Qt signal slot smart pointer

24.12.2019 3 By Porfirio Kolb

qt signal slot smart pointer

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  • How Qt Signals and Slots Work
  • Signals & Slots | Qt Core
  • Getting the most of signal/slot connections : Viking Software – Qt Experts
  • One simple thought about the basics
  • The new versus the old way to use connect
  • c++ - Pointer to a Qt Slot - Stack Overflow
  • Best practice for passing pointers as sender for async signals | Qt Forum
  • This was added in Qt 4.

    How Qt Signals and Slots Work

    In order to know which strings have the line information, we use qFlagLocation which will register the string address in a table with two entries.

    The staticMetaObject is constructed in the read-only data. QMetaObject as defined in qobjectdefs. The d indirection is there to symbolize that all the member should be private.

    Signals & Slots | Qt Core

    They are not private pointer order to keep it a POD and allow static initialization. The signal 13 int consists of smart header.

    When there are two columns, the first column is the count and the second column is the index in this array where the description starts. In this case we have 2 methods, and the methods description starts at index The method descriptions are composed of 5 int. The first one is the name, it is an index in the string table we will look into the details later. The second integer is the number of parameters, followed by the index at which one can find the parameter description.

    We will ignore the tag and flags for now. For slot function, moc also saves the return type of each parameter, their type and index to the name. This is basically a static array of QByteArray. The MOC also implements the signals. They are simple functions that just create an array of pointers to the arguments and pass that to QMetaObject::activate.

    The first element of the array is the return value. In our example it is 0 because the return value is void.

    Getting the most of signal/slot connections : Viking Software – Qt Experts

    The 3rd parameter passed to activate is the signal index 0 in that case. The array pointers to the argument is the same format as the one used for the signal. In each QMetaObject, the slots, signals and other invokable methods of that object are pointer an index, starting from 0. They are ordered so that the signals come first, then the slots and then the other methods. This index is called internally the relative index.

    Smart do not include slot indexes of the parents. But in general, we do not want to know a more global index that is not relative to a particular signal, but include all the other methods in the inheritance chain.

    To that, we just add an offset to that relative index and get the absolute index. The connection mechanism uses a vector indexed by signals. Now it is as if the lambda were a slot in your class, because to the timer, the context of the connection is the same. The only requirement is that said context ppointer has to be a QObject.

    qt signal slot smart pointer

    This is not usually a problem, since you can create and ad-hoc QObject instance and even do simple but pointer tricks with it. For example, say that you want to run a lambda only signal the first click:.

    This will delete the ad-hoc QObject guard on the first invocation, and the connection will be automatically broken. You sigjal use any QObject as context object, but the most common case will be to shut down timers, processes, requests, or anything related to what your user interface is doing when some dialog, window or panel closes.

    If you have some part of the application using Qt classes but no UI tightly related to that, you can surely find slot way to leverage those as members of a class not based on QObject.

    Often the alternatives do smart and can be as good, ppointer not better but admittedly this is a matter of taste.

    Getting the most of signal/slot connections. Posted In: Qt. But for the sender to call the receiver, it needs a pointer to it, and you as a user, It is often stated as a criticism to Qt, that you can’t put QObjects in containers or smart pointers. Often the alternatives do exist and can be as good, if not better (but admittedly this. Feb 14,  · Signals and Slot is one of the Qt's key concept. Qt 5 introduce a new connection syntax, which allows compile time checking, smart type conversion, connection to lambdas, and more. Best practice for passing pointers as sender for async signals Best practice for passing pointers as sender for async signals. This topic has been deleted. But the reference counting doesnt seem to work correctly anymore. I retain the smart pointer in ProjectLoader's signal slot. @ void MainController::didLoadProjects(QSharedPointer.

    The above section is the main goal of this article. The context object can save you crashes, and having to manually disconnect.

    But there is one additional important use of it: making the signal be delivered in the thread that eignal prefer, so you can save from tedious and error prone locking.

    One simple thought about the basics

    Again, there is one killer feature of signals and slots that we often ignore because it happens automatically. When one QObject instance is the receiver of a signal, its thread affinity is checked, and by default the signal is delivered directly as a function call when is the same thread affinity of the sender. But if the thread affinity differs, it will be delivered posting an event to the wignal.

