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POIs it possible to make a factory in C++ that complies with the open/closed principle?
 

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  1. POIs it possible to make a factory in C++ that complies with the open/closed principle?
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    <p>In a project I'm working on in C++, I need to create objects for messages as they come in over the wire. I'm currently using the <a href="http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.12" rel="nofollow noreferrer">factory method pattern</a> to hide the creation of objects:</p> <pre><code>// very psuedo-codey Message* MessageFactory::CreateMessage(InputStream&amp; stream) { char header = stream.ReadByte(); switch (header) { case MessageOne::Header: return new MessageOne(stream); case MessageTwo::Header: return new MessageTwo(stream); // etc. } } </code></pre> <p>The problem I have with this is that I'm lazy and don't like writing the names of the classes in two places! </p> <p>In C# I would do this with some reflection on first use of the factory (bonus question: that's an OK use of reflection, right?) but since C++ lacks reflection, this is off the table. I thought about using a registry of some sort so that the messages would register themselves with the factory at startup, but this is hampered by the <a href="http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.12" rel="nofollow noreferrer">non-deterministic (or at least implementation-specific) static initialization order problem</a>. </p> <p>So the question is, is it possible to implement this type of factory in C++ while respecting the open/closed principle, and how?</p> <p>EDIT: Apparently I'm overthinking this. I intended this question to be a "how would you do this in C++" since it's really easy to do with reflection in other languages. </p>
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    1. USNeil Williams
    1. USNeil Williams
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      1. POIs it possible to make a factory in C++ that complies with the open/closed principle?
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    1. COIt's easy with python too because you can read string (like message tag/id) and lookup a class name in your file and then instantiate the class from which you can instantiate objects. I don't think there's an 'easy' way to do it in C++ though.
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    2. COI don't see why static initialization order would prevent a registry. First, it doesn't have to (and probably shouldn't be) static, and second, even if it is, the initialization order only becomes a problem if it is accessed from other statics. And if it is, you have far greater problems (overuse of globals) than this.
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    3. COWhat I meant is that you couldn't do something akin to what epatel suggests without using a singleton or relying on static initialization order. But it's all moot now since it's overengineering.
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