Qt Create Slots And Signals

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Build complex application behaviours using signals and slots, and override widget event handling with custom events.

Automatic Connections: using Qt signals and slots the easy way One key and distinctive feature of Qt framework is the use of signals and slots to connect widgets and related actions. Is it better practice to create an object of each class in mainwindow (and instantiate it) of ALL the classes that would require signals and slots and then to define all the connections there. In each class, only the signal and slot needs to be defined. This would eliminate the need to create a class (and instantiate it) in each individual class. Qt provide list of widgets that we can use with signal and slot as we used in last blog. We used QWidget as a container in our last example. We can make combination of Qt widgets, that can be used as a single object or widget having its own signal and slots. It is possible by inheriting QWidget class and adding components to it.

As already described, every interaction the user has with a Qt application causes an Event. There are multiple types of event, each representing a difference type of interaction — e.g. mouse or keyboard events.

Events that occur are passed to the event-specific handler on the widget where the interaction occurred. For example, clicking on a widget will cause a QMouseEvent to be sent to the .mousePressEvent event handler on the widget. This handler can interrogate the event to find out information, such as what triggered the event and where specifically it occurred.

Qt connects widgets by means of a nice designed scheme based on the idea that objectS may send signalS of different typeS to a single object instance: This is a screenshot of the example code running. The main application creates dialogs A and B and then connects the signal from A to the signal slot in B. A developer can choose to connect to a signal by creating a function (a 'slot') and calling the connect function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes.

You can intercept events by subclassing and overriding the handler function on the class, as you would for any other function. You can choose to filter, modify, or ignore events, passing them through to the normal handler for the event by calling the parent class function with super().

However, imagine you want to catch an event on 20 different buttons. Subclassing like this now becomes an incredibly tedious way of catching, interpreting and handling these events.

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Thankfully Qt offers a neater approach to receiving notification of things happening in your application: Signals.

Signals

Instead of intercepting raw events, signals allow you to 'listen' for notifications of specific occurrences within your application. While these can be similar to events — a click on a button — they can also be more nuanced — updated text in a box. Data can also be sent alongside a signal - so as well as being notified of the updated text you can also receive it.

The receivers of signals are called Slots in Qt terminology. A number of standard slots are provided on Qt classes to allow you to wire together different parts of your application. However, you can also use any Python function as a slot, and therefore receive the message yourself.

Load up a fresh copy of `MyApp_window.py` and save it under a new name for this section. The code is copied below if you don't have it yet.

Basic signals

First, let's look at the signals available for our QMainWindow. You can find this information in the Qt documentation. Scroll down to the Signals section to see the signals implemented for this class.

Qt 5 Documentation — QMainWindow Signals

As you can see, alongside the two QMainWindow signals, there are 4 signals inherited from QWidget and 2 signals inherited from Object. If you click through to the QWidget signal documentation you can see a .windowTitleChanged signal implemented here. Next we'll demonstrate that signal within our application.

Qt 5 Documentation — Widget Signals

The code below gives a few examples of using the windowTitleChanged signal.

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Try commenting out the different signals and seeing the effect on what the slot prints.

We start by creating a function that will behave as a ‘slot’ for our signals.

Then we use .connect on the .windowTitleChanged signal. We pass the function that we want to be called with the signal data. In this case the signal sends a string, containing the new window title.

If we run that, we see that we receive the notification that the window title has changed.

Events

Next, let’s take a quick look at events. Thanks to signals, for most purposes you can happily avoid using events in Qt, but it’s important to understand how they work for when they are necessary.

As an example, we're going to intercept the .contextMenuEvent on QMainWindow. This event is fired whenever a context menu is about to be shown, and is passed a single value event of type QContextMenuEvent.

To intercept the event, we simply override the object method with our new method of the same name. So in this case we can create a method on our MainWindow subclass with the name contextMenuEvent and it will receive all events of this type.

If you add the above method to your MainWindow class and run your program you will discover that right-clicking in your window now displays the message in the print statement.

Sometimes you may wish to intercept an event, yet still trigger the default (parent) event handler. You can do this by calling the event handler on the parent class using super as normal for Python class methods.

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This allows you to propagate events up the object hierarchy, handling only those parts of an event handler that you wish.

However, in Qt there is another type of event hierarchy, constructed around the UI relationships. Widgets that are added to a layout, within another widget, may opt to pass their events to their UI parent. In complex widgets with multiple sub-elements this can allow for delegation of event handling to the containing widget for certain events.

However, if you have dealt with an event and do not want it to propagate in this way you can flag this by calling .accept() on the event.

Alternatively, if you do want it to propagate calling .ignore() will achieve this.

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Qt creator slots and signals

In this section we've covered signals, slots and events. We've demonstratedsome simple signals, including how to pass less and more data using lambdas.We've created custom signals, and shown how to intercept events, pass onevent handling and use .accept() and .ignore() to hide/show eventsto the UI-parent widget. In the next section we will go on to takea look at two common features of the GUI — toolbars and menus.

