If i remember correctly in earlier versions of Qt a slot was also public automatically, but i did not find a reference for that now. Any other class can connect to a signal from a foreign class, as long the QOBJECT macro is given in the class and the foreign class is known (header included). The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function. In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques − QtCore.QObject.connect(widget, QtCore.SIGNAL(‘signalname’), slot.

• Qt Signal Slot Not Called Two
• Qt Signal Slot Not Called Money

This is the sequel of my previous article explaining the implementation details of the signals and slots.In the Part 1, we have seenthe general principle and how it works with the old syntax.In this blog post, we will see the implementation details behind thenew function pointerbased syntax in Qt5.

New Syntax in Qt5

The new syntax looks like this:

Why the new syntax?

I already explained the advantages of the new syntax in adedicated blog entry.To summarize, the new syntax allows compile-time checking of the signals and slots. It also allowsautomatic conversion of the arguments if they do not have the same types.As a bonus, it enables the support for lambda expressions.

New overloads

There was only a few changes required to make that possible.
The main idea is to have new overloads to QObject::connect which take the pointersto functions as arguments instead of char*

There are three new static overloads of QObject::connect: (not actual code)

The first one is the one that is much closer to the old syntax: you connect a signal from the senderto a slot in a receiver object.The two other overloads are connecting a signal to a static function or a functor object withouta receiver.

They are very similar and we will only analyze the first one in this article.

Pointer to Member Functions

Before continuing my explanation, I would like to open a parenthesis totalk a bit about pointers to member functions.

Here is a simple sample code that declares a pointer to member function and calls it.

Pointers to member and pointers to member functions are usually part of the subset of C++ that is not much used and thus lesser known.
The good news is that you still do not really need to know much about them to use Qt and its new syntax. All you need to remember is to put the & before the name of the signal in your connect call. But you will not need to cope with the ::*, .* or ->* cryptic operators.

These cryptic operators allow you to declare a pointer to a member or access it.The type of such pointers includes the return type, the class which owns the member, the types of each argumentand the const-ness of the function.

You cannot really convert pointer to member functions to anything and in particular not tovoid* because they have a different sizeof.
If the function varies slightly in signature, you cannot convert from one to the other.For example, even converting from void (MyClass::*)(int) const tovoid (MyClass::*)(int) is not allowed.(You could do it with reinterpret_cast; but that would be an undefined behaviour if you callthem, according to the standard)

Pointer to member functions are not just like normal function pointers.A normal function pointer is just a normal pointer the address where thecode of that function lies.But pointer to member function need to store more information:member functions can be virtual and there is also an offset to apply to thehidden this in case of multiple inheritance.
sizeof of a pointer to a member function can evenvary depending of the class.This is why we need to take special care when manipulating them.

Type Traits: QtPrivate::FunctionPointer

Let me introduce you to the QtPrivate::FunctionPointer type trait.
A trait is basically a helper class that gives meta data about a given type.Another example of trait in Qt isQTypeInfo.

What we will need to know in order to implement the new syntax is information about a function pointer.

The template<typename T> struct FunctionPointer will give us informationabout T via its member.

• ArgumentCount: An integer representing the number of arguments of the function.
• Object: Exists only for pointer to member function. It is a typedef to the class of which the function is a member.
• Arguments: Represents the list of argument. It is a typedef to a meta-programming list.
• call(T &function, QObject *receiver, void **args): A static function that will call the function, applying the given parameters.

Qt still supports C++98 compiler which means we unfortunately cannot require support for variadic templates.Therefore we had to specialize our trait function for each number of arguments.We have four kinds of specializationd: normal function pointer, pointer to member function,pointer to const member function and functors.For each kind, we need to specialize for each number of arguments. We support up to six arguments.We also made a specialization using variadic templateso we support arbitrary number of arguments if the compiler supports variadic templates.

The implementation of FunctionPointer lies inqobjectdefs_impl.h.

QObject::connect

The implementation relies on a lot of template code. I am not going to explain all of it.

