Re: Populating CString in Win32 dll interface that accepts LPCTSTR

From:

"Tom Serface" <tom.nospam@camaswood.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Fri, 27 Jun 2008 07:53:03 -0700

Message-ID:

<C06D9ED3-F8D8-4133-A842-CEC0FAE3C5FC@microsoft.com>

Hi Joe,

I'm confused from the code, but it seems to me the call to GetBuffer() and
to ReleaseBuffer() is unneeded altogether. I've only had to call
ReleaseBuffer() if I actually modify the buffer after GetBuffer is called
and I only call GetBuffer() if I want to modify the string directly or the
calling parameter demands a LPTSTR (which I admit is hardly ever). Since
the parameter is a pointer to a const I think it's unlikely they can modify
the buffer supplied anyway.

I liked your repsonse though. I didn't even consider that OP would want to
perhaps fill the string. If that's the case then making the parametere a
LPCTSTR is definitely wrong, but if the DLL is using the string without
modifying it then that would be the better way to do it just to be sure. I
think we need to see some more of the code if possible.

Tom

"Joseph M. Newcomer" <newcomer@flounder.com> wrote in message
news:sar964l2uh4j4s05h072dfn19fujchsco6@4ax.com...

See below...
On Fri, 27 Jun 2008 05:18:33 -0700 (PDT), "divya_rathore_@gmail.com"
<divyarathore@gmail.com> wrote:

I have a Win32 dll interface:

typedef bool (__cdecl *TITLEPTR)(LPCTSTR);

and I want a string to be populated by calling this inside a UNICODE
enabled MFC App (VS2008):

CString str;
if (!(*pfn3)( str.GetBuffer(256)) )

****
This is totally weird syntax. If pfn3 is a pointer to a function, you can
call it with
pfn3(str.GetBuffer(256);
The silly (*pfn3) notation has not been required since K&R C and is
considered obsolete,
used only for backward compatibility with old programs written before
ANSI/ISO C.
Therefore, there is no reason to use this obsolete and clumsy notation.

Note that in any sensible interface, you would pass in a pointer to the
string AND A
LENGTH; how is it that you know for certain that 256 is the correct amount
of space to
allocate (and if you "it's a file name" you're in trouble, because file
names by default
are longer than 256; take a look at MAX_PATH)

{
MessageBox(_T("Some Problemo!"));
}

..All I am getting in str is jumbled characters (ANSI?).

****
I see no ReleaseBuffer here. It won't work if you don't call
ReleaseBuffer
****

How can I get the string str populated properly? I am clearly missing
some Unicode concepts here.

****
If your app is Unicode, and your DLL returns 8-bit characters, you would
declare
CStringA str;
which gives you an 8-bit string, but then you wouldn't be using LPCTSTR as
the prototype
of the function, if it returned 8-bit strings; it would LPSTR. LPCTSTR
says that it will
take 8-bit strings in an 8-bit app, and Unicode strings in a Unicode app,
AND THAT THIS
PARAMETER REPRESENTS CONSTANT DATA AND WILL THEREFORE NOT BE MODIFIED. If
the purpose is
to fill a buffer, it would have to be LPTSTR (for ANSI/Unicode), LPSTR
(for ANSI) or
LPWSTR (for Unicode). Since it is a DLL, you have to declare data of the
correct type
that matches the prototype of the function. Because it is a DLL, you have
to use the
header file it was compiled with to determine the correct typedef for your
function. You
can't just invent one out of thin air and expect it to work.
joe
****

warm regards,
Divya Rathore

Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm