about "auto_ptr"

[greghua]

class A
{
public:
	A(int a,int b):a1(a),a2(b){}
	void output(){cout<<"a1="<<a1<<";a2="<<a2<<endl;}
private:
	int a1;
	int a2;
};

template<class T>
class auto_ptr {
public:
  auto_ptr(T *p = 0): ptr(p) {}       
  ~auto_ptr() {  delete ptr;}   
  T* operator ->(){return ptr;}
private:
  T *ptr;                             
};


class B
{
public:
	B(int a,int b):ca(new A(a,b)){}
	void output() {ca->output();}
private:
	const auto_ptr<A> ca;
};

void main()
{
   B b(3,4);
   b.output();
}

In B, its member data is pointer which point to a class A. with the use of auto_ptr, the point will be delete automatically after object b goes out of range. my compiler doesn't support auto_ptr in standard libary, so I write it by myself.

my quest is, the program can not be compiled, but if I get of the "const" before auto_ptr<A>, it can be compiled and run right. why?