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    <p>After considering some other aspects of the system as well such as interfaces in .NET, I am starting to think maybe it's better - perhaps even more-or-less necessary - to not even use the C++ virtual calling mechanism at all. The more I consider, the messier using that mechanism is getting.</p> <p>In this approach, each user object class would have a separate struct for the vtable (perhaps kept in a separate namespace like <code>vtabletype::</code>. The generated class would have a pointer member that would be initialized through some trickery to point to a static instance of the vtable. Virtual calls would explicitly use a member pointer from that vtable.</p> <p>If done properly this should have the same performance as the compiler's own implementation would. I've confirmed it does on VC2008. (By contrast, just using straight C, which is what I was planning on earlier, would likely <em>not</em> perform as well, since compilers often optimize <code>this</code> into a register.)</p> <p>It would be hellish to write code like this manually, but of course this isn't a concern for a generator. This approach does have some advantages in this application:</p> <ul> <li><p>Because it's a much more explicit approach, one can be more sure that it's doing exactly what .NET specifies it should be doing with respect to <code>newslot</code> as well as selection of interface implementations.</p></li> <li><p>It might be more efficient (depending on some internal details) than a more traditional C++ approach to interfaces, which would tend to invoke multiple inheritance.</p></li> <li><p>In .NET, objects are considered to be fully constructed when their <code>.ctor</code> runs. This impacts how virtual functions behave. With explicit knowledge of the vtables, this could be achieved by writing it in during allocation. (Although putting the <code>.ctor</code> code into a normal member function is another option.)</p></li> <li><p>It might avoid redundant data when implementing reflection.</p></li> <li><p>It provides better control and knowledge of object layout, which could be useful for the garbage collector.</p></li> </ul> <p>On the downside, it totally loses the C++ compiler's overloading feature with regard to the vtable entries: those entries are data members, not functions, so there is no overloading. In this case it would be tempting to just number the members (say <code>_0</code>, <code>_1</code>...) This may not be so bad when debugging, since once the pointer is followed, you'll see an actual, properly-named member function anyway.</p> <p>I think I may end up doing it this way but by all means I'd like to hear if there are better options, as this is admittedly a rather complex approach (and problem.)</p>
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