Qt signal slot call by reference

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qt signal slot call by reference

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  • Signals & Slots | Qt
  • Introduction
  • SLOT/SIGNAL safety with QByteArray &references | Qt Forum
  • c++ - stack object Qt signal and parameter as reference - Stack Overflow
  • Signals & Slots | Qt Core
  • I'm sorry to continue a subject years old but it came up on Google. I want to clarify HostileFork's answer as it may mislead future readers. No, you won't encounter a dangling reference. At least, not unless your slot does the sort of things that would cause problems in regular functions too.

    We can generally accept that this won't be a problem for direct connections as those slots are called immediately. Your signal emission call until all slots have been called. Once that happens, emit recerence fooStackObject ; will return just like a regular function. Signa, fact, myQtSignal fooStackObject ; reference a regular function!

    The emit keyword is entirely for your benefit--it does nothing. The signal function is just special because its code is generated by Qt's compiler: the moc. Benjamin T has pointed out in the documentation that arguments are copied, but I think it's enlightening to explore how this works under the hood at least in Qt 4.

    If we start by compiling our project and searching around for our signal moc file, we can find something like this:. So basically, we pass a number of things to QMetaObject::activate : our QObject, slot metaObject for our QObject's type, our signal id, and a pointer to each of the arguments our signal received.

    Signals & Slots | Qt

    If we investigate QMetaObject::activatewe'll find it's skot in qobject. This is something integral to how QObjects work. After browsing through some stuff that's irrelevant to this question, we find the behaviour for queued connections.

    Qt is well known for its signals and slots mechanism. But how does it work? In this blog post, we will explore the internals of QObject and QMetaObject and discover how signals and slot work under the hood. In this blog article, I show portions of Qt5 code, sometimes edited for formatting and brevity. Passing a reference to a Qt signal is not dangerous thanks to the way signal/slot connections work: If the connection is direct, connected slots are directly called directly, e.g. when emit MySignal(my_string) returns all directly connected slots have been executed. If the the connection is queued, Qt creates a copy of the referencees. So when. Mar 13,  · Using C++11 Lambdas As Qt Slots. IDEs can identify the methods used in a connect call when you search for uses of a method; In the signal/slot case (the focus of this post) the return type is void. As I mentioned, this is inferred if you leave it out. Reply to Andy. Dmitry.

    This means two things:. Reefrence are normalized. Once the event has been queued, the signal returns. If the scope in which an object exists ends and it is then used, it will refer to a destroyed refeernce which will cause undefined behaviour. Listen now. Learn more. Asked 8 years, 1 month ago. Active 2 years ago. Viewed 19k times. Guillaume07 Guillaume07 9, 11 11 gold badges 52 refegence silver badges bronze badges.

    If you change to Qt::DirectConnectionit doesn't make any copies: transmitting with 0x7ffebf as parameter receive called with 0x7ffebf as parameter receive called with 0x7ffebf as parameter signal called with 0x7ffebf as parameter And slot you switched to passing by valueyou'd actually get a more slot copies, especially in the Qt::QueuedConnection case: Calling Copy Constructor Calling Copy Constructor Calling Copy Constructor Calling Copy Constructor Calling Copy Constructor receive called with 0x7fffecf as parameter Calling Signal Cxll receive called with 0x7fffecf as parameter Calling Copy Constructor receive called with 0x7fffecf as parameter But passing by pointer doesn't do any reference magic.

    HostileFork HostileFork Since Qt makes a copy anyway, wouldn't it be call style to pass xignal by value? Since the actual semantics of what's going on is rather obscure, I think pass-by-argument expresses the fact that copies are going to be made in a more clear way slot pass-by-reference. And the intermediate copies problem sounds like something that should be solved by Qt developers rather than its users.

    If the the connection is queued, Qt creates a copy of the referencees. So when the slot is called it has its own valid copy of reference variables passed by reference. However this means that parameters must be of a type that Qt knows about in order to copy it.

    Benjamin T Benjamin T 6, 12 12 silver badges 29 29 bronze badges. The whole point of having StackOverflow be referencf wiki-style and let people come up with later answers is for it to get better! So don't worry about ny. The link you cite does mention copying, but it reference explicitly state that it copies the objects pointed to by reference which I'd think it should! That's a pretty major thing, and seems sivnal a documentation bug call me!

    Signal and Benjamin T seem to have opposing arguments.


    Is is safe or not to use references everywhere when defining signals and slots? It is safe. I would even call it good style. It's all fine and well to sslot proven wrong, and systems are complicated, etc. I'm busy and would like to see proof-positive that's how it works, vs.

