• Each PyQt widget, which is derived from QObject class, is designed to emit ‘ signal ’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘ slot ’. The slot can be any callable Python function.
• A slot is a function called as a response to an emitted signal. For example, a slot findClicked is executed as a response to a button click. All subclasses of QObject or one of its subclasses can contain signals and slots. In practice, two things are required for the signal and slot mechanism to work. All classes that contain signals or slots.

1. Qt Signal Slot Thread
2. Qt Signal Slot Class

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1. Subclassing QDialog

The code inside the Worker's slot would then execute in a separate thread. However, you are free to connect the Worker's slots to any signal, from any object, in any thread. It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections. Signals and Slots Across Threads Qt supports these signal-slot connection types: Auto Connection (default) If the signal is emitted in the thread which the receiving object has affinity then the behavior is the same as the Direct Connection. Otherwise, the behavior is the same as the Queued Connection.' I believe the signal/slot mechanism has found its soul mate in C11 lambda functions. What’s this signal/slot thingy? If you don’t work in Qt you probably don’t care anyway but the fundamental communication mechanism between objects in the Qt framework is defined by signals (events that can be emitted) and slots (handlers for events).

2. Creating Dialogs

• Subclassing QDialog
• Signals and Slots in Depth
• Rapid Dialog Design
• Shape-Changing Dialogs
• Dynamic Dialogs
• Built-in Widget and Dialog Classes

This chapter will teach you how to create dialog boxes using Qt. Dialog boxes present users with options and choices, and allow them to set the options to their preferred values and to make their choices. They are called dialog boxes, or simply 'dialogs', because they provide a means by which users and applications can 'talk to' each other.

Most GUI applications consist of a main window with a menu bar and toolbar, along with dozens of dialogs that complement the main window. It is also possible to create dialog applications that respond directly to the user's choices by performing the appropriate actions (e.g., a calculator application).

We will create our first dialog purely by writing code to show how it is done. Then we will see how to build dialogs using Qt Designer, Qt's visual design tool. Using Qt Designer is a lot faster than hand-coding and makes it easy to test different designs and to change designs later.

Subclassing QDialog

Our first example is a Find dialog written entirely in C++. It is shown in Figure 2.1. We will implement the dialog as a class in its own right. By doing so, we make it an independent, self-contained component, with its own signals and slots.

Figure 2.1 The Find dialog

The source code is spread across two files: finddialog.h and finddialog.cpp. We will start with finddialog.h.

Lines 1 and 2 (and 27) protect the header file against multiple inclusions.

Line 3 includes the definition of QDialog, the base class for dialogs in Qt. QDialog is derived from QWidget.

Lines 4 to 7 are forward declarations of the Qt classes that we will use to implement the dialog. A forward declaration tells the C++ compiler that a class exists, without giving all the detail that a class definition (usually located in a header file of its own) provides. We will say more about this shortly.

Next, we define FindDialog as a subclass of QDialog:

The Q_OBJECT macro at the beginning of the class definition is necessary for all classes that define signals or slots.

The FindDialog constructor is typical of Qt widget classes. The parent parameter specifies the parent widget. The default is a null pointer, meaning that the dialog has no parent.

The signals section declares two signals that the dialog emits when the user clicks the Find button. If the Search backward option is enabled, the dialog emits findPrevious(); otherwise, it emits findNext().

The signals keyword is actually a macro. The C++ preprocessor converts it into standard C++ before the compiler sees it. Qt::CaseSensitivity is an enum type that can take the values Qt::CaseSensitive and Qt::CaseInsensitive.

In the class's private section, we declare two slots. To implement the slots, we will need to access most of the dialog's child widgets, so we keep pointers to them as well. The slots keyword is, like signals, a macro that expands into a construct that the C++ compiler can digest.

For the private variables, we used forward declarations of their classes. This was possible because they are all pointers and we don't access them in the header file, so the compiler doesn't need the full class definitions. We could have included the relevant header files (<QCheckBox>, <QLabel>, etc.), but using forward declarations when it is possible makes compiling somewhat faster.

We will now look at finddialog.cpp, which contains the implementation of the FindDialog class.

First, we include <QtGui>, a header file that contains the definition of Qt's GUI classes. Qt consists of several modules, each of which lives in its own library. The most important modules are QtCore, QtGui, QtNetwork, QtOpenGL, QtScript, QtSql, QtSvg, and QtXml. The <QtGui> header file contains the definition of all the classes that are part of the QtCore and QtGui modules. Including this header saves us the bother of including every class individually.

In finddialog.h, instead of including <QDialog> and using forward declarations for QCheckBox, QLabel, QLineEdit, and QPushButton, we could simply have included <QtGui>. However, it is generally bad style to include such a big header file from another header file, especially in larger applications.

On line 4, we pass on the parent parameter to the base class constructor. Then we create the child widgets. The tr() function calls around the string literals mark them for translation to other languages. The function is declared in QObject and every subclass that contains the Q_OBJECT macro. It's a good habit to surround user-visible strings with tr(), even if you don't have immediate plans for translating your applications to other languages. We cover translating Qt applications in Chapter 18.

