(Qt) Signals and Slots. Slots Description The moc that connects objects Why Inter-object communication Exmaple Simple example of signals and slots look at. (Qt) Signals and Slots signals and slots basics. GetCode; 기본적 signals and slots 사용법. The PunchingBag inherits from QObject so it can emit signals; it has a signal called punched, which carries no data; and it has a punch method which does nothing but emit the punched signal. To make our PunchingBag useful, we need to connect its punched signal to a slot that does something. In contrast to slots, signals may be handled by none, one or many components. There is no guarantee that triggering a signal in C will actually run QML code, unless there’s a handler defined. Properties work both ways: Properties are read- and write-able from both C and QML.

Hi
I have a Qt 'signal-slot connection'.
I want to test the class with the signal
The class with the slot is mocked/stubbed.
Now I would like to verify that when I emit signal, the slot is
called.
I wonder if it is possible to use gmock to verify this.
// class Tx contains the signal, which is protected so I inherit
// to get access to it
class Test : public ::testing::Test, public Tx
{ public:
MockRx *rx;
void SetUp()
{ rx = new MockRx;
rx->rxsignal(); // Just to see it compiles
rx->gmock_rxsignal(); // Compiles, gmock generated function
QObject::connect(this, SIGNAL(txsignal(), rx, SLOT(rxsignal()));
}
void TearDown() { delete rx; }
};
TEST_F(Test, Signal)
{
EXPECT_CALL(*rx, rxsignal()).Times(1).WillOnce(Return());
this->txsignal();
}
class MockRx : public QWidget
{ Q_OBJECT
public:
MOCK_METHOD0(rxsignal, void());
};
When running, I get the error:
QObject::connect: No such slot as RxMock::rxsignal()
If I remove Q_OBJECT in class MockRx, I get: No such slot as
QWidget::rxsignal()
I also tried to QObject::conntect(.. SLOT(gmock_rxsignal()));
which gives No such slot as RxMock::rxsignal()
If I replace the MockRx class with a class Rx with rxsignal()
everything works.
I wonder why QObject::connect cannot find RxText::rxsignal() when
- it works if I replace with a regular class Rx instead of class
MockRx
- rx->rxsignal() can be called
Is gmock doing something under the hood that prevents this?
Hope someone can help, thanks a lot
Paul

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 Signals And Slots Tutorial

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. Quotes instagram post.

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 e

Qt Signal Slot Two Classes