ATL / WTL / STL

20-Sep-04 23:22

Hi all,
Someone please explain me why COM doesnot support Multiple Inheritance incase of Interfaces
I mean why I shouldn't have
IDerived:IBase1,IBase2

I did read about it in Don Box book, but I would appreciate if someone explains it in detail (A link to some online resource would work fine too )

Thanks

-------------------------
Have a great day ahead!
Regards,
Sohail Kadiwala

modified 21-Apr-21 21:01pm.

Jörgen Sigvardsson21-Sep-04 12:46

Jörgen Sigvardsson

21-Sep-04 12:46

I can't really answer your question. My guess is that it may be quite a restriction on the ABI if multiple inheritance is allowed. A restriction too high for other compiler vendors to cope with.

But, one has to ask oneself: is multiple interface inheritance really needed? I don't see why it is needed at all. Why not let the class implement the interfaces one by one...?

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Arigato gozaimashita!

User 21559722-Sep-04 1:34

22-Sep-04 1:34

Its all about the binary layout thats results due to multiple inheritance among interfaces...
I wanted some details about it. Hope someone does help regarding this

Thanks Jörgen Sigvardsson

-------------------------
Have a great day ahead!
Regards,
Sohail Kadiwala

modified 21-Apr-21 21:01pm.

ThatsAlok29-Oct-04 21:18

ThatsAlok

29-Oct-04 21:18

Hain Sohail,
according to me,Com is Binary standared that can be used by every language,as there are many language which don't support the Multiple Inheritance,then they can't use this type component and that the failure for COM.

so i think M$ try to find a middle way to stop Multiple Inheitence in COM COmponent.

-----------------------------
"I Think It Will Help"
-----------------------------
Alok Gupta
visit me at http://www.thisisalok.tk

Nathan Holt at EMOM29-Sep-04 5:32

Nathan Holt at EMOM

29-Sep-04 5:32

sohailkadiwala wrote:
Hi all,
Someone please explain me why COM doesnot support Multiple Inheritance incase of Interfaces
I mean why I shouldn't have
IDerived:IBase1,IBase2

I did read about it in Don Box book, but I would appreciate if someone explains it in detail (A link to some online resource would work fine too )

Com interfaces are required to have a certain binary structure. Specificly, an interface referes to a table (simple array) of function pointers. The first three members of this table are always QueryInterface, AddRef, and Release. Additional interface functions come right after these. Therefore, both IBase1 and IBase2 would have members in the fourth slot (unless one is IUnknown). Multiple inheritence of interfaces would require the fourth slot to contain both the IBase1 member and the *Base2 member, which is not possible.

When you use C++ multiple inheritance with classes that have virtual functions, it creates multiple tables of function pointers. This makes it easy to create an object with multiple interfaces, but isn't useful for creating interfaces with multiple base classes.

It seems to me that what I've explained so far would all be in your book, so please feel free to ask for clarification.

Nathan Holt

User 21559730-Sep-04 0:35

30-Sep-04 0:35

Hi Nathan,
Thanks for the detailed explanation. I agree about the Binary Structure that must be standard for all interfaces and thats why COM avoids multiple inheritance among interfaces

Nathan Holt at CCEI wrote:
Multiple inheritence of interfaces would require the fourth slot to contain both the IBase1 member and the *Base2 member, which is not possible

Can you please explain me more on this. I am confused abt the above quote. What I know is there can be multiple inheritance possible among structures. COM interface is nothing but a Structure. So why would it be not possible to have multiple inhertance amongst interface ?

Thanks again

-------------------------
Have a great day ahead!
Regards,
Sohail Kadiwala

modified 21-Apr-21 21:01pm.

Nathan Holt at EMOM30-Sep-04 4:49

Nathan Holt at EMOM

30-Sep-04 4:49

sohailkadiwala wrote:
Can you please explain me more on this. I am confused abt the above quote. What I know is there can be multiple inheritance possible among structures. COM interface is nothing but a Structure. So why would it be not possible to have multiple inhertance amongst interface ?

Sure. Multiple inheritance in C++ can rely on detailed information about the object. If both base classes have virtual functions, then the derived class will have two pointers to two different virtual function tables. (The virtual function tables are the tables I described in the previous message.) I don't know if C++ specifies the order the pointers occur in, and I'm pretty sure it doesn't specify how far apart the pointers are. This would make it difficult to specify a binary standard for multiple inheritance in interfaces.

In practice, I suspect that Microsoft could have defined their own standards for windows C++ compilers that would have made multiple inheritance of interfaces possible, but they didn't. In fact, there are a disturbing number of people who seem to think multiple inheritance was a mistake.

Nathan Holt

User 2155971-Oct-04 4:00

1-Oct-04 4:00

Thanks Nathan.
Very informative. Helped me understand the concept in a much better way.
Thanks Again Smile | :)

-------------------------
Have a great day ahead!
Regards,
Sohail Kadiwala

modified 21-Apr-21 21:01pm.

BSTR to const char[]

AssemblySoft20-Sep-04 7:10

20-Sep-04 7:10

I've created a get / put property inside an activeX control. I know the functionality is working ok because i've tested it with script <param =="" />

What i now need to do is copy the BSTR into a char* or c style array.
I've tried:

char str[MAX_PATH];
CComBSTR m_bstrString;

put:
ATLTRACE(_T("String assigned to: %ls\n"),newVal);
m_bstrFileString = newVal; //assign btr to CComBSTR

//do conversion
BSTR var = m_bstrString.Copy();
strcat(str,(LPCTSTR)var);

It only seems to be copying the first char?

Please help D'Oh! | :doh:

Anonymous20-Sep-04 9:19

Anonymous

20-Sep-04 9:19

I'm sure this will help:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclib/html/_atl_String_Conversion_Macros.asp[^]

SandeepN20-Sep-04 10:21

SandeepN

20-Sep-04 10:21

The following line has undefined variable m_bstrFileString (could be typo)
m_bstrFileString = newVal; //assign btr to CComBSTR

Note : W2A macros has limitation, can not be used in loop. It can convert max 500KB of chars

One more soln would be using _bstr_t class..
For Ex:

char str[MAX_PATH];
_bstr_t str1(newVal);
strcpy(str,(char *)str1);

Sandeep Naik

AssemblySoft20-Sep-04 12:56

20-Sep-04 12:56

thanks i will try that.

Could you please have a look at my thread 'ATL activeX control Initialize', directly below this one.

AssemblySoft20-Sep-04 12:56

20-Sep-04 12:56

thanks i will try that.

Could you please have a look at my thread 'ATL activeX control Initialize', directly below this one.

Michael Dunn20-Sep-04 10:56

Michael Dunn

20-Sep-04 10:56

See The Complete Guide to C++ Strings, Part II - String Wrapper Classes[^]

--Mike--
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AssemblySoft20-Sep-04 12:57

ATL activeX control Initialize

20-Sep-04 12:57

thanks i will try that.

Could you please have a look at my thread 'ATL activeX control Initialize', directly below this one.

AssemblySoft20-Sep-04 7:02