This is the sequel of my previous article explaining the implementation details of the signals and slots.In the Part 1, we have seenthe general principle and how it works with the old syntax.In this blog post, we will see the implementation details behind thenew function pointerbased syntax in Qt5.

New Syntax in Qt5

The new syntax looks like this:

Why the new syntax?

I already explained the advantages of the new syntax in adedicated blog entry.To summarize, the new syntax allows compile-time checking of the signals and slots. It also allowsautomatic conversion of the arguments if they do not have the same types.As a bonus, it enables the support for lambda expressions.

New overloads

There was only a few changes required to make that possible.
The main idea is to have new overloads to QObject::connect which take the pointersto functions as arguments instead of char*

There are three new static overloads of QObject::connect: (not actual code)

The first one is the one that is much closer to the old syntax: you connect a signal from the senderto a slot in a receiver object.The two other overloads are connecting a signal to a static function or a functor object withouta receiver.

They are very similar and we will only analyze the first one in this article.

Pointer to Member Functions

Before continuing my explanation, I would like to open a parenthesis totalk a bit about pointers to member functions.

Here is a simple sample code that declares a pointer to member function and calls it.

Pointers to member and pointers to member functions are usually part of the subset of C++ that is not much used and thus lesser known.
The good news is that you still do not really need to know much about them to use Qt and its new syntax. All you need to remember is to put the & before the name of the signal in your connect call. But you will not need to cope with the ::*, .* or ->* cryptic operators.

These cryptic operators allow you to declare a pointer to a member or access it.The type of such pointers includes the return type, the class which owns the member, the types of each argumentand the const-ness of the function.

You cannot really convert pointer to member functions to anything and in particular not tovoid* because they have a different sizeof.
If the function varies slightly in signature, you cannot convert from one to the other.For example, even converting from void (MyClass::*)(int) const tovoid (MyClass::*)(int) is not allowed.(You could do it with reinterpret_cast; but that would be an undefined behaviour if you callthem, according to the standard)

Pointer to member functions are not just like normal function pointers.A normal function pointer is just a normal pointer the address where thecode of that function lies.But pointer to member function need to store more information:member functions can be virtual and there is also an offset to apply to thehidden this in case of multiple inheritance.
sizeof of a pointer to a member function can evenvary depending of the class.This is why we need to take special care when manipulating them.

Type Traits: QtPrivate::FunctionPointer

Let me introduce you to the QtPrivate::FunctionPointer type trait.
A trait is basically a helper class that gives meta data about a given type.Another example of trait in Qt isQTypeInfo.

What we will need to know in order to implement the new syntax is information about a function pointer.

The template<typename T> struct FunctionPointer will give us informationabout T via its member.

  • ArgumentCount: An integer representing the number of arguments of the function.
  • Object: Exists only for pointer to member function. It is a typedef to the class of which the function is a member.
  • Arguments: Represents the list of argument. It is a typedef to a meta-programming list.
  • call(T &function, QObject *receiver, void **args): A static function that will call the function, applying the given parameters.

Qt still supports C++98 compiler which means we unfortunately cannot require support for variadic templates.Therefore we had to specialize our trait function for each number of arguments.We have four kinds of specializationd: normal function pointer, pointer to member function,pointer to const member function and functors.For each kind, we need to specialize for each number of arguments. We support up to six arguments.We also made a specialization using variadic templateso we support arbitrary number of arguments if the compiler supports variadic templates.

The implementation of FunctionPointer lies inqobjectdefs_impl.h.

QObject::connect

The implementation relies on a lot of template code. I am not going to explain all of it.

Here is the code of the first new overload fromqobject.h:

You notice in the function signature that sender and receiverare not just QObject* as the documentation points out. They are pointers totypename FunctionPointer::Object instead.This uses SFINAEto make this overload only enabled for pointers to member functionsbecause the Object only exists in FunctionPointer ifthe type is a pointer to member function.

We then start with a bunch ofQ_STATIC_ASSERT.They should generate sensible compilation error messages when the user made a mistake.If the user did something wrong, it is important that he/she sees an error hereand not in the soup of template code in the _impl.h files.We want to hide the underlying implementation from the user who should not needto care about it.
That means that if you ever you see a confusing error in the implementation details,it should be considered as a bug that should be reported.

We then allocate a QSlotObject that is going to be passed to connectImpl().The QSlotObject is a wrapper around the slot that will help calling it. It alsoknows the type of the signal arguments so it can do the proper type conversion.
We use List_Left to only pass the same number as argument as the slot, which allows connectinga signal with many arguments to a slot with less arguments.

Signals

QObject::connectImpl is the private internal functionthat will perform the connection.It is similar to the original syntax, the difference is that instead of storing amethod index in the QObjectPrivate::Connection structure,we store a pointer to the QSlotObjectBase.

The reason why we pass &slot as a void** is only tobe able to compare it if the type is Qt::UniqueConnection.

We also pass the &signal as a void**.It is a pointer to the member function pointer. (Yes, a pointer to the pointer)

Signal Index

We need to make a relationship between the signal pointer and the signal index.
We use MOC for that. Yes, that means this new syntaxis still using the MOC and that there are no plans to get rid of it :-).

