Qt5 Signal Slot Example
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This page was used to describe the new signal and slot syntax during its development. The feature is now released with Qt 5.
A slot is a function that is called when a signal is emitted. For example, a push button emits a clicked signal when clicked by a user. A slot that is attached to that signal is called when the clicked signal is emitted. Multiple signals can be connected to any slot. Qt5 Tutorial Signals and Slots - 2020, Introduction#. Signals and slots are. Qt has a unique signal and slot mechanism. This signal and slot mechanism is an extension to the C programming language. Signals and slots are used for communication between objects. A signal is emitted when a particular event occurs. A slot is a normal C method; it is called when a signal connected to it is emitted. Nd the index of the signal and of the slot Keep in an internal map which signal is connected to what slots When emitting a signal, QMetaObject::activate is called. It calls qt metacall (generated by moc) with the slot index which call the actual slot.
- 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.
- 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.
published at 07.08.2014 15:45 by Jens Weller
Last week I started to work on an old project again: My own feed reader. I found the code 2 weeks a go on an old USB Stick, and decided to refactor it into a useful state. This involved dealing with HTTP via QNetworkAccessManager.
QNetworkAccessManager: HTTP in Qt
The QNetworkAccessManager class replaces the old QHttp classes. If you work with HTML, there is also the webkit module, but for raw HTTP, QNetworkAccessManger is the fitting choice. The API is asynchronous, your program will not block during the HTTP Request. A simple example:
Each request is done via the class QNetworkRequest, you can set various parameters for such a request. For my feed reader it was important to give the request a user agent. Otherwise some pages don't accept the connection and you will receive an RemoteHostClosed error. When you initiate the HTTP request with the QNetworkAccessManager(QNAM), you get a Pointer to a QNetworkReply object, corresponding to this request. You then either can connect to the Signals of QNetworkReply, or to the finished(QNetworkReply*) Signal of the QNAM. When dealing with multiple requests it can be better to bind to the QNAM signal, as you otherwise would have to dyncast QObjects::sender() for getting the context.
Which brings me back to my feed reader. None of the above is interesting for the feed reader. It has some feed url, and is actually interested in reading this feed. Its not interested to deal with QNAMs issues, as downloading via HTTP is such a common task, that this should be handled by a different class. So I created HttpDownloader, a class which currently downloads via get, and hands the result as a QByteArray in a signal to the caller. The interesting things then happen in the finished(QNetworkReply*) slot.
First the error handling:
So, Qt does not use exceptions, and first one has to check if there is an error, currently all I do then is some logging and erasing the reply from an internal map.
But we aren't done yet, that there is no error does not mean that the call was successful. The HTTP Response also could be a redirect. QNAM does not automatically handle this, so the handler has to test for the redirect, and then issue a new request:
Currently all I do is following the redirect. This is a working solution, but maybe not the best. It does not prevent redirect loops. QNetworkReply and QNetworkRequest have a few more attributes, but I only see the need to handle redirects currently.
Now, that errors and redirects are handled, the code for handling the content and request are rather short:
The content part of the HTTP call is easily read with readAll, as this class only deals with downloading it, it simply emits a signal that the content now can be processed. It is important also to delete the QNetworkReply instance now, first I have to delete it from a local map which binds a QVariant id to all active requests. Instead of using a local map I also could store this id directly as an attribute inside QNetworkReply.
This id is very important, as once the content is downloaded from the net, I need to do something with it. This id servers this purpose, the actual content handling then happens in the clients slot.
HTTPS and Qt
I hoped to get around this, as for the first version I would not need to rely on HTTPS, later when building apps with my feed reader, SSL support would be a must. But some of the feeds I want to have in my feed reader work over HTTPS, and one even is only available via HTTPS. Qt does not come with OpenSSL binaries, and when searching the net all kind of things show up. For example that you need to build Qt your self, as HTTPS is not enabled by default.
So, when requesting a HTTPS url, the following errors show up with QNetworkAccessManager in Qt5:
So, Qt5 does recognize the https url correctly, and tries to open a SSL Socket for it, which fails. The reason for this behavior is the missing OpenSSL binaries, which you need to provide to Qt in order to be able to properly open a https link. On windows this are the DLLs libeay32 and ssleay32.
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Qt5 Signal Slot Examples
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