Click on the banner to return to the Class Reference home page.
©Copyright 1996 Rogue Wave Software
RWDDEstreambufRWCLIPstreambuf
#include <rw/winstrea.h>
#include <iostream.h> iostream str( new RWDDEstreambuf(CF_TEXT, TRUE, TRUE, TRUE) ) ;
Class RWDDEstreambuf is a specialized streambuf that gets and puts sequences of characters to Microsoft Windows global memory that has been allocated with the GMEM_DDESHARE flag. It can be used to exchange data through the Windows Dynamic Data Exchange (DDE) facility.
The class has two modes of operation: dynamic and static. In dynamic mode, memory is allocated and reallocated on an as-needed basis. If too many characters are inserted into the internal buffer for its present size, then it will be resized and old characters copied over into any new memory as necessary. This is transparent to the user. It is expected that this mode would be used primarily by the DDE server. In static mode, the buffer streambuf is constructed from a specific piece of memory. No reallocations will be done. It is expected that this mode would be used primarily by the DDE client.
In dynamic mode, the RWDDEstreambuf "owns" any allocated memory until the member function str() is called, which "freezes" the buffer and returns an unlocked Windows handle to it. The effect of any further insertions is undefined. Until str() has been called, it is the responsibility of the RWDDEstreambuf destructor to free any allocated memory. After the call to str(), it becomes the user's responsibility.
In static mode, the user always has the responsibility for freeing the memory handle. However, because the constructor locks and dereferences the handle, you should not free the memory until either the destructor or str() has been called, either of which will unlock the handle.
Note that although the user may have the "responsibility" for freeing the memory, whether it is the client or the server that actually does the call to GlobalFree() will depend on the DDE "release" flag.
None
This is an example of how the class might be used by a DDE server.
#include <rw/winstrea.h>
#include <iostream.h>
#include <windows.h>
#include <dde.h>
BOOL
postToDDE(HWND hwndServer, HWND hwndClient) {
RWDDEstreambuf* buf =
new RWDDEstreambuf(CF_TEXT, TRUE, TRUE, TRUE);
ostream ostr(buf);
double d = 12.34;
ostr << "Some text to be exchanged through the DDE.\n";
ostr << "The double you requested is: " << d << endl;
ostr.put(0); // Include the terminating null
// Lock the streambuf, get its handle:
HANDLE hMem = buf->str();
// Get an identifying atom:
ATOM aItem = GlobalAddAtom("YourData");
if(!PostMessage(hwndClient, WM_DDE_DATA, hwndServer,
MAKELONG(hMem, aItem))){
// Whoops! The message post failed, perhaps because
// the client terminated. Now we are responsible
// for deallocating the memory:
if( hMem != NULL )
GlobalFree(hMem);
GlobalDeleteAtom(aItem);
return FALSE;
}
return TRUE;
}
The handle of the DDE server is passed in as parameter hwndServer, the handle of the client as parameter hwndClient. An ostream is created, using an RWDDEstreambuf as its associated streambuf. The results can be used much like any other ostream, such as cout, except that characters will be inserted into Windows global memory, from where they can be transferred through the DDE. Note the parameters used in the constructor. These should be studied below as they have important ramifications on how memory allocations are handled through the DDE. In particular, parameter fRelease, if TRUE, states that the client will be responsible for deallocating the memory when done. The defaults also specify fAckReq TRUE, meaning that the client will acknowledge receiving the message: you must be prepared to receive it.
Some text and a double is inserted into the ostream. Member function str() is then called which unlocks and returns a Windows HANDLE. Once we have called str(), we are responsible for this memory and must either free it when done, or pass on that responsibility to someone else. In this case, it will be passed on to the client.
An atom is then constructed to identify the data. The DDE data, along with its identifying atom, is then posted. If the post fails, then we have been unable to foist our responsbility for the global memory onto someone else and will have to free it (along with the atom) ourselves.
RWDDEstreambuf(WORD cfFormat = CF_TEXT,
BOOL fResponse = TRUE
BOOL fAckReq = TRUE
BOOL fRelease = TRUE);
Constructs an empty RWDDEstreambuf in dynamic mode. The results can be used anywhere any other streambuf can be used. Memory to accomodate new characters will be allocated as needed.
The four parameters are as defined by the Windows Reference, Volume 2 (in particular, see the section DDE Message Directory). Parameter cfFormat specifies the format of the data being inserted into the streambuf. These formats are the same as used by SetClipboardData(). If a specializing virtual streams class such as RWbostream or RWpostream is used to perform the actual character insertions instead of a simple ostream, the format may not be so simple. In this case, the user might want to register his or her own format, using the Windows function RegisterClipboardFormat().
For the meaning of the other three parameters see below, and/or the Windows reference manuals.
RWDDEstreambuf(HANDLE hMem);
Constructs an RWDDEstreambuf in static mode, using the memory block with global handle hMem. The effect of gets and puts beyond the size of this block is unspecified. The format of the DDE transfer, and the specifics of DDE acknowledgments, memory allocations, etc., can be obtained by using the member functions defined below.
~RWDDEstreambuf();
If member function str() has not been called, the destructor unlocks the handle and, if in dynamic mode, also frees it.
Because RWDDEstreambuf inherits from streambuf, any of the latter's member functions can be used. Furthermore, RWDDEstreambuf has been designed to be analogous to streambuf. However, note that the return type of str() is a HANDLE, rather than a char*.
BOOL ackReq() const;
Returns whether this DDE exchange requests an acknowledgement. See the Windows Reference, Volume 2, for more information.
WORD format() const;
Returns the format of this DDE exchange (e.g., CF_TEXT for text exchange, etc.). See the Windows Reference, Volume 2, for more information.
BOOL release() const;
Returns TRUE if the client is responsible for the release of of the memory returned by str(). See the Windows Reference, Volume 2, for more information.
BOOL response() const;
Returns TRUE if this data is in response to a WM_DDE_REQUEST message. Otherwise, it is in response to a WM_DDE_ADVISE message. See the Windows Reference, Volume 2, for more information.
HANDLE str();
Returns an (unlocked) HANDLE to the global memory being used. The RWDDEstreambuf should now be regarded as "frozen": the effect of inserting any more characters is undefined. If the RWDDEstreambuf was constructed in dynamic mode, and nothing has been inserted, then the returned HANDLE may be NULL. If it was constructed in static mode, then the returned handle will be the handle used to construct the RWDDEstreambuf.