    HostileFork Quite the opposite. It doesn't notice the difference between a reference and a value. Either way, it's just making a copy of the argument you gave it. I ran out of room in this comment field, so I've posted more detail on how it works as an answer.

    Qt::DirectionConnection We can generally accept that this won't be a problem for direct connections as those slots are called immediately. Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

    SLOT/SIGNAL safety with QByteArray &references | Qt Forum

    When a cal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned.

    The situation is slightly calp when using queued connections ; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later. If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted. Signals are automatically generated by the moc and must not be signwl in the.

    qt signal slot call by reference

    They can never have return types i. A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. Connecting different input widgets together would be impossible.

    A slot is called when a signal connected to it is emitted. However, as slots, they can be invoked bu any component, regardless of its access level, hy a signal-slot connection.

    This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class. Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for reference applications is insignificant.

    In general, emitting a signal that is connected to some slots, is approximately ten times slower call calling the receivers directly, with non-virtual function calls. Slot is the overhead required to locate the connection signal, to safely iterate over all connections i.

    While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or deletethe signals and slots overhead is only responsible for a very small proportion of the complete function call costs. The same is true whenever you do a system call in a slot; or indirectly call more than ten functions.

    The simplicity and flexibility of the signals and slots mechanism is well worth the rederence, which your users won't even notice. Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application.

    To solve this problem, undef the offending preprocessor symbol.

    It's also worth mentioning that for non-const references (i.e. "out" parameters) it's possible to use a Qt::BlockingQueuedConnection that will make the calling signal block until all slots return. This way the reference remains valid through all slot execution. New-style Signal and Slot Support¶ This section describes the new style of connecting signals and slots introduced in PyQt4 v One of the key features of Qt is its use of signals and slots to communicate between objects. Their use encourages the development of reusable components. A signal is emitted when something of potential interest. 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). connect(sender, SIGNAL(valueChanged(QString, QString)), receiver, SLOT(updateValue(QString)));.

    The QObject -based version has the same internal state, and provides public methods to access the state, call in addition it has support for component programming using reference and slots.

    This class can tell the outside world signal its state has changed by emitting a signal, valueChangedand it has a slot which other objects can send signals to. They call also derive directly or indirectly from QObject. Slots are implemented by the clal programmer. Here is a possible implementation of the Counter::setValue slot:. The emit line emits the signal valueChanged from the object, with the new value as argument.

    In the following code snippet, we create two Counter objects and connect the first object's valueChanged signal to the second object's setValue slot using QObject::connect :. Calling a. Then b emits the same valueChanged signal, but since no slot has been connected to b slot valueChanged signal, the signal alot ignored.

    Note that the setValue function sets the value and emits the signal only if value! This prevents infinite looping in the case of cyclic connections e. By default, for every connection you make, a signal sitnal emitted; two signals are sifnal for duplicate connections. You can break all of these slot with a single disconnect call.

    c++ - stack object Qt signal and parameter as reference - Stack Overflow

    If you pass the Qt::UniqueConnection typecall connection will only be made if it is not a duplicate. If there is already a duplicate exact same signal to the exact same slot on the same objectsthe connection will fail and connect will return false.

    This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect function calls, signal with uic 's automatic connections feature. The following is an example of the header of a simple widget class without member functions.

    The purpose is to show how you can utilize signals and slots in your own applications. After the class constructor and public members, we declare reference class signals. The LcdNumber class emits a signal, overflowwhen it is asked to show an impossible value. If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow signal, i. If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to slot different slots.

    Qt will call both in the order they were connected. A slot is a receiving function used to get information about state changes in other widgets.

    LcdNumber uses it, as the code above indicates, to set the displayed number.

    Signals & Slots | Qt Core

    Since display is part of the class's interface with the rest of the program, the slot is public. Several of the example programs connect the valueChanged signal of a QScrollBar to the display slot, so referencs LCD number continuously shows the value of the scroll bar.

    Note that display is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself. The signatures of signals and slots may contain arguments, and the arguments can have default values.

    Consider QObject::destroyed :. Calk a QObject is deleted, it emits rference QObject::destroyed signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObjectso we can clean it up. A suitable slot signature might be:.

    About the Author: Percy Gatts


    1. 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.

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    3. By using our site, you acknowledge that you have read and understand our Cookie Policy , Privacy Policy , and our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. May I have a "dangling reference" with the following code in an eventual slot connected to the myQtSignal?

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