C / C++ / MFC

3-Apr-08 4:47

Hello everyone,

In the book ATL Internals, it is mentioned, the properties of CComPtr is,

--------------------
Release the encapsulated interface pointer when the class destructor executes;
Automatically releases its interface pointer during exception handling, ...
--------------------

They both means invoking the Relase method (inherited from IUnknown) of the interface?

thanks in advance,
George

Re: CComPtr

led mike3-Apr-08 5:14

led mike

3-Apr-08 5:14

Yes

led mike

Re: CComPtr

George_George3-Apr-08 21:16

3-Apr-08 21:16

Thanks led mike,

Question answered.

regards,
George

A2WBSTR

George_George3-Apr-08 4:40

3-Apr-08 4:40

Hello everyone,

1.

BSTR is always wide character buffer. I do not know why the name of the macro A2WBSTR is so special and add a needless 'W'?

For others dealing with BSTR, since it is already wide character buffer, 'W' is no need. Like W2BSTR, and A2WSTR.

2.

What is differences between A2WBSTR and A2BSTR? Can not find answer from search.

thanks in advance,
George

James R. Twine3-Apr-08 4:54

3-Apr-08 4:54

   Yes, AFAIK, BSTRs were (and are) always wide-character strings.

   Perhaps the reason they did this was to specify the encoding of the characters in the BSTR? Likely not, but just a guess.

   Of course, you could always just look at how those macros/functions are implemented to try and find differences! Smile | :)

Peace!

-=- James
Please rate this message - let me know if I helped or not!<hr></hr>If you think it costs a lot to do it right, just wait until you find out how much it costs to do it wrong!
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See DeleteFXPFiles

George_George3-Apr-08 21:29

3-Apr-08 21:29

Thanks James,

I have found out and write my conclusions here, any comments?

http://www.codeproject.com/script/Forums/View.aspx?fid=1647&msg=2493091[^]

regards,
George

CPallini3-Apr-08 5:47

3-Apr-08 5:47

On my system, provided _ATL_EX_CONVERSION_MACROS_ONLY symbol is not defined, they are the same

#if defined(_UNICODE)
// in these cases the default (TCHAR) is the same as OLECHAR
	inline BSTR T2BSTR_EX(__in_opt LPCTSTR lp) {return ::SysAllocString(lp);}
	inline BSTR A2BSTR_EX(__in_opt LPCSTR lp) {return A2WBSTR(lp);}
	inline BSTR W2BSTR_EX(__in_opt LPCWSTR lp) {return ::SysAllocString(lp);}

#ifndef _ATL_EX_CONVERSION_MACROS_ONLY

	inline BSTR T2BSTR(__in_opt LPCTSTR lp) {return ::SysAllocString(lp);}
	inline BSTR A2BSTR(__in_opt LPCSTR lp) {return A2WBSTR(lp);}
	inline BSTR W2BSTR(__in_opt LPCWSTR lp) {return ::SysAllocString(lp);}
	
#endif	// _ATL_EX_CONVERSION_MACROS_ONLY

#else // !defined(_UNICODE)
	inline BSTR T2BSTR_EX(__in_opt LPCTSTR lp) {return A2WBSTR(lp);}
	inline BSTR A2BSTR_EX(__in_opt LPCSTR lp) {return A2WBSTR(lp);}
	inline BSTR W2BSTR_EX(__in_opt LPCWSTR lp) {return ::SysAllocString(lp);}
	
#ifndef _ATL_EX_CONVERSION_MACROS_ONLY

	inline BSTR T2BSTR(__in_opt LPCTSTR lp) {return A2WBSTR(lp);}
	inline BSTR A2BSTR(__in_opt LPCSTR lp) {return A2WBSTR(lp);}
	inline BSTR W2BSTR(__in_opt LPCWSTR lp) {return ::SysAllocString(lp);}

#endif	// _ATL_EX_CONVERSION_MACROS_ONLY

#endif // defined(_UNICODE)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

George_George3-Apr-08 21:27

3-Apr-08 21:27

Hi CPallini,

After checking implementation in my system (MSVC 2008), my conclusion,

A2BSTR is for null terminated string input, and A2WBSTR can be non-null terminated and assign an additional length field.

Correct?