In the string literals, we use ampersands ('&') to indicate shortcut keys. For example, line 11 creates a Find button, which the user can activate by pressing Alt+F on platforms that support shortcut keys. Ampersands can also be used to control focus: On line 6 we create a label with a shortcut key (Alt+W), and on line 8 we set the label's buddy to be the line editor. A buddy is a widget that accepts the focus when the label's shortcut key is pressed. So when the user presses Alt+W (the label's shortcut), the focus goes to the line editor (the label's buddy).

On line 12, we make the Find button the dialog's default button by calling setDefault(true). The default button is the button that is pressed when the user hits Enter. On line 13, we disable the Find button. When a widget is disabled, it is usually shown grayed out and will not respond to user interaction.

The private slot enableFindButton(const QString &) is called whenever the text in the line editor changes. The private slot findClicked() is called when the user clicks the Find button. The dialog closes itself when the user clicks Close. The close() slot is inherited from QWidget, and its default behavior is to hide the widget from view (without deleting it). We will look at the code for the enableFindButton() and findClicked() slots later on.

Since QObject is one of FindDialog's ancestors, we can omit the QObject:: prefix in front of the connect() calls.

Next, we lay out the child widgets using layout managers. Layouts can contain both widgets and other layouts. By nesting QHBoxLayouts, QVBoxLayouts, and QGridLayouts in various combinations, it is possible to build very sophisticated dialogs.

For the Find dialog, we use two QHBoxLayouts and two QVBoxLayouts, as shown in Figure 2.2. The outer layout is the main layout; it is installed on the FindDialog on line 35 and is responsible for the dialog's entire area. The other three layouts are sub-layouts. The little 'spring' at the bottom right of Figure 2.2 is a spacer item (or 'stretch'). It uses up the empty space below the Find and Close buttons, ensuring that these buttons occupy the top of their layout.

One subtle aspect of the layout manager classes is that they are not widgets. Instead, they are derived from QLayout, which in turn is derived from QObject. In the figure, widgets are represented by solid outlines and layouts are represented by dashed outlines to highlight the difference between them. In a running application, layouts are invisible.

When the sublayouts are added to the parent layout (lines 25, 33, and 34), the sublayouts are automatically reparented. Then, when the main layout is installed on the dialog (line 35), it becomes a child of the dialog, and all the widgets in the layouts are reparented to become children of the dialog. The resulting parent–child hierarchy is depicted in Figure 2.3.

Figure 2.3 The Find dialog's parent–child relationships

Finally, we set the title to be shown in the dialog's title bar and we set the window to have a fixed height, since there aren't any widgets in the dialog that can meaningfully occupy any extra vertical space. The QWidget::sizeHint() function returns a widget's 'ideal' size.

This completes the review of FindDialog's constructor. Since we used new to create the dialog's widgets and layouts, it would seem that we need to write a destructor that calls delete on each widget and layout we created. But this isn't necessary, since Qt automatically deletes child objects when the parent is destroyed, and the child widgets and layouts are all descendants of the FindDialog.

Now we will look at the dialog's slots:

The findClicked() slot is called when the user clicks the Find button. It emits the findPrevious() or the findNext() signal, depending on the Search backward option. The emit keyword is specific to Qt; like other Qt extensions it is converted into standard C++ by the C++ preprocessor.

The enableFindButton() slot is called whenever the user changes the text in the line editor. It enables the button if there is some text in the editor, and disables it otherwise.

These two slots complete the dialog. We can now create a main.cpp file to test our FindDialog widget:

To compile the program, run qmake as usual. Since the FindDialog class definition contains the Q_OBJECT macro, the makefile generated by qmake will include special rules to run moc, Qt's meta-object compiler. (We cover Qt's meta-object system in the next section.)

For moc to work correctly, we must put the class definition in a header file, separate from the implementation file. The code generated by moc includes this header file and adds some C++ boilerplate code of its own.

Classes that use the Q_OBJECT macro must have moc run on them. This isn't a problem because qmake automatically adds the necessary rules to the makefile. But if you forget to regenerate your makefile using qmake and moc isn't run, the linker will complain that some functions are declared but not implemented. The messages can be fairly obscure. GCC produces error messages like this one:

Visual C++'s output starts like this:

If this ever happens to you, run qmake again to update the makefile, then rebuild the application.

Now run the program. If shortcut keys are shown on your platform, verify that the shortcut keys Alt+W, Alt+C, Alt+B, and Alt+F trigger the correct behavior. Press Tab to navigate through the widgets with the keyboard. The default tab order is the order in which the widgets were created. This can be changed using QWidget::setTabOrder().

Providing a sensible tab order and keyboard shortcuts ensures that users who don't want to (or cannot) use a mouse are able to make full use of the application. Full keyboard control is also appreciated by fast typists.

In Chapter 3, we will use the Find dialog inside a real application, and we will connect the findPrevious() and findNext() signals to some slots.

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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.

Qt Signal Slot Thread

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.

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.

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.

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).

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.

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.

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.

Qt Signal Slot Class

Update:The part 3 is available.