Qt5 alpha has been released. One of the features which I have been working on is a new syntax for signals and slot.This blog entry will present it.
The QTimer class provides a high-level programming interface for timers. To use it, create a QTimer, connect its timeout signal to the appropriate slots, and call start. From then on, it will emit the timeout signal at constant intervals. Example for a one second (1000 millisecond) timer (from the Analog Clock example). 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. The QTimer class provides a high-level programming interface for timers. To use it, create a QTimer, connect its timeout signal to the appropriate slots, and call start. From then on, it will emit the timeout signal at constant intervals. Example for a one second (1000 millisecond) timer (from the Analog Clock example). The QTimer class provides a high-level programming interface for: 59: timers. To use it, create a QTimer, connect its timeout signal: 60: to the appropriate slots, and call start. From then on, it will: 61: emit the timeout signal at constant intervals. 62: 63: Example for a one second (1000 millisecond) timer (from the: 64.
Here is how you would connect a signal to a slot:
What really happens behind the scenes is that the SIGNAL and SLOT macros will convert their argument to a string. Then QObject::connect() will compare those strings with the introspection data collected by the moc tool.
What's the problem with this syntax?
While working fine in general, we can identify some issues:
- No compile time check: All the checks are done at run-time by parsing the strings. That means if you do a typo in the name of the signal or the slot, it will compile but the connection will not be made, and you will only notice a warning in the standard output.
- Since it operates on the strings, the type names of the slot must match exactly the ones of the signal. And they also need to be the same in the header and in the connect statement. This means it won't work nicely if you want to use
typedefor namespaces
In the upcoming Qt5, an alternative syntax exist. The former syntax will still work. But you can now also use this new way of connecting your signals to your slots:
Which one is the more beautiful is a matter of taste. One can quickly get used to the new syntax.
So apart from the aesthetic point of view, let us go over some of the things that it brings us:
Compile-time checking
You will get a compiler error if you misspelled the signal or slot name, or if the arguments of your slot do not match those from the signal.
This might save you some time while you are doing some re-factoring and change the name or arguments of signals or slots.
An effort has been made, using static_assert to get nice compile errors if the arguments do not match or of you miss a Q_OBJECT
Arguments automatic type conversion
Not only you can now use typedef or namespaces properly, but you can also connect signalsto slots that take arguments of different types if an implicit conversion is possible
In the following example, we connect a signal that has a QString as a parameter to a slot that takes a QVariant. It works because QVariant has an implicit constructor that takes a QString
Connecting to any function
As you might have seen in the previous example, the slot was just declared as publicand not as slot. Qt will indeed call directly the function pointer of the slot, andwill not need moc introspection anymore. (It still needs it for the signal)
But what we can also do is connecting to any function or functor:
This can become very powerful when you associate that with boost or tr1::bind.
C++11 lambda expressions
Qtimer Start
Everything documented here works with the plain old C++98. But if you use compiler that supportsC++11, I really recommend you to use some of the language's new features.Lambda expressions are supportedby at least MSVC 2010, GCC 4.5, clang 3.1. For the last two, you need to pass -std=c++0x asa flag.
You can then write code like:
This allows you to write asynchronous code very easily.
Qtimer Timeout
Update: Also have a look what other C++11 features Qt5 offers.
What's the problem with this syntax?
While working fine in general, we can identify some issues:
- No compile time check: All the checks are done at run-time by parsing the strings. That means if you do a typo in the name of the signal or the slot, it will compile but the connection will not be made, and you will only notice a warning in the standard output.
- Since it operates on the strings, the type names of the slot must match exactly the ones of the signal. And they also need to be the same in the header and in the connect statement. This means it won't work nicely if you want to use
typedefor namespaces
In the upcoming Qt5, an alternative syntax exist. The former syntax will still work. But you can now also use this new way of connecting your signals to your slots:
Which one is the more beautiful is a matter of taste. One can quickly get used to the new syntax.
So apart from the aesthetic point of view, let us go over some of the things that it brings us:
Compile-time checking
You will get a compiler error if you misspelled the signal or slot name, or if the arguments of your slot do not match those from the signal.
This might save you some time while you are doing some re-factoring and change the name or arguments of signals or slots.
An effort has been made, using static_assert to get nice compile errors if the arguments do not match or of you miss a Q_OBJECT
Arguments automatic type conversion
Not only you can now use typedef or namespaces properly, but you can also connect signalsto slots that take arguments of different types if an implicit conversion is possible
In the following example, we connect a signal that has a QString as a parameter to a slot that takes a QVariant. It works because QVariant has an implicit constructor that takes a QString
Connecting to any function
As you might have seen in the previous example, the slot was just declared as publicand not as slot. Qt will indeed call directly the function pointer of the slot, andwill not need moc introspection anymore. (It still needs it for the signal)
But what we can also do is connecting to any function or functor:
This can become very powerful when you associate that with boost or tr1::bind.
C++11 lambda expressions
Qtimer Start
Everything documented here works with the plain old C++98. But if you use compiler that supportsC++11, I really recommend you to use some of the language's new features.Lambda expressions are supportedby at least MSVC 2010, GCC 4.5, clang 3.1. For the last two, you need to pass -std=c++0x asa flag.
You can then write code like:
This allows you to write asynchronous code very easily.
Qtimer Timeout
Update: Also have a look what other C++11 features Qt5 offers.
Qtimer Python
It is time to try it out. Check out the alpha and start playing. Don't hesistate to report bugs.