// atlconv.h

_Check_return_ inline BSTR A2WBSTR(_In_opt_ LPCSTR lp, int nLen = -1)
{
	if (lp == NULL || nLen == 0)
		return NULL;
	USES_CONVERSION_EX;
	BSTR str = NULL;
#pragma warning(push)
#pragma warning(disable: 6385)
	int nConvertedLen = MultiByteToWideChar(_acp_ex, 0, lp,
		nLen, NULL, NULL);
#pragma warning(pop)
	int nAllocLen = nConvertedLen;
	if (nLen == -1)
		nAllocLen -= 1;  // Don't allocate terminating '\0'
	str = ::SysAllocStringLen(NULL, nAllocLen);

	if (str != NULL)
	{
		int nResult;
		nResult = MultiByteToWideChar(_acp_ex, 0, lp, nLen, str, nConvertedLen);
		ATLASSERT(nResult == nConvertedLen);
		if(nResult != nConvertedLen)
		{
			SysFreeString(str);
			return NULL;
		}

	}
	return str;
}

regards,
George

CPallini3-Apr-08 22:38

3-Apr-08 22:38

I don't know, because you did not post the A2BSTR definition (in your MSSVC 2008).
Smile | :)

George_George3-Apr-08 23:04

3-Apr-08 23:04

Here it is, CPallini.

Do you think my previous analysis is correct? Smile | :)

inline BSTR A2BSTR(_In_opt_ LPCSTR lp) {return A2WBSTR(lp);}

regards,
George

CPallini3-Apr-08 23:15

3-Apr-08 23:15

You analysis is wrong.
A2BSTR do no more and no less then calling A2WBSTR, i.e. their effect is identical (there is even no difference in involved stack levels, since A2BSTR it is declared inline).

BTW all BSTR must have a length field (this is the reason why you typically call SysAllocString to alloc a BSTR rather than doing it directly).
Smile | :)

George_George3-Apr-08 23:18

3-Apr-08 23:18

Thanks CPallini,

CPallini wrote:
A2BSTR do no more and no less then calling A2WBSTR, i.e. their effect is identical

I do not agree with you. I think for A2WBSTR you can specify the 2nd argument, which is the length of the ANSI string input, but for A2BSTR, you can not, you can only use the implicit default parameter -1.

Agree? Any comments?

regards,
George

CPallini3-Apr-08 23:29

3-Apr-08 23:29

You're right (and I'm blind Blush | :O

).
A2WBSTR seems more flexible: you may convert only a chunk of the original ANSI string.
Smile | :)

George_George3-Apr-08 23:32

esoteric use of the "new" keyword

3-Apr-08 23:32

Thanks CPallini,

Question answered.

regards,
George

Dave Calkins3-Apr-08 4:12

Dave Calkins

3-Apr-08 4:12

I came across this use of the "new" keyword in a library recently. I had to look it up to see what was going on. Apparently its valid, I've just never seen it before in any code examples, books, articles, etc.

If you write a custom new operator, you can write it to take an extra argument. When callers use the new keyword they supply an additional argument which gets passed to the new operator.

So you can call

MyClass* instance = new 128 MyClass;

This creates an instance of MyClass as expected but when invoking the custom new operator passes along the argument 128.

Anyway, I'm just curious if anyone else has seen or used this. It caught me off guard.

James R. Twine3-Apr-08 4:51

3-Apr-08 4:51

   Have not seen it done like that before, but I could see it being used for things like allocating additional memory at the end of the object. Remember how variable-length structures like DDEDATA, PDH_COUNTER_INFO and some device driver ones worked?

   Also for custom heap management - I want this object on this heap and that object created on on that heap, or created in a pre-reserved (shared?) block of memory.

   Peace!

David Crow3-Apr-08 4:56

David Crow

3-Apr-08 4:56

Dave Calkins wrote:
Anyway, I'm just curious if anyone else has seen or used this. It caught me off guard.

Placement new, perhaps?

"Love people and use things, not love things and use people." - Unknown

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

James R. Twine3-Apr-08 5:05

3-Apr-08 5:05

Placement new required the use of parenthesis:

new (value) Type

Although now that I think about it, you may be correct. I think that placement new is the only way to pass parameters to new...?

Peace!

Cedric Moonen3-Apr-08 5:10

Cedric Moonen

3-Apr-08 5:10

James R. Twine wrote:
I think that placement new is the only way to pass parameters to new...?

AFAIK, you can override the new operator. I think that's what is done for the MFC: the file and line are passed to the new operator in order to track memory leaks easier.

Cédric Moonen
Software developer

Charting control [v1.3]

James R. Twine3-Apr-08 5:41

3-Apr-08 5:41

Yep - yer right...

Mark Salsbery3-Apr-08 6:10

Mark Salsbery

3-Apr-08 6:10

Cedric Moonen wrote:
AFAIK, you can override the new operator. I think that's what is done for the MFC

GDI+ does it too Smile | :)

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]