Wrapping a PropertySheet; how to handle callbacks?

Question

I'm writing an (unmanaged) C++ class to wrap the Windows PropertySheet. Essentially, something like this:

class PropSheet {
    PROPSHEETHEADER d_header;
    public:
        PropSheet(/* parameters */);
        INT_PTR show();
    private:
        static int CALLBACK *propSheetProc(HWND hwnd, UINT msg, LPARAM lParam);
};

The constructor just initializes the d_header member:

PropSheet::PropSheet(/* parameters */) {
    d_header.dwSize = sizeof(PROPSHEETHEADER);
    d_header.dwFlags = PSH_USECALLBACK;
    // ...
    d_header.pfnCallback = &propSheetProc;
    // ...
}

After which I can show it, modally, with:

INT_PTR PropSheet::show() {
    return PropertySheet(&d_header);
}

Now the problem is, because the callback is static, that it cannot access the wrapper class. If this were a normal window, with a WindowProc instead of a PropSheetProc, I could attach some extra data to the window using cbWndExtra in WNDCLASS, in which I could store a pointer back to the wrapper, like in this article. But property sheets do not offer this functionality.

Furthermore, because the property sheet is shown modally, I can execute no code between the creation and destruction of the actual window, except when that code is executed through the callback or one of the sheets's window procedures.

The best solution I've come up with so far is to, right before showing the property sheet, store a pointer to the wrapper class inside a global variable. But this assumes that I'll only be showing one property sheet at a time, and is quite ugly anyway.

Does anyone have a better idea how to work around this?

Answer 1

As you are showing the property sheet modally, you should be able to use the parent window (i.e. its handle) of the property sheet to map to an instance, using ::GetParent() on the hwndDlg parameter of PropSheetProc().

Answer 2

Awesome, yet another Win32 API that uses callbacks without a user-defined context parameter. It is not the only one, alas. e.g. CreateWindow is bad (it gives you user-defined context, but that context isn't available for the first few window messages), SetWindowsHookEx is even worse (no context at all).

The only "solution" that is general-purpose and effective is to emit a small piece of executable code with a 'this' pointer hardcoded. Something like this: http://episteme.arstechnica.com/eve/forums/a/tpc/f/6330927813/m/848000817831?r=848000817831#848000817831

It's horrible.

Answer 3

The PROPSHEETPAGE structure has an lParam field available for callbacks. In your PROPSHEETHEADER, you can include the PSH_PROPSHEETPAGE flag to pass an array of PROPSHEETPAGE items describing your pages, or omit the flag to pass an array of preallocated HPROPSHEETPAGE handles instead (which means using CreatePropertySheetPage(), and thus using PROPSHEETPAGE anyway).

Answer 4

You've already admitted "I can execute no code between the creation and destruction of the actual window". It seems that a global variable wouldn't be a terrible hack.

Answer 5

I've found another option: using SetProp to add a property that stores the pointer to the wrapper. Only requires the global variable once, to be able call SetProp from the property sheet callback.