Here is the code of the first new overload fromqobject.h:

You notice in the function signature that sender and receiverare not just QObject* as the documentation points out. They are pointers totypename FunctionPointer::Object instead.This uses SFINAEto make this overload only enabled for pointers to member functionsbecause the Object only exists in FunctionPointer ifthe type is a pointer to member function.

We then start with a bunch ofQ_STATIC_ASSERT.They should generate sensible compilation error messages when the user made a mistake.If the user did something wrong, it is important that he/she sees an error hereand not in the soup of template code in the _impl.h files.We want to hide the underlying implementation from the user who should not needto care about it.
That means that if you ever you see a confusing error in the implementation details,it should be considered as a bug that should be reported.

We then allocate a QSlotObject that is going to be passed to connectImpl().The QSlotObject is a wrapper around the slot that will help calling it. It alsoknows the type of the signal arguments so it can do the proper type conversion.
We use List_Left to only pass the same number as argument as the slot, which allows connectinga signal with many arguments to a slot with less arguments.

QObject::connectImpl is the private internal functionthat will perform the connection.It is similar to the original syntax, the difference is that instead of storing amethod index in the QObjectPrivate::Connection structure,we store a pointer to the QSlotObjectBase.

The reason why we pass &slot as a void** is only tobe able to compare it if the type is Qt::UniqueConnection.

We also pass the &signal as a void**.It is a pointer to the member function pointer. (Yes, a pointer to the pointer)

Signal Index

We need to make a relationship between the signal pointer and the signal index.
We use MOC for that. Yes, that means this new syntaxis still using the MOC and that there are no plans to get rid of it :-).

MOC will generate code in qt_static_metacallthat compares the parameter and returns the right index.connectImpl will call the qt_static_metacall function with thepointer to the function pointer.

Once we have the signal index, we can proceed like in the other syntax.

The QSlotObjectBase

QSlotObjectBase is the object passed to connectImplthat represents the slot.

Before showing the real code, this is what QObject::QSlotObjectBasewas in Qt5 alpha:

It is basically an interface that is meant to be re-implemented bytemplate classes implementing the call and comparison of thefunction pointers.

It is re-implemented by one of the QSlotObject, QStaticSlotObject orQFunctorSlotObject template class.

Fake Virtual Table

The problem with that is that each instantiation of those object would need to create a virtual table which contains not only pointer to virtual functionsbut also lot of information we do not need such asRTTI.That would result in lot of superfluous data and relocation in the binaries.

In order to avoid that, QSlotObjectBase was changed not to be a C++ polymorphic class.Virtual functions are emulated by hand.

The m_impl is a (normal) function pointer which performsthe three operations that were previously virtual functions. The 're-implementations'set it to their own implementation in the constructor.

Please do not go in your code and replace all your virtual functions by such ahack because you read here it was good.This is only done in this case because almost every call to connectwould generate a new different type (since the QSlotObject has template parameterswich depend on signature of the signal and the slot).

Protected, Public, or Private Signals.

Signals were protected in Qt4 and before. It was a design choice as signals should be emittedby the object when its change its state. They should not be emitted fromoutside the object and calling a signal on another object is almost always a bad idea.

However, with the new syntax, you need to be able take the addressof the signal from the point you make the connection.The compiler would only let you do that if you have access to that signal.Writing &Counter::valueChanged would generate a compiler errorif the signal was not public.

In Qt 5 we had to change signals from protected to public.This is unfortunate since this mean anyone can emit the signals.We found no way around it. We tried a trick with the emit keyword. We tried returning a special value.But nothing worked.I believe that the advantages of the new syntax overcome the problem that signals are now public.

Sometimes it is even desirable to have the signal private. This is the case for example inQAbstractItemModel, where otherwise, developers tend to emit signalfrom the derived class which is not what the API wants.There used to be a pre-processor trick that made signals privatebut it broke the new connection syntax.
A new hack has been introduced.QPrivateSignal is a dummy (empty) struct declared private in the Q_OBJECTmacro. It can be used as the last parameter of the signal. Because it is private, only the objecthas the right to construct it for calling the signal.MOC will ignore the QPrivateSignal last argument while generating signature information.See qabstractitemmodel.h for an example.

