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copied!<p><strong>Answer One</strong></p> <p>The general technique of deriving a class like this is the <a href="http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern" rel="noreferrer">Curiously Recurring Template Pattern (CRTP)</a>:</p> <pre><code>class PingMessage: public MessageTmpl < 10, PingMessage > </code></pre> <p>Your specific technique of using a template class's static member initialization to register subclasses of that class is (IMO) simply brilliant, and I've never seen that before. A more common approach, used by unit test frameworks like <a href="http://unittest-cpp.sourceforge.net/" rel="noreferrer">UnitTest++</a> and <a href="http://code.google.com/p/googletest/" rel="noreferrer">Google Test</a>, is to provide macros that declare both a class and a separate static variable initializing that class.</p> <p><strong>Answer Two</strong></p> <p>Static variables are initialized in the order listed. If you move your m_List declaration before your MessageFactory::Register calls, you should be safe. Also keep in mind that if you start declaring Message subclasses in more than one file, you'll have to wrap m_List as a singleton and check that it's initialized before each use, due to the <a href="http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.12" rel="noreferrer">C++ static initialization order fiasco</a>.</p> <p><strong>Answer Three</strong></p> <p>C++ compilers will only instantiate template members that are actually used. Static members of template classes is not an area of C++ that I've used much, so I could be wrong here, but it looks like providing the constructor is enough to make the compiler think that MESSAGE_ID is used (thus ensuring that MessageFactory::Register is called).</p> <p>This seems very unintuitive to me, so it may be a compiler bug. (I was testing this in g++ 4.3.2; I'm curious to know how Comeau C++, for example, handles it.)</p> <p>Explicitly instantiating MESSAGE_ID also suffices, at least in g++ 4.3.2:</p> <pre><code>template const uint16_t PingMessage::MESSAGE_ID; </code></pre> <p>But that's even more unnecessary work than providing an empty default constructor.</p> <p>I can't think of a good solution using your current approach; I'd personally be tempted to switch to a technique (such as macros or using a script to generate part of your source files) that relied less on advanced C++. (A script would have the added advantage of easing maintenance of MESSAGE_IDs.)</p> <p><strong>In response to your comments:</strong></p> <p>Singletons are <i>generally</i> to be avoided because they're often overused as poorly disguised global variables. There are a few times, however, when you really do need a global variable, and a global registry of available Message subclasses is one of those times.</p> <p>Yes, the code that you provided is initializing MESSAGE_ID, but I was talking about <a href="http://msdn.microsoft.com/en-us/library/by56e477%28VS.80%29.aspx" rel="noreferrer">explicitly instantiating</a> each subclass's instance of MESSAGE_ID. Explicit instantiation refers to instructing the compiler to instantiate a template even if it thinks that that template instance won't otherwise be used.</p> <p>I suspect that the static function with the volatile assignment is there to trick or force the compiler into generating the MESSAGE_ID assignment (to get around the problems that dash-tom-bang and I pointed out with the compiler or linker dropping or not instantiating the assignment).</p>

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