Abstract class (IDispatch) in a C structure

This is a discussion on Abstract class (IDispatch) in a C structure within the Windows Programming forums, part of the Platform Specific Boards category; I have a C structure (not my code), which compiles without errors on a C compiler Code: typedef struct{ IDispatch ...

  1. #1
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    Abstract class (IDispatch) in a C structure

    I have a C structure (not my code), which compiles without errors on a C compiler
    Code:
    typedef struct{
    	IDispatch		dispatchObj;
    	DWORD			refCount;
    } _IDispatchEx;
    But when compiling in C++, I get this error:
    Code:
    'IDispatch' : cannot instantiate abstract class
    because IDispatch is an abstract class (COM). I don't understand how it can be declared in C, since C doesn't support classes at all, but the problem is that I need to use this structure in my C++ program.

    How should I change it to work in my C++ project?

    p.s. If I declared dispatchObj as a pointer (IDispatch *dispatchObj; ), the structure wouldn't work as intended (or would it?).

    Thanks for any help.

  2. #2
    Captain Crash brewbuck's Avatar
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    All COM objects are intended to be accessed through interface pointers. Declaring an instance of an interface makes no sense, since an interface is not an object but merely a gateway to the functionality of an object.

    I can understand how such a piece of code might get created (by someone confused about COM) but it's not correct.

    It looks like somebody (you?) is trying to create a new interface called IDispatchEx which aggregates an IDispatch interface on some object. If you can give more details about what this is for I might be able to suggest something.

    EDIT: Looking at the "C" structure, this looks like a basic implementation of IDispatch, not an extension. You might look into the various ATL helper classes which let you easily define classes which implement a single interface. Be aware that the way COM is done in C is not at all how it's done in C++, which might be the cause of your confusion.
    Last edited by brewbuck; 12-29-2008 at 04:59 PM.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    Thanks for the reply, the code is from an article on codeproject, it's from a header file for a DLL. I can compile the DLL (it's in C), but I also need that header file for my C++ project.

    The full structure with comments from its author is here:
    Code:
    // Our _IDispatchEx struct. This is just an IDispatch with some
    // extra fields appended to it for our use in storing extra
    // info we need for the purpose of reacting to events that happen
    // to some element on a web page.
    typedef struct {
    	IDispatch		dispatchObj;	// The mandatory IDispatch.
    	DWORD			refCount;		// Our reference count.
    	IHTMLWindow2 *	htmlWindow2;	// Where we store the IHTMLWindow2 so that our IDispatch's Invoke() can get it.
    	HWND			hwnd;			// The window hosting the browser page. Our IDispatch's Invoke() sends messages when an event of interest occurs.
    	short			id;				// Any numeric value of your choosing that you wish to associate with this IDispatch.
    	unsigned short	extraSize;		// Byte size of any extra fields prepended to this struct.
    	IUnknown		*object;		// Some object associated with the web page element this IDispatch is for.
    	void			*userdata;		// An extra pointer.
    } _IDispatchEx;
    I've also attached the full header file from that article, the structure is on line 34.
    Sorry for my weak understanding of COM, I just need this structure to work in my C++ project.
    Attached Files Attached Files

  4. #4
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    Quote Originally Posted by Overlord View Post
    Thanks for the reply, the code is from an article on codeproject, it's from a header file for a DLL. I can compile the DLL (it's in C), but I also need that header file for my C++ project.

    The full structure with comments from its author is here:
    Code:
    // Our _IDispatchEx struct. This is just an IDispatch with some
    // extra fields appended to it for our use in storing extra
    // info we need for the purpose of reacting to events that happen
    // to some element on a web page.
    typedef struct {
    	IDispatch		dispatchObj;	// The mandatory IDispatch.
    	DWORD			refCount;		// Our reference count.
    	IHTMLWindow2 *	htmlWindow2;	// Where we store the IHTMLWindow2 so that our IDispatch's Invoke() can get it.
    	HWND			hwnd;			// The window hosting the browser page. Our IDispatch's Invoke() sends messages when an event of interest occurs.
    	short			id;				// Any numeric value of your choosing that you wish to associate with this IDispatch.
    	unsigned short	extraSize;		// Byte size of any extra fields prepended to this struct.
    	IUnknown		*object;		// Some object associated with the web page element this IDispatch is for.
    	void			*userdata;		// An extra pointer.
    } _IDispatchEx;
    I've also attached the full header file from that article, the structure is on line 34.
    Sorry for my weak understanding of COM, I just need this structure to work in my C++ project.
    Judging from the Codeproject code, a good starting point for conversion to C++ would be:

    Code:
    typedef struct {
    	IDispatch * dispatchObj;	// The mandatory IDispatch.
    	DWORD			refCount;		// Our reference count.
    	IHTMLWindow2 *	htmlWindow2;	// Where we store the IHTMLWindow2 so that our IDispatch's Invoke() can get it.
    	HWND			hwnd;			// The window hosting the browser page. Our IDispatch's Invoke() sends messages when an event of interest occurs.
    	short			id;				// Any numeric value of your choosing that you wish to associate with this IDispatch.
    	unsigned short	extraSize;		// Byte size of any extra fields prepended to this struct.
    	IUnknown		*object;		// Some object associated with the web page element this IDispatch is for.
    	void			*userdata;		// An extra pointer.
    } _IDispatchEx;
    It should be private data in your class declaration. So, give it a try to see if it works.

  5. #5
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    Thank you, it worked flawlessly. I had declared dispatchObj as a pointer previously, but I thought that would not work, since dispatchObj is not the same as *dispatchObj. But it still works, thanks again and happy New Year.

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