Does it make any difference, using public slots instead of private slots in Qt
Does it make any difference, using public slots instead of private slots in Qt?
In C++, public means those members that are accessible from anywhere where the object is visible, private means that members are accessible only from within other members of the same class or from their friends.
But in Qt, the difference in private slots and public slots seem not to exist. I have begun writing Qt in recent days, and I used private slots all the time.
Someone told me I should use public slots instead. So now I am puzzled. I can not find reference info in the Qt`s docs.
What's the actual difference between the two types?
3 Answers 3
Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of 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.
What this means: From another class, you can't call a private slot as a function, but if you emit a signal connected to that private slot, you can invoke it.
The private/public access is checked by the compiler at compile time, but the signal-slot connection is performed at run-time and slots are invoked by some QMetaObject mechanisms (something like for example invokeMethod).
So the difference is this: private slots are private if called as regular member functions but always "public" for signals to invoke, a good reason is because slots conceptually are public interface, since their main purpose is inter-object communication
Another example about some related weird stuff is the call of private virtual functions if they are public in the static type of the pointer that is used to call the method.
@user2448027 answer is correct, but there is a missing point in Qt's design pattern: different applications of private slots vs public slots .
By making slot private you force users of the object to use connect function to call the slot, rather than member access operators( . or - ).
Imagine you have a slow or blocking code in one of the slots of your class. You expect users of the class to move the created object to another thread, so the owner of this object (GUI or some object's related to GUI) would not freeze or block by calling this slot. Here is the point: if the slot is called directly by . or - operators, it will block. Because the current thread of caller method uses the slot, it can only happen if you use public slot. The solution is to make the slot private , so the user could only call it with connect , but not member access operators( . or - ).
Qt ???:public slots ? private slots
class MainWindow : public QMainWindow
<
Q OBJECT
public:
MainWindow(QWidget *parent = 0);
public slots: （//???????private ，????slots）
void selectDir();
void next();
void prev();
private : (//???????private slots)
void createActions();
void createMenus();
void createToolBars();
void iniCentralWidget();
private:
ImageWidget *imagewidget;
QMenu *FileMenu;
QToolBar *toolBar;
QAction *dirAct;
QAction *nextAct;
QAction *prevAct;
QDir imageDir;
QStringList imageList;
int index;
>;
#endif
??private ???private slots ??，?????????????（? ）
????? C++ ????，???????，??????????????????????????????????，???????????????，????????????
???????????，??????????，??????????????????????????????? C++ ??????，??????????，? public slots?private slots ? protected slots?
- public slots：??????????????????????????????????????，??????????????，????????????????????????
- protected slots：?????????????????????????????????????，??????????，?????????????
- private slots：?????????????????????????????????????????
???????????????slots ?????，?public private protected ???C++??
Private slots question
I noticed that a private slot can be called by any signal even if the object emmting it is not a class member toghether with the slot (as a normal method it can not be invoked by non member objects). This is mentioned in the docs but wouldnt it make more sense to be able to invoke a private slot only be member objects or friends and not by any object?
Not really, the private/public access is checked by the compiler at compile time, and the signal-slot connection is performed at run-time and slots are invoked by some QMetaObject mechanisms (invokeMethod():http://doc.qt.nokia.com/4.7/qmetaobject.php#invokeMethod).
So private slots are private if called as regular member functions but always public for connection.
Another example about some weird stuff can be the call of private virtual functions.
True it makes sense private protected and such are non existing at runtime i should have seen this earlier :D. Thanks.
Yes, and another thing is that private slots doesn't make sense for connections. Slots conceptually are public interface, since their main purpose is inter-object communication.
If you really need a function to be private that function shouldn't be a slot too, it should be just a private member function (if call that function internally you you just call the function, you don't need a connection for that)
Private/protected slots may make sense if you need them internally for a kind of more sophisticated widget. You want to connect them with a signal of a subwidget internally but won't expose them to the public API.
PIMPL:http://www.qtcentre.org/wiki/index.php?title=Private implementation does not help here, as the d-pointer objects are not QObject derived and thus cannot have slots.
Making the method simply private/protected (without slot keyword) would make it impossible to connect() too.
Of course, the private/protected slots can be connected to via signal/slots connection. But, you must look into the header files or ignore the protected in the documentation. In this case you should know what you do.
And as you mentioned, private/protected/public is checked at compile time[1]. So you always can fool the compiler by grabbing the original header file, move the private method to the public part and compile :-)
fn1. I'm not sure if some compiler (MS VC++?) includes the access level into the name mangling scheme.
