Re: Something like a final method

"Alf P. Steinbach" <>
Sat, 21 Mar 2009 18:43:10 +0100
* Marcel M?ller:

I am seeking for a solution for the following performance issue:

class Interface
  virtual int GetStatus() = 0;

class AbstractBase : public Interface
  int Status;
  virtual int GetStatus() { return Status; } // Never overridden

If I call GetStatus() through a pointer to AbstractBase (or any derived
class) the call cannot be inlined because it is a virtual function. Of
course, there is no other way when using a pointer to Interface. But
there are many calls via pointers to AbstractBase or some of the derived
classes. I would like to avoid these function calls since almost any
method of derived classes do it and the implementation of GetStatus is
really trivial.

Is there any better solution than renaming the method of Interface to
GetStatusByInterface() or something like that?


Which you probably need to do anyway, because "status", "get" and lack of
"const" are all design smells.

If you decide to keep the status (it would probably be better to get rid of it
because status checking implies that it's needed implies that valid operations
depend on it implies that you have states that are so distinct they're more like
distinct types, hence e.g. Envelope/Letter pattern would help), then, keeping
it, it might go like this:

   class Interface
       virtual ~Interface() {}
       virtual int status() const = 0;

   class SimpleStatusImpl: public virtual Interface
       int myStatus;
       SimpleStatusImpl(): myStatus(0) {}
       virtual int status() const { return nonVirtualStatus(); }
       int nonVirtualStatus() const { return myStatus; }

   class AbstractBase: public virtual Interface
       // Nothing about status here.

   class SimpleConcrete
       : public AbstractBase
       , public SimpleStatusImpl

The client code that wants fast non-virtual status checking can just call
nonVirtualStatus. Which can be redefined in derived classes. It's up to the
client code to decide whether it knows enough to be able to use it.

I think that if I'd been forced to do this status thing, I'd keep the name
"nonVirtualStatus", because it communicates the purpose of that accessor.

Cheers & hth.,

- Alf

PS: Don't forget the first law of optimization: *measure*. ;-)

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