More Template Code

The rest of the code is inqobjectdefs_impl.h andqobject_impl.h.It is mostly standard dull template code.

I will not go into much more details in this article,but I will just go over few items that are worth mentioning.

Meta-Programming List

As pointed out earlier, FunctionPointer::Arguments is a listof the arguments. The code needs to operate on that list:iterate over each element, take only a part of it or select a given item.

That is why there isQtPrivate::List that can represent a list of types. Some helpers to operate on it areQtPrivate::List_Select andQtPrivate::List_Left, which give the N-th element in the list and a sub-list containingthe N first elements.

The implementation of List is different for compilers that support variadic templates and compilers that do not.

With variadic templates, it is atemplate<typename... T> struct List;. The list of arguments is just encapsulatedin the template parameters.
For example: the type of a list containing the arguments (int, QString, QObject*) would simply be:

Without variadic template, it is a LISP-style list: template<typename Head, typename Tail > struct List;where Tail can be either another List or void for the end of the list.
The same example as before would be:

ApplyReturnValue Trick

In the function FunctionPointer::call, the args[0] is meant to receive the return value of the slot.If the signal returns a value, it is a pointer to an object of the return type ofthe signal, else, it is 0.If the slot returns a value, we need to copy it in arg[0]. If it returns void, we do nothing.

The problem is that it is not syntaxically correct to use thereturn value of a function that returns void.Should I have duplicated the already huge amount of code duplication: once for the voidreturn type and the other for the non-void?No, thanks to the comma operator.

In C++ you can do something like that:

You could have replaced the comma by a semicolon and everything would have been fine.

Where it becomes interesting is when you call it with something that is not void:

There, the comma will actually call an operator that you even can overload.It is what we do inqobjectdefs_impl.h

ApplyReturnValue is just a wrapper around a void*. Then it can be usedin each helper. This is for example the case of a functor without arguments:

This code is inlined, so it will not cost anything at run-time.

Conclusion

This is it for this blog post. There is still a lot to talk about(I have not even mentioned QueuedConnection or thread safety yet), but I hope you found thisinterresting and that you learned here something that might help you as a programmer.

Update:The part 3 is available.

The one thing that confuses the most people in the beginning is the Signal & Slot mechanism of Qt. But it’s actually not that difficult to understand. In general Signals & Slots are used to loosely connect classes. Illustrated by the keyword emit, Signals are used to broadcast a message to all connected Slots. If no Slots are connected, the message 'is lost in the wild'. So a connection between Signals & Slots is like a TCP/IP connection with a few exceptions, but this metaphor will help you to get the principle. A Signal is an outgoing port and a Slot is an input only port and a Signal can be connected to multiple Slots.

For me one of the best thins is, that you don’t have to bother with synchronization with different threads. For example you have one QObject that’s emitting the Signal and one QObject receiving the Signal via a Slot, but in a different thread. You connect them via QObject::connect(...) and the framework will deal with the synchronization for you. But there is one thing to keep in mind, if you have an object that uses implicitly sharing (like OpenCV’s cv::Mat) as parameter, you have to deal with the synchronization yourself.The standard use-case of Signals & Slots is interacting with the UI from the code while remaining responsive. This is nothing more than a specific version of 'communicating between threads'.Another benefit of using them is loosely coupled objects. The QObject emitting the Signal does not know the Slot-QObject and vice versa. This way you are able to connect QObjects that are otherwise only reachable via a full stack of pointer-calls (eg. this->objA->...->objZ->objB->recieveAQString()). Alone this can save you hours of work if someone decides to change some structure, eg. the UI.

Right now I only mentioned Signal- & Slot-methods. But you are not limited to methods - at least on the Slots side. You can use lambda functions and function pointers here. This moves some of the convenience from languages like Python or Swift to C++.

