c++问题，为什么发生错误

Question

#include<iostream>
using namespace std;

class Vector
{

float v[4];
float sum;

friend class vectorContainer;

public:
Vector(float ve[]){
  sum=0;
  for(int i=0;i<4;i++)
  {
   v[i]=ve[i];
   sum=sum+v[i];
  }
 
}

Vector(){
for(int i=0;i<4;i++)
         v[i]=0;
}

~Vector(){}

void output();

Vector& operator=(Vector &a);

};

void Vector::output()
{

for(int i=0;i<4;i++)
{
  cout << v[i] << " " ;
 
}
cout << sum << endl ;

}

Vector& Vector::operator=(Vector &a)
{
      for(int i=0;i<4;i++)
   {
         v[i]=a.v[i];
   }
   sum=a.sum;
      return *this;

}

class vectorContainer
{
Vector* _v;
int _size;

public:
vectorContainer(Vector *ve,int size){
  _v=new Vector[size];
  _size=size;
  for(int i=0;i<size;i++)
   _v[i]=ve[i];

}
vectorContainer(){}
~vectorContainer(){}

Vector& operator[](int idx);
Vector* operator->();
void operator()();
Vector& operator()(int i);
float& operator()(int i,int j);
};

Vector& vectorContainer::operator[](int idx)
{
_v=_v+idx;
for(int j=0;j<4;j++)
  cout << _v->v[j] << " " ;
cout << endl;
_v=_v-idx;
return *_v;
}

Vector* vectorContainer::operator->()
{
Vector *big;

big=_v[0];
for(int i=1;i<_size;i++)
{
  if(big.sum<_v[i].sum)
        big=_v[i];
}
   
return big;
}

int main()
{

Vector a0[4];
float k1[]={1,2,3,4};
float k2[]={2,3,4,5};
float k3[]={3,4,5,6};
float k4[]={4,5,6,7};

Vector a1(k1);
Vector a2(k2);
Vector a3(k3);
Vector a4(k4);

a0[0]=a1;
a0[1]=a2;
a0[2]=a3;
a0[3]=a4;

vectorContainer c(a0,4);
c[0];
c[1];
c[2];
c[3];

c->output();

}
在重载—>的时候发生了不知道什么错误，麻烦帮帮我改1下，请详细解释，我已经重载了=号，并且在重载【】时验证了是正确的