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/*!

    \page templates.html

    \title Why Doesn't Qt Use Templates for Signals and Slots?

    \brief The reasoning behind Qt's implementation of signals and slots.

    Templates are a builtin mechanism in C++ that allows the compiler to

    generate code on the fly, depending on the type of the arguments

    passed. As such, templates are highly interesting to framework

    creators, and we do use advanced templates in many places

    in Qt. However, there are limitations: There are things that you can

    easily express with templates, and there are things that are

    impossible to express with templates. A generic vector container class

    is easily expressible, even with partial specialisation for pointer

    types, while a function that sets up a graphical user interface based

    on a XML description given as a string is not expressible as

    template. And then there is gray area in between. Things that you can

    hack with templates at the cost of code size, readability,

    portability, usability, extensability, robustness and ultimately

    design beauty. Both templates and the C preprocessor can be stretched

    to do incredibility smart and mind boggling things. But just because

    those things can be done, does not necessarily mean doing them is the

    right design choice.

    There is an important practical challenge we have to mention: due to

    the inadequacies of various compilers it is still not possible to

    fully exploit the template mechanism in cross-platform

    applications. Code unfortunately is not meant to be published in

    books, but compiled with real-world compilers on real-world operating

    system. Even today, many widely used C++ compilers have problems with

    advanced templates. For example, you cannot safely rely on partial

    template specialisation, which is essential for some non-trivial

    problem domains. Some compilers also have limitations with regards to

    template member functions, which make it hard to combine generic

    programming with object orientated programming. However, we do not

    perceive these problems as a serious limitation in our work. 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 meta object compiler for a template

    based signals and slots system. Here are five reasons why:

    \section1 Syntax matters

    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.

    \section1 Code Generators are Good

    Qt's \c{moc} (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 \c{moc} 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 \c{moc} must be compiled and

    linked with the implementation of the class (or it can be

    \c{#included} into the class's source file). Typically \c{moc}

    is not called manually, but automatically by the build system, so it

    requires no additional effort by the programmer.

    The \c{moc} is not the only code generator Qt is using. Another

    prominent example is the \c{uic} (User Interface Compiler). It

    takes a user interface description in XML and creates C++ code that

    sets up the form. Outside Qt, code generators are common as well. Take

    for example \c{rpc} and \c{idl}, that enable programs or

    objects to communicate over process or machine boundaries. Or the vast

    variety of scanner and parser generators, with \c{lex} and

    \c{yacc} being the most well-known ones. They take a grammar

    specification as input and generate code that implements a state

    machine. The alternatives to code generators are hacked compilers,

    proprietary languages or graphical programming tools with one-way

    dialogs or wizards that generate obscure code during design time

    rather than compile time. 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.

    \section1 GUIs are Dynamic

    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.

    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 \c{moc}, 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.

    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. 

    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 UI files.

    \section1 Calling Performance is Not Everything

    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, queued calls between different

    threads, 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.

    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.

    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.

    \section1 No Limits

    Because we had the \c{moc} for signals and slots, we could add

    other useful things to it that could not be done with templates.

    Among these are scoped translations via a generated \c{tr()}

    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. But it does not

    end here. We also provide a dynamic qobject_cast<T>() mechanism

    that does not rely on the system's RTTI and thus does not share its

    limitations. We use it to safely query interfaces from dynamically

    loaded components. Another application domain are dynamic meta

    objects. We can e.g. take ActiveX components and at runtime create a

    meta object around it. Or we can export Qt components as ActiveX

    components by exporting its meta object. You cannot do either of these

    things with templates.

    C++ with the \c{moc} 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.

*/