 cin>>data[i];
        }
        friend void output( stack &b);
};
void test(stack a)
{
    cout<<"in the function!"<<endl;
}

void output( stack &b)            //为什么这里不可以用普通变量b调用拷贝？
{
    for(int i=0;i<b.pos;i++)
       cout<<b.data[i]<<endl;
}

int main()
{
    int i;
    cout<<"plecse input number:";
    cin>>i;
    stack x(i);
    x.input();
    stack y;
    y=x;
    output(y);
    return 0;
}
为什么总是报错：
   [Error] ld returned 1 exit status
我的目的是验证拷贝函数的调用情况
        还有一个问题：为什么不可以 stack y=x;初始化式的调用拷贝函数？

还这是只有这一句