MOC will generate code in qt_static_metacallthat compares the parameter and returns the right index.connectImpl will call the qt_static_metacall function with thepointer to the function pointer.

Qt Create Slots And Signals Play

Once we have the signal index, we can proceed like in the other syntax.

The QSlotObjectBase

QSlotObjectBase is the object passed to connectImplthat represents the slot.

Before showing the real code, this is what QObject::QSlotObjectBasewas in Qt5 alpha:

It is basically an interface that is meant to be re-implemented bytemplate classes implementing the call and comparison of thefunction pointers.

Qt Create Slots And Signals Post

It is re-implemented by one of the QSlotObject, QStaticSlotObject orQFunctorSlotObject template class.

Fake Virtual Table

The problem with that is that each instantiation of those object would need to create a virtual table which contains not only pointer to virtual functionsbut also lot of information we do not need such asRTTI.That would result in lot of superfluous data and relocation in the binaries.

In order to avoid that, QSlotObjectBase was changed not to be a C++ polymorphic class.Virtual functions are emulated by hand.

The m_impl is a (normal) function pointer which performsthe three operations that were previously virtual functions. The 're-implementations'set it to their own implementation in the constructor.

Please do not go in your code and replace all your virtual functions by such ahack because you read here it was good.This is only done in this case because almost every call to connectwould generate a new different type (since the QSlotObject has template parameterswich depend on signature of the signal and the slot).

Qt create slots and signals play

Protected, Public, or Private Signals.

Signals were protected in Qt4 and before. It was a design choice as signals should be emittedby the object when its change its state. They should not be emitted fromoutside the object and calling a signal on another object is almost always a bad idea.

However, with the new syntax, you need to be able take the addressof the signal from the point you make the connection.The compiler would only let you do that if you have access to that signal.Writing &Counter::valueChanged would generate a compiler errorif the signal was not public.

In Qt 5 we had to change signals from protected to public.This is unfortunate since this mean anyone can emit the signals.We found no way around it. We tried a trick with the emit keyword. We tried returning a special value.But nothing worked.I believe that the advantages of the new syntax overcome the problem that signals are now public.

Sometimes it is even desirable to have the signal private. This is the case for example inQAbstractItemModel, where otherwise, developers tend to emit signalfrom the derived class which is not what the API wants.There used to be a pre-processor trick that made signals privatebut it broke the new connection syntax.
A new hack has been introduced.QPrivateSignal is a dummy (empty) struct declared private in the Q_OBJECTmacro. It can be used as the last parameter of the signal. Because it is private, only the objecthas the right to construct it for calling the signal.MOC will ignore the QPrivateSignal last argument while generating signature information.See qabstractitemmodel.h for an example.

More Template Code

The rest of the code is inqobjectdefs_impl.h andqobject_impl.h.It is mostly standard dull template code.

I will not go into much more details in this article,but I will just go over few items that are worth mentioning.

Meta-Programming List

As pointed out earlier, FunctionPointer::Arguments is a listof the arguments. The code needs to operate on that list:iterate over each element, take only a part of it or select a given item.

That is why there isQtPrivate::List that can represent a list of types. Some helpers to operate on it areQtPrivate::List_Select andQtPrivate::List_Left, which give the N-th element in the list and a sub-list containingthe N first elements.

The implementation of List is different for compilers that support variadic templates and compilers that do not.

With variadic templates, it is atemplate<typename... T> struct List;. The list of arguments is just encapsulatedin the template parameters.
For example: the type of a list containing the arguments (int, QString, QObject*) would simply be:

Without variadic template, it is a LISP-style list: template<typename Head, typename Tail > struct List;where Tail can be either another List or void for the end of the list.
The same example as before would be:

ApplyReturnValue Trick

In the function FunctionPointer::call, the args[0] is meant to receive the return value of the slot.If the signal returns a value, it is a pointer to an object of the return type ofthe signal, else, it is 0.If the slot returns a value, we need to copy it in arg[0]. If it returns void, we do nothing.

Qt Create Slots And Signals Online

The problem is that it is not syntaxically correct to use thereturn value of a function that returns void.Should I have duplicated the already huge amount of code duplication: once for the voidreturn type and the other for the non-void?No, thanks to the comma operator.

In C++ you can do something like that:

You could have replaced the comma by a semicolon and everything would have been fine.

Where it becomes interesting is when you call it with something that is not void:

There, the comma will actually call an operator that you even can overload.It is what we do inqobjectdefs_impl.h

ApplyReturnValue is just a wrapper around a void*. Then it can be usedin each helper. This is for example the case of a functor without arguments:

This code is inlined, so it will not cost anything at run-time.

Qt Create Slots And Signals Free

Conclusion

This is it for this blog post. There is still a lot to talk about(I have not even mentioned QueuedConnection or thread safety yet), but I hope you found thisinterresting and that you learned here something that might help you as a programmer.

Update:The part 3 is available.