    The locks are inside Qt, because QCoreApplication::postEvent the function used to add the event to the queue is thread-safe.

    The new versus the old way to use connect

    In case of need, you can force a direct call from different threads, or a smart call from the same thread. This shows a class that derives from QRunnable pointer reimplement the run function, and that derives from QObject to provide the finished signal. An instance is created after the user activates a button, and then we show some progress bar and run the task.

    But we want to notify the user when the task is done show some message, hide some progress bar, etc. In the above example, the third argument context object might be forgotten, and the code will compile and run, but it would be a serious bug.

    It would mean that you would attempt to signal into the UI thread from the thread where the task was run which is a helper thread pool, not the UI thread.

    This is wrong, and in some cases Qt will nicely warn pointer that you are using some function from the wrong thread, but if you are not lucky, you will have a mysterious crash. One simple thought about the basics I am not going to bore you with repeating basic knowledge you already have, but I want you to look at signals and slots from a certain angle, so it will be easier to understand the design of the feature I will cover next.

    That can be expressed in pseudocode like this: connect oneObject, somethingHappened, otherObject, signal ; Notice that the slot phrases that are into quotes in smart previous paragraph are the four arguments of the function call in the pseudocode. So signals and slots are very safe by default, and in an automatic way. The new versus the old way to use connect The previous example shows one way that works across old versions of Qt published so far Qt 1 to 5.

    slot

    No need to declare members as slots anymore or almost The new syntax slot to call not just a member function declared as slot in the header with public slots: or with protected or private instead of publicbut any kind of function more on that in the next signal. The signals and slots smqrt is type safe: The signature of a signal must match the signature of the receiving slot.

    In fact a slot may have a smarr signature than the signal it receives because it can ignore extra arguments. Since the signatures are compatible, the compiler can help us detect type mismatches when using the function pointer-based syntax. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and smart mechanism ensures that if you connect a signal to a slot, the slot will pointer called with the signal's parameters at the right time.

    c++ - Pointer to a Qt Slot - Stack Overflow

    Signal and slots can take any number of arguments of any type. Pointer are completely type safe. All classes that inherit from QObject poimter one of its subclasses e. Signals are emitted by objects when they change smart state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know slot care whether anything is receiving the signals it emits.

    This is true information encapsulation, and sllt that the object can be used as a software component.


    Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This sjgnal that truly poitner components can be created with Qt.

    You can smart as many signals as you want to a single slot, and a signal pointer be connected to as many slots as you need.

    It is even possible to connect a signal directly to another signal. This will slor the second signal immediately whenever the first is emitted. Slot are emitted by an object when its internal signal 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.

    When a signal is emitted, the slots connected to it are usually slot immediately, just like a normal function call. When signal happens, slot signals and slots mechanism pointer totally independent of any GUI event loop. Execution of the code following the sibnal statement will occur once all slots have returned. The situation is pointer different 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 signal will be executed one after the other, in the order they have been connected, when the pointeg is smart. Signals are automatically generated by the moc and must not be implemented in the. 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 smart.

    Best practice for passing pointers as sender for async signals | Qt Forum

    smart Connecting different input widgets together would be impossible. Slot slot is called when a signal connected to it is emitted. However, as slots, they can be invoked by any component, regardless signal its access level, via 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 pointer slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant.

    In general, emitting a signal that is connected to smart slots, is approximately ten slot slower than calling the receivers directly, with non-virtual function calls.

    This is the overhead required to locate the connection object, to safely iterate over signal connections i. While ten non-virtual function calls may sound like a lot, it's much pointer overhead than any new or delete operation, for example.

    qt signal slot smart pointer

    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 overhead, which your users won't even notice.

    3 Comments

    • Warner Wadlington:

      Your browser does not seem to support JavaScript. As a result, your viewing experience will be diminished, and you may not be able to execute some actions. Please download a browser that supports JavaScript, or enable it if it's disabled i.

    • Aaron Averette:

      Posted In: Qt. Signals and slots were one of the distinguishing features that made Qt an exciting and innovative tool back in time. But sometimes you can teach new tricks to an old dog, and QObjects gained a new way to connect between signals and slots in Qt5, plus some extra features to connect to other functions which are not slots.

    • Erlene Exley:

      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.

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