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POUsing a Qt-based DLL in a non-Qt application
 

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  1. POUsing a Qt-based DLL in a non-Qt application
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    <p>Am I doing it right?</p> <p>A client of mine has a group where I'm developing Qt-based client-server stuff with a lot of fun widget stuff and sockets.</p> <p>Another group within the company wants to use a wrapped version of the QTcpSocket-based client data provider classes. (Which does basically what it sounds like, provides data from the server to the client displays)</p> <p>However, that group has a huge application built mostly with MFC, and that is simply not going to change any time soon. The Qt-based DLL is also delay-loading so that it can be deployed without this feature in certain configurations. </p> <p>I've got it working, but it's a little hacky. Here's my solution at the moment:</p> <p>The DLL wrapper class constructor calls QCoreApplication::instance() to see if it's NULL or not. If it's NULL, it assumes it's in a non-Qt app, and creates a QCoreApplication instance of it's own:</p> <pre><code>if (QCoreApplication::instance() == NULL) { int argc = 1; char* argv[] = { "dummy.exe", NULL }; d-&gt;_app = new QCoreApplication(argc, argv); // safe? } else d-&gt;_app = NULL; </code></pre> <p>It then will set up a windows timer to occasionally call processEvents():</p> <pre><code>if (eventTimerInterval &gt; 0) { // STATE: start a timer to occasionally process the Qt events in the event queue SetTimer(NULL, (UINT_PTR)this, eventTimerInterval, CDatabaseLayer_TimerCallback); } </code></pre> <p>The callback simply calls the processEvents() function using the timerID as a pointer to the class instance. The SetTimer() docs say when HWND is NULL it ignores the timerID, so this appears to be perfectly valid.</p> <pre><code>VOID CALLBACK BLAHBLAH_TimerCallback(HWND hwnd, UINT uMsg, UINT_PTR idEvent, DWORD dwTime) { ((BLAHBLAH*)idEvent)-&gt;processEvents(); // basically just calls d-&gt;_app-&gt;processEvents(); } </code></pre> <p>I then destroy the QCoreApplication instance as the very last thing in the destructor.</p> <pre><code>BLAHBLAH::~BLAHBLAH() { .. other stuff QCoreApplication* app = d-&gt;_app; d-&gt;_app = NULL; delete d; if (app != NULL) delete app; } </code></pre> <p>If the hosting application wishes to time the calls to processEvents() itself, it can pass 0 in for eventTimerInterval and call BLAHBLAH::processEvents() itself.</p> <p>Any thoughts on this? Porting that app to Qt is not an option. It's not ours.</p> <p>It appears to work, but there are probably several assumptions being broken here. Can I just construct a QCoreApplication with dummy arguments like that? Is the event queue safe to operate in this manner?</p> <p>I don't want this blowing up in my face later. Thoughts?</p>
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    1. COI am resorting to doing this myself. Qt 3 apparently had support for creating plugin dlls - e.g. npapi type plugins for chrome safari and mozilla based browsers. But that seems to have been dropped from Qt 4.
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    2. COThanks for asking the question! do you know if this is the best approach now in 4.8, or there is a better way. Actually I run a separate QThread and create and exec the QCoreApplication there (global argc and argv). QThread itself does not need QCoreApplication, only some system level messages such as timer need the dispatcher.
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