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.
Qt will indeed call directly the function pointer of the slot, and will not need moc introspection anymore. (It still needs it for the signal) (It still needs it for the signal) But what we can also do is connecting to any function or functor. 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.
New Syntax in Qt5
Hi, I have a QTabWidget with many page. When the user is clicking on the first tab, I would like to update informations. Moreover, I don't want to use a thread for it, to not overthread my applcation. This is what I did: void MaFenetre::MaFenetre: QWi. There is no way to safely iterate the list of signal-slot connections without holding Qt's internal mutexes/semaphores. The signals and slots can come and go at any time, so at best you'd get a list. 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.
The new syntax looks like this:
Why the new syntax?
Dans windows 10. 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.
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
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
->* 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 to
void* because they have a different
If the function varies slightly in signature, you cannot convert from one to the other.For example, even converting from
void (MyClass::*)(int) const to
void (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.
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.
Qt Signals Slots Lambda
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.
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
receiverare not just
QObject* as the documentation points out. They are pointers to
typename 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 of
Q_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
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.
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
The reason why we pass
&slot as a
void** is only tobe able to compare it if the type is
We also pass the
&signal as a
void**.It is a pointer to the member function pointer. (Yes, a pointer to the pointer)
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.
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
QFunctorSlotObject 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.
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.
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
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 in
QAbstractItemModel, 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.
Qt Signal Slot Parameter
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 is
QtPrivate::List that can represent a list of types. Some helpers to operate on it are
QtPrivate::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 a
template<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
void for the end of the list.
The same example as before would be:
In the function
args 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. 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
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.
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.