For some demonstrations I will use the following classes:

Using Connections

To connect a Signal to a Slot you can simply call QObject::connect(a, &AObject::signalSometing, b, &BObject::recieveAQString) or QObject::connect(a, SIGNAL(signalSometing(QString), b, SLOT(recieveAQString(QString)) if you want to use the 'old' syntax. The main difference is, if you use the new syntax, you have compile-time type-checking and -converting. But one big advantage of the 'old' method is that you don’t need to bother with inheritance and select the most specialized method.Lambdas can be a very efficient way of using Signals & Slots. If you just want to print the value, e.g. if the corresponding property changes, the most efficient way is to use lambdas. So by using lambdas you don’t have to blow up your classes with simple methods. But be aware, that if you manipulate any object inside the lambda you have to keep in mind, that synchronization issues (in a multithreaded environment) might occur.

You will get an idea of how to use the different methods in the following example:

As you see, recived a QString: 'Hello' is printed two times. This happens because we connected the same Signals & Slots two times (using different methods). In the case, you don’t want that, you see some methods to prohibit that and other options in the next section Connection Types.

One side note: if you are using Qt::QueuedConnection and your program looks like the following example, at some point you will probably wonder, why calling the Signal will not call the Slots until app.exec() is called. The reason for this behavior is that the event queue, the Slot-call is enqueued, will start with this call (and block until program exits).

And before we start with the next section here is a little trick to call a method of another thread inside the context of the other thread. This means, that the method will be executed by the other thread and not by the 'calling' one.

To learn more about that here is your source of truth: https://doc.qt.io/qt-5/qmetamethod.html#invoke

Connection Types

Qt::AutoConnection

Qt::AutoConnection is the default value for any QObject::connect(...) call. If both QObjects that are about to be connected are in the same thread, a Qt::DirectConnection is used. But if one is in another thread, a Qt::QueuedConnection is used instead to ensure thread-safety. Please keep in mind, if you have both QObjects in the same thread and connected them the connection type is Qt::DirectConnection, even if you move one QObject to another thread afterwards. I generally use Qt::QueuedConnection explicitly if I know that the QObjects are in different threads.

Qt::DirectConnection

A Qt::DirectConnection is the connection with the most minimal overhead you can get with Signals & Slots. You can visualize it that way: If you call the Signal the method generated by Qt for you calls all Slots in place and then returns.

Qt::QueuedConnection

The Qt::QueuedConnection will ensure that the Slot is called in the thread of the corresponding QObject. It uses the fact, that every thread in Qt (QThread) has a Event-queue by default. So if you call the Signal of the QObject the method generated by Qt will enqueue the command to call the Slot in the Event-queue of the other QObjects thread. The Signal-method returns immediately after enqueuing the command. To ensure all parameters exist within the other threads scope, they have to be copied. The meta-object system of Qt has to know all of the parameter types to be capable of that (see qRegisterMetaType).

Qt::BlockingQueuedConnection

Qt Signal Slot Not Called Two

A Qt::BlockingQueuedConnection is like a Qt::QueuedConnection but the Signal-method will block until the Slot returns. If you use this connection type on QObjects that are in the same thread you will have a deadlock. And no one likes deadlocks (at least I don’t know anyone).

Qt::UniqueConnection

Qt::UniqueConnection is not really a connection type but a modifier. If you use this flag you are not able to connect the same connection again. But if you try it QObject::connect(...) will fail and return false.

This is not everything you will ever need to know about Signals & Slots but with this information you can cover about 80% of all use-cases (in my opinion).If it happens and you need the other 20% of information, I’ll give you some good links to search your specific problem on:

The Qt documentation:

Very deep understanding:

Part1: https://woboq.com/blog/how-qt-signals-slots-work.html

Part2: https://woboq.com/blog/how-qt-signals-slots-work-part2-qt5.html

Part3: https://woboq.com/blog/how-qt-signals-slots-work-part3-queuedconnection.html

Qt Signal Slot Not Called Money

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