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    <p>I think I should add the following.</p> <p>There is <a href="https://stackoverflow.com/questions/2008033/how-does-qt-implement-signals-and-slots">another linked question</a> -- and there is <a href="http://www.ntcore.com/files/qtrev.htm" rel="nofollow noreferrer">a very good article</a> that can be considered as a quite detailed expansion to it's <a href="https://stackoverflow.com/questions/2008033/how-does-qt-implement-signals-and-slots/2008542#2008542">answer</a>; <a href="http://www.codeproject.com/Articles/31330/Qt-Internals-Reversing" rel="nofollow noreferrer">here is this article again</a>, with improved (though still not perfect) code syntax highlighting.</p> <p>Here's my short retelling of it, that may be prone to mistakes ) </p> <p>Basically when we insert the <code>Q_OBJECT</code> macro in our class definition, the preprocessor expands it to a static <code>QMetaObject</code> instance declaration, one that would be shared by all instances of the same class:</p> <pre><code>class ClassName : public QObject // our class definition { static const QMetaObject staticMetaObject; // &lt;--= Q_OBJECT results to this // ... signal and slots definitions, other stuff ... } </code></pre> <p>This instance, in turn, on initialization will store the <em>signatures</em> (<code>"methodname(argtype1,argtype2)"</code>) of the signals and the slots, what will allow to implement the <code>indexOfMethod()</code> call, which returns, well, method's index by it's signature string : </p> <pre><code>struct Q_CORE_EXPORT QMetaObject { // ... skip ... int indexOfMethod(const char *method) const; // ... skip ... static void activate(QObject *sender, int signal_index, void **argv); // ... skip ... struct { // private data const QMetaObject *superdata; // links to the parent class, I guess const char *stringdata; // basically, "string1\0string2\0..." that contains signatures and other names const uint *data; // the indices for the strings in stringdata and other stuff (e.g. flags) // skip } d; }; </code></pre> <p>Now when the <code>moc</code> creates the <code>moc_headername.cpp</code> file for the Qt class header <code>headername.h</code>, it puts there the signature strings and other data that are necessary for correct initialization of the <code>d</code> structure, and then writes the initialization code for the <code>staticMetaObject</code> singleton using this data. </p> <p>Another important thing it does is generation of the code for the object's <code>qt_metacall()</code> method, that takes an object's method id and an array of argument pointers and calls the method via a long <code>switch</code> like this: </p> <pre><code>int ClassName::qt_metacall(..., int _id, void **_args) { // ... skip ... switch (_id) { case 0: signalOrSlotMethod1(_args[1], _args[2]); break; // for a method with two args case 1: signalOrSlotMethod2(_args[1]); break; // for a method with a single argument // ... etc ... } // ... skip ... } </code></pre> <p>Last, for every signal <code>moc</code> generates an implementation, which contains a <code>QMetaObject::activate()</code> call :</p> <pre><code>void ClassName::signalName(argtype1 arg1, argtype2 arg2, /* ... */) { void *_args[] = { 0, // this entry stands for the return value &amp;arg1, // actually, there's a (void*) type conversion &amp;arg2, // in the C++ style // ... }; QMetaObject::activate( this, &amp;staticMetaObject, 0, /* this is the signal index in the qt_metacall() map, I suppose */ _args ); } </code></pre> <p>Finally, the <code>connect()</code> call translates the string method signatures to their integer ids (the ones used by <code>qt_metacall()</code>) and maintains a list of signal-to-slot connections; when the signal is emitted, the <code>activate()</code> code goes through this list and calls the appropriate object "slots" via their <code>qt_metacall()</code> method. </p> <p>To sum up, the static <code>QMetaObject</code> instance stores the "meta-information" (method signature strings etc), a generated <code>qt_metacall()</code> method provides "a method table" that allows any signal/slot to be called by an index, signal implementations generated by <code>moc</code> use these indexes via <code>activate()</code>, and finally <code>connect()</code> does the job of maintaining a list of signal-to-slot index maps.</p> <p>*Note: there's a complication of this scheme used for the case when we want to deliver signals between different threads ( I suspect that one has to look at the <code>blocking_activate()</code> code ), but I hope the general idea remains the same )</p> <p>This is my very rough understanding of the linked article, which easily may be wrong, so I do recommend to go and read it directly )</p> <p>PS. As I'd like to improve my understanding of Qt implementation -- please let me know of any inconsistencies in my retelling !</p> <hr> <p>Since my other (earlier) answer was deleted by some zealous editor, I will append the text here ( I am missing few details that were <em>not</em> incorporated in Pavel Shved's post, and I doubt the person who deleted the answer cared. ) </p> <p>@Pavel Shved: </p> <blockquote> <p>I am pretty sure that somewhere in Qt headers there exists a line:</p> <p><code>#define emit</code></p> </blockquote> <p>Just to confirm: found it in old Qt code by Google Code Search. It is quite likely that it is still there ) ; the found location path was : </p> <p><a href="ftp://ftp.slackware-brasil.com.br" rel="nofollow noreferrer">ftp://ftp.slackware-brasil.com.br</a>› slackware-7.1› contrib› kde-1.90› qt-2.1.1.tgz› usr› lib› qt-2.1.1› src› kernel› <strong>qobjectdefs.h</strong></p> <hr> <p>Another complementory link: <a href="http://lists.trolltech.com/qt-interest/2007-05/thread00691-0.html" rel="nofollow noreferrer">http://lists.trolltech.com/qt-interest/2007-05/thread00691-0.html</a> -- see the answer by Andreas Pakulat </p> <hr> <p>And here is another piece of the answer : <a href="https://stackoverflow.com/questions/1368593/qt-question-how-do-signals-and-slots-work/1371958#1371958">Qt question: How do signals and slots work?</a></p>
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