Re: LPCTSTR and CString

From:
=?Utf-8?B?bWlkbzE5NzE=?= <mido1971@discussions.microsoft.com>
Newsgroups:
microsoft.public.vc.mfc
Date:
Wed, 25 Jun 2008 02:34:01 -0700
Message-ID:
<76B546F9-A404-46A5-A291-5CD3732D9B77@microsoft.com>
thanks its work

"Giovanni Dicanio" wrote:

"mido1971" <mido1971@discussions.microsoft.com> ha scritto nel messaggio
news:32D88001-5DA4-4CB4-A97A-BF26F7AE4B36@microsoft.com...

extern "C" AFX_EXT_API LPCTSTR WINAPI GetString()
{
        LPCTSTR lpcstr ;
        CString str = _T("xxx");
        lpcstr =str;

        return lpcstr ;
}


The above code is wrong.

In fact, CString 'str' instance is built on the stack (it is a local
automatic variable). So, when function GetString() terminates, this variable
'str' (which is local to function body) is *destroyed*.
So, its content (the string) is lost.

What is safe to do is to return a CString instance:

  CString GetString()
  {
      CString str = ...
      ....

      return str;
  }

another thing why i cant use CString in post of LPCTSTR if i use it i got
this warning
warning C4190: '<Unknown>' has C-linkage specified, but returns UDT
'CString' which is incompatible with C


The warning is clear: you say that your function as a C interface, but you
are using a C++ class (CString) in that interface, and so you are
introducing a C++ element (CString class) into a pure C interface, and that
is not possible.

If you really want a C interface, you should consider passing the buffer
from the caller (i.e. the caller allocates a string buffer, and the called
function fills it):

<code>

extern "C" AFX_EXT_API void WINAPI GetString( TCHAR * destBuffer, size_t
destSizeInBytes )
{
   // Check destination buffer
   ASSERT( destBuffer != NULL );
   ASSERT( destSizeInBytes != 0 );

   CString str;
   ....
   work with your str instance ...
   ...

   //
   // Return string to the caller,
   // copying the string into caller's buffer.
   //
   // Use a safe string function to avoid buffer overruns.
   //
   StringCbCopy(
    destBuffer, // destination buffer
    destSizeInBytes, // its size in bytes
    str // source string
   );

}

</code>

You can use the above code like this:

  // Get string into local buffer
  TCHAR theString[ 300 ];
  GetString( theString, sizeof(theString) );

HTH,
Giovanni

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