[quote]
Private/protected slots may make sense if you need them internally for a kind of more sophisticated widge. You want to connect them with a signal of a subwidget internally but won t expose them to the public API.
[/quote]
There is a possibility to apply a runtime check too:
You can do this in a private slot:
check if the sender of the signal is this
check if the sender is a friend class, in this case an object of type Object3 may call this slot.
@void Object1::mySlot()
<
if(sender() != this sender()-metaObject()-className() != QString(Object3))
<
qFatal(Object1::mySlot() is private and can not be called from %s, sender()-metaObject()-className());
>
This is of course not fun to debug as you might only hit the fatal message by accident.
Edit: I'm sure I made a couple of mistakes here.
But if you do this,m internal slots will not work.
I use private slots for connections to shild objects. For example:
I have a widget with a button. The button is cobnnected to a protected or private slot. The sender will not be this. For this, you normally do not need signals, only if you want to achieve some threadding stuff with queued connections.
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Qt slots public or private
Allow attributes within the access specifier (public [[qt::slot]]:) traits to get the attributes (constexpr std::has attribute("qt::signal"); traits to get the access of a function (public, private, protected) (for qmetamethod::access) a way to declare functions. Getting default value of arguments. Qt's widgets have many pre-defined signals, but we can always subclass to add our own. A slot is a function that is called in reponse to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to add your own slots so that you can handle the signals that you are interested in. Qt public versus private slots, pokerkaarten amersfoort, large square poker chips, blackjack roofing products prize pool: 100% up to 400 + 200 free spins over 600 casino games. Signals, slots, q object, emit, signal, slot. Those are known as the qt extension to c++. They are in fact simple macros, defined in qobjectdefs. #define signals public #define slots /* nothing */ that is right, signals and slots are simple functions: the compiler will handle them them like any other functions. Make sure that your slot function is declared as a slot, e. Private slots not private. Use qsignalspy to verify signal arguments. These functions were made public in qt 5. Note: wait() and the sleep() functions should be unnecessary in general, since qt is an event-driven framework. Instead of wait(), consider listening for the finished() signal. Com/c-tutorial/qt-tutorials-for-beginners-qt-signal-and-slots/ in this video we will learn how qt signals and slots w. Please read qt documentation: in a nutshell "since slots are normal member functions, they follow the normal c++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. 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 slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. Qt lambda slots i ve toyed with qt connections trying to create anonymous function when connecting a slot to a signal. For instance, i am trying to do the following:. This is rather intuitive and easy to used. But when slots and qt event loop are used in the worker thread, some users do it wrong. Hughes, one of the qt core developers, recommend that use worker objects by moving them to the thread using qobject::movetothread. Unfortunately, some users went on a crusade against the former usage
This is not an official categorization, but I believe that they describe the actual state of affairs, qt slots public or private.
Qt slots public or private
In qt, we have an alternative to the callback technique: we use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. @user2448027 answer is correct, but there is a missing point in qt's design pattern: different applications of private slots vs public slots. By making slot private you force users of the object to use connect function to call the slot, rather than member access operators(. This is only done in this case because almost every call to connect would generate a new different type (since the qslotobject has template parameters wich depend on signature of the signal and the slot). Protected, public, or private signals. Signals were protected in qt4 and before. Com/c-tutorial/qt-tutorials-for-beginners-qt-signal-and-slots/ in this video we will learn how qt signals and slots w. This is rather intuitive and easy to used. But when slots and qt event loop are used in the worker thread, some users do it wrong. Hughes, one of the qt core developers, recommend that use worker objects by moving them to the thread using qobject::movetothread. Unfortunately, some users went on a crusade against the former usage. Make sure that your slot function is declared as a slot, e. Private slots not private. Use qsignalspy to verify signal arguments. Stack overflow for teams is a private, secure spot for you and your coworkers to find and share information. Signals, slots, q object, emit, signal, slot. Those are known as the qt extension to c++. They are in fact simple macros, defined in qobjectdefs. #define signals public #define slots /* nothing */ that is right, signals and slots are simple functions: the compiler will handle them them like any other functions. In addition previous posts, private or public slots have no significance with qt c++ environment if you are using slots in the context of signal-to-slot communication. If you are interested to call this slots as normal member function then public/private is applicable. One way is to add a slot to your public class then have it call the method you want in the private class, but this means you have to add a method to public api which means you break binary compatibility. Another is to have your private class extend qobject, but this adds overhead. Qt private slots public slots, incredibile hulk slot machine gratis, como ganar en poker texas holdem, slot machine construction Depending on how quickly payments are being confirmed, some sites do say it can take a little longer (as long as 15 minutes), qt slots public or private.
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