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    <p>Basically Patrick's suggestion is the same as David's which is also the same as mine. Use NVI (non-virtual interface idiom) for this purpose. Pure virtual interfaces lack any kind of centralized control. You could alternatively create a separate abstract base class that all commands inherit, but why bother?</p> <p>For detailed discussion about why NVIs are desirable, see C++ Coding Standards by Herb Sutter. There he goes so far as to suggest making all public functions non-virtual to achieve a strict separation of overridable code from public interface code (which should not be overridable so that you can always have some centralized control and add instrumentation, pre/post-condition checking, and whatever else you need).</p> <pre><code>class Command { public: void operator()() { do_command(); add_to_undo_stack(this); } void undo() { // This might seem pointless now to just call do_undo but // it could become beneficial later if you want to do some // error-checking, for instance, without having to do it // in every single command subclass's undo implementation. do_undo(); } private: virtual void do_command() = 0; virtual void do_undo() = 0; }; </code></pre> <p>If we take a step back and look at the general problem instead of the immediate question being asked, I think Pete offers some very good advice. Making Command responsible for adding itself to the undo stack is not particularly flexible. It can be independent of the container in which it resides. Those higher-level responsibilities should probably be a part of the actual container which you can also make responsible for executing and undoing the command.</p> <p>Nevertheless, it should be very helpful to study NVI. I've seen too many developers write pure virtual interfaces like this out of the historical benefits they had only to add the same code to every subclass that defines it when it need only be implemented in one central place. It is a very handy tool to add to your programming toolbox.</p>
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