Where’d my interface go?

One of the joys of modern development is that you get to work in pretty much any language there is.

I’ve been doing a lot of C# development lately (Phone, WinRT and WPF desktop), but occasionally have to drop back to some C++/MFC software.

Yesterday, I had to refactor some code, that made some glaring assumptions about it’s use (yes, I used reinterpret_cast to convert a CWnd derived object to a class derived from CPropertyPage so that I could call one of it’s public members).

So here’s what I did:

First, I declared an interface that represented exactly what I wanted to do:

interface IDoSomethingAwesome
{
  public:
    virtual void DoSomethingAwesome()=0;
};

Next, I modified my class, which needed to expose the interface

public class MyAwsomePropertyPage : 
  public CPropertyPage, IDoSomethingAwesome
{
  /// ... etc.
  virtual void DoSomethingAwesome() override;
}

and provided an implementation in the class.

Then in an unrelated class, I made a call like this

CChildOfAwsome::SomeEventHander()
{
   IDoSomethingAwesome *pInt = dynamic_cast<IDoSomethingAwesome>(GetParent());
   if (pInt != NULL)
   {
       pInt->DoSomethingAwesome();
   }
}

Only one problem:  pInt was always NULL!  That was odd, because looking at the pointer returned by GetParent() gave me an object that the debugger knew was of type MyAwesomePropertyPage, yet I couldn’t for the life of me figure out why this wasn’t working.

I started writing this up as a question on StackOverflow, but while doing so, ended up talking to the duck and discovered the error in my ways.

My C# had blinded me to the joys of C++ inheritance.

I had inadvertently declared my IDoSomethingAwesome interface not public.  As a result, asking for the interface dutifully returns null, as private inheritance is an implementation detail, and not visible in the inheritance tree.

So, the correct declaration is

public class MyAwsomePropertyPage : 
  public CPropertyPage, public IDoSomethingAwesome
{
  /// ... etc.
  virtual void DoSomethingAwesome() override;
}