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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Why doesn't Qt use templates for signals and slots?</h1>
<p> A simple answer is that when Qt was designed, it was not possible to
fully exploit the template mechanism in multi-platform applications due
to the inadequacies of various compilers. Even today, many widely used
C++ compilers have problems with advanced templates. For example, you
cannot safely rely on partial template instantiation, which is essential
for some non-trivial problem domains. Thus Qt's usage of templates has
to be rather conservative. Keep in mind that Qt is a multi-platform
toolkit, and progress on the Linux/g++ platform does not necessarily
improve the situation elsewhere.
<p> Eventually those compilers with weak template implementations will
improve. But even if all our users had access to a fully standards
compliant modern C++ compiler with excellent template support, we would
not abandon the string-based approach used by our <a href="metaobjects.html#meta-object">meta object</a> compiler.
Here are five reasons why:
<p> <h3>1. Syntax matters</h3>
<p> Syntax isn't just sugar: the syntax we use to express our algorithms can
significantly affect the readability and maintainability of our code.
The syntax used for Qt's signals and slots has proved very successful in
practice. The syntax is intuitive, simple to use and easy to read.
People learning Qt find the syntax helps them understand and utilize the
signals and slots concept -- despite its highly abstract and generic
nature. Furthermore, declaring signals in class definitions ensures that
the signals are protected in the sense of protected C++ member
functions. This helps programmers get their design right from the very
beginning, without even having to think about design patterns.
<p> <h3>2. Precompilers are good</h3>
<p> Qt's <tt><a href="moc.html#moc">moc</a></tt> (Meta Object Compiler) provides a clean way to go
beyond the compiled language's facilities. It does so by generating
additional C++ code which can be compiled by any standard C++ compiler.
The <tt>moc</tt> reads C++ source files. If it finds one or more class
declarations that contain the "Q_OBJECT" macro, it produces another C++
source file which contains the meta object code for those classes. The
C++ source file generated by the <tt>moc</tt> must be compiled and
linked with the implementation of the class (or it can be
<tt>#included</tt> into the class's source file). Typically <tt>moc</tt>
is not called manually, but automatically by the build system, so it
requires no additional effort by the programmer.
<p> There are other precompilers, for example, <tt>rpc</tt> and
<tt>idl</tt>, that enable programs or objects to communicate over
process or machine boundaries. The alternatives to precompilers are
hacked compilers, proprietary languages or graphical programming tools
with dialogs or wizards that generate obscure code. Rather than locking
our customers into a proprietary C++ compiler or into a particular
Integrated Development Environment, we enable them to use whatever tools
they prefer. Instead of forcing programmers to add generated code into
source repositories, we encourage them to add our tools to their build
system: cleaner, safer and more in the spirit of UNIX.
<p> <h3>3. Flexibility is king</h3>
<p> C++ is a standarized, powerful and elaborate general-purpose language.
It's the only language that is exploited on such a wide range of
software projects, spanning every kind of application from entire
operating systems, database servers and high end graphics
applications to common desktop applications. One of the keys to C++'s
success is its scalable language design that focuses on maximum
performance and minimal memory consumption whilst still maintaining
ANSI-C compatibility.
<p> For all these advantages, there are some downsides. For C++, the static
object model is a clear disadvantage over the dynamic messaging approach
of Objective C when it comes to component-based graphical user interface
programming. What's good for a high end database server or an operating
system isn't necessarily the right design choice for a GUI frontend.
With <tt>moc</tt>, we have turned this disadvantage into an advantage,
and added the flexibility required to meet the challenge of safe and
efficient graphical user interface programming.
<p> Our approach goes far beyond anything you can do with templates. For
example, we can have object properties. And we can have overloaded
signals and slots, which feels natural when programming in a language
where overloads are a key concept. Our signals add zero bytes to the
size of a class instance, which means we can add new signals without
breaking binary compatibility. Because we do not rely on excessive
inlining as done with templates, we can keep the code size smaller.
Adding new connections just expands to a simple function call rather
than a complex template function.
<p> Another benefit is that we can explore an object's signals and slots at
runtime. We can establish connections using type-safe call-by-name,
without having to know the exact types of the objects we are connecting.
This is impossible with a template based solution. This kind of runtime
introspection opens up new possibilities, for example GUIs that are
generated and connected from Qt Designer's XML <tt>ui</tt> files.
<p> <h3>4. Calling performance is not everything</h3>
<p> Qt's signals and slots implementation is not as fast as a template-based
solution. While emitting a signal is approximately the cost of four
ordinary function calls with common template implementations, Qt
requires effort comparable to about ten function calls. This is not
surprising since the Qt mechanism includes a generic marshaller,
introspection and ultimately scriptability. It does not rely on
excessive inlining and code expansion and it provides unmatched runtime
safety. Qt's iterators are safe while those of faster template-based
systems are not. Even during the process of emitting a signal to several
receivers, those receivers can be deleted safely without your program
crashing. Without this safety, your application would eventually crash
with a difficult to debug free'd memory read or write error.
<p> Nonetheless, couldn't a template-based solution improve the performance
of an application using signals and slots? While it is true that Qt adds
a small overhead to the cost of calling a slot through a signal, the
cost of the call is only a small proportion of the entire cost of a
slot. Benchmarking against Qt's signals and slots system is typically
done with empty slots. As soon as you do anything useful in your slots,
for example a few simple string operations, the calling overhead becomes
negligible. Qt's system is so optimized that anything that requires
operator new or delete (for example, string operations or
inserting/removing something from a template container) is significantly
more expensive than emitting a signal.
<p> Aside: If you have a signals and slots connection in a tight inner loop
of a performance critical task and you identify this connection as the
bottleneck, think about using the standard listener-interface pattern
rather than signals and slots. In cases where this occurs, you probably
only require a 1:1 connection anyway. For example, if you have an object
that downloads data from the network, it's a perfectly sensible design
to use a signal to indicate that the requested data arrived. But if you
need to send out every single byte one by one to a consumer, use a
listener interface rather than signals and slots.
<p> <h3>5. No limits</h3>
<p> Because we had the <tt>moc</tt> for signals and slots, we could add
other useful things to it that could not not be done with templates.
Among these are scoped translations via a generated <tt>tr()</tt>
function, and an advanced property system with introspection and
extended runtime type information. The property system alone is a great
advantage: a powerful and generic user interface design tool like Qt
Designer would be a lot harder to write - if not impossible - without a
powerful and introspective property system.
<p> C++ with the <tt>moc</tt> preprocessor essentially gives us the
flexibility of Objective-C or of a Java Runtime Environment, while
maintaining C++'s unique performance and scalability advantages. It is
what makes Qt the flexible and comfortable tool we have today.
<p>
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