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:
Next, I modified my class, which needed to expose the interface
and provided an implementation in the class.
Then in an unrelated class, I made a call like this
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;