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• Qt5 Signal Slot Overload
• Qt5 Signals Slots Emit
• Qt Signal Slot Dll

This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.

• Differences between String-Based and Functor-Based Connections (Official documentation)
• Introduction (Woboq blog)
• Implementation Details (Woboq blog)

Note: This is in addition to the old string-based syntax which remains valid.

Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget) connect (sender, SIGNAL (valueChanged (QString, QString)), receiver, SLOT (updateValue (QString))); New: connecting to QObject member. Signals are protected in Qt4 but are public in Qt5, thus the contradictory information. Slots are functions and public/protected/private is honored when calling them as such, when connecting to a signal, the metaobject system ignores it though. As signals is defined as public:, prepending them with e.g.

Qt5插件使用信号槽机制通信 / Qt 5 plugin with signal-slots. Contribute to 3dBPoint/Qt5PluginSignalSlots development by creating an account on GitHub. PyQt5 signals and slots Graphical applications (GUI) are event-driven, unlike console or terminal applications. A users action like clicks a button or selecting an item in a list is called an event. If an event takes place, each PyQt5 widget can emit a signal.

Signals and Slots in Qt5 Qt5 alpha has been released. One of the features which I have been working on is a new syntax for signals and slot. This blog entry will present it.

• 1Connecting in Qt 5
• 2Disconnecting in Qt 5
• 4Error reporting
• 5Open questions

Connecting in Qt 5

There are several ways to connect a signal in Qt 5.

Old syntax

Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget)

New: connecting to QObject member

Here's Qt 5's new way to connect two QObjects and pass non-string objects:

Pros

• Compile time check of the existence of the signals and slot, of the types, or if the Q_OBJECT is missing.
• Argument can be by typedefs or with different namespace specifier, and it works.
• Possibility to automatically cast the types if there is implicit conversion (e.g. from QString to QVariant)
• It is possible to connect to any member function of QObject, not only slots.

Cons

• More complicated syntax? (you need to specify the type of your object)
• Very complicated syntax in cases of overloads? (see below)
• Default arguments in slot is not supported anymore.

New: connecting to simple function

The new syntax can even connect to functions, not just QObjects:

Pros

• Can be used with std::bind:

• Can be used with C++11 lambda expressions:

Cons

• There is no automatic disconnection when the 'receiver' is destroyed because it's a functor with no QObject. However, since 5.2 there is an overload which adds a 'context object'. When that object is destroyed, the connection is broken (the context is also used for the thread affinity: the lambda will be called in the thread of the event loop of the object used as context).

Disconnecting in Qt 5

As you might expect, there are some changes in how connections can be terminated in Qt 5, too.

Old way

You can disconnect in the old way (using SIGNAL, SLOT) but only if

• You connected using the old way, or
• If you want to disconnect all the slots from a given signal using wild card character

Symetric to the function pointer one

Only works if you connected with the symmetric call, with function pointers (Or you can also use 0 for wild card)In particular, does not work with static function, functors or lambda functions.

New way using QMetaObject::Connection

Works in all cases, including lambda functions or functors.

Asynchronous made easier

With C++11 it is possible to keep the code inline

Here's a QDialog without re-entering the eventloop, and keeping the code where it belongs:

Another example using QHttpServer : http://pastebin.com/pfbTMqUm

Error reporting

Tested with GCC.

Fortunately, IDEs like Qt Creator simplifies the function naming

Missing Q_OBJECT in class definition

Type mismatch

Open questions

Default arguments in slot

If you have code like this:

The old method allows you to connect that slot to a signal that does not have arguments.But I cannot know with template code if a function has default arguments or not.So this feature is disabled.

There was an implementation that falls back to the old method if there are more arguments in the slot than in the signal.This however is quite inconsistent, since the old method does not perform type-checking or type conversion. It was removed from the patch that has been merged.

Overload

As you might see in the example above, connecting to QAbstractSocket::error is not really beautiful since error has an overload, and taking the address of an overloaded function requires explicit casting, e.g. a connection that previously was made as follows:

connect(mySpinBox, SIGNAL(valueChanged(int)), mySlider, SLOT(setValue(int));

cannot be simply converted to:

...because QSpinBox has two signals named valueChanged() with different arguments. Instead, the new code needs to be:

Unfortunately, using an explicit cast here allows several types of errors to slip past the compiler. Adding a temporary variable assignment preserves these compile-time checks:

Some macro could help (with C++11 or typeof extensions). A template based solution was introduced in Qt 5.7: qOverload

The best thing is probably to recommend not to overload signals or slots …

… but we have been adding overloads in past minor releases of Qt because taking the address of a function was not a use case we support. But now this would be impossible without breaking the source compatibility.

Disconnect

Should QMetaObject::Connection have a disconnect() function?

The other problem is that there is no automatic disconnection for some object in the closure if we use the syntax that takes a closure.One could add a list of objects in the disconnection, or a new function like QMetaObject::Connection::require

Callbacks

Function such as QHostInfo::lookupHost or QTimer::singleShot or QFileDialog::open take a QObject receiver and char* slot.This does not work for the new method.If one wants to do callback C++ way, one should use std::functionBut we cannot use STL types in our ABI, so a QFunction should be done to copy std::function.In any case, this is irrelevant for QObject connections.

Introduction

Remarks

Official documentation on this topic can be found here.

A Small Example

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.

The minimal example requires a class with one signal, one slot and one connection:

counter.h

The main sets a new value. We can check how the slot is called, printing the value.

Finally, our project file:

Qt5 Signal Slot Overload

The new Qt5 connection syntax

The conventional connect syntax that uses SIGNAL and SLOT macros works entirely at runtime, which has two drawbacks: it has some runtime overhead (resulting also in binary size overhead), and there's no compile-time correctness checking. The new syntax addresses both issues. Before checking the syntax in an example, we'd better know what happens in particular.

Let's say we are building a house and we want to connect the cables. This is exactly what connect function does. Signals and slots are the ones needing this connection. The point is if you do one connection, you need to be careful about the further overlaping connections. Whenever you connect a signal to a slot, you are trying to tell the compiler that whenever the signal was emitted, simply invoke the slot function. This is what exactly happens.

Here's a sample main.cpp:

Hint: the old syntax (SIGNAL/SLOT macros) requires that the Qt metacompiler (MOC) is run for any class that has either slots or signals. From the coding standpoint that means that such classes need to have the Q_OBJECT macro (which indicates the necessity to run MOC on this class).

The new syntax, on the other hand, still requires MOC for signals to work, but not for slots. If a class only has slots and no signals, it need not have the Q_OBJECT macro and hence may not invoke the MOC, which not only reduces the final binary size but also reduces compilation time (no MOC call and no subsequent compiler call for the generated *_moc.cpp file).

Connecting overloaded signals/slots

While being better in many regards, the new connection syntax in Qt5 has one big weakness: Connecting overloaded signals and slots. In order to let the compiler resolve the overloads we need to use static_casts to member function pointers, or (starting in Qt 5.7) qOverload and friends:

Multi window signal slot connection

A simple multiwindow example using signals and slots.

There is a MainWindow class that controls the Main Window view. A second window controlled by Website class.

Qt5 Signals Slots Emit

The two classes are connected so that when you click a button on the Website window something happens in the MainWindow (a text label is changed).

I made a simple example that is also on GitHub:

mainwindow.h

mainwindow.cpp

website.h

Qt Signal Slot Dll

website.cpp

Project composition:

Consider the Uis to be composed:

• Main Window: a label called 'text' and a button called 'openButton'
• Website Window: a button called 'changeButton'

So the keypoints are the connections between signals and slots and the management of windows pointers or references.