We are trying to use a .NET form ("Form") from a non-managed C++
class ("Engine").
The Engine class needs to create the form once (e.g. at construction),
then will be displaying the Form and running a modal loop a few
times. At destruction, the form will be closed and disposed of.

So I wanted to store a kind of pointer or reference to the
garbage-collected Form within the Engine class.

How do we achieve that?
The following naive solution is rejected by the compiler:

using MyNetTest::Form; // a .NET form generated using the Designer

class Engine {
public:
  //...

private:
   Form^  dlg;  //ERROR C3265
};

Error message:
error C3265: cannot declare a managed 'dlg' in an unmanaged 'Engine'
may not declare a global or static variable, or a member of a native
 type that refers to objects in the gc heap

There has to be some kind of built-in smart pointer/wrapper
that I can use to make Form a member of Engine, right?
What is the solution?

Thanks,
Ivan