Forcing a derived class to overload a virtual method in a non-abstract base class

Question

I'm writing some code using a simple clone pattern, I'd like it if I were able to force derived classes to override that clone pattern, but retain the ability to use my base class. (So I don't want to declare the clone method to be pure virtual.)

Is there anyway to enforce this restriction at the compiler level?

Even if your base class `clone` method was pure virtual that wouldn't force the child of a child to implement it. You could achieve this if you required derived classes to include a macro of some sort in their declaration (that declared the overridden method so they have to implement it), but that's not ideal. — paddy, Sep 28 '12 at 02:34
`virtual` implies dynamic polymorphism, which is inherantly a runtime operation, and invariants must be enforced with runtime checks. If you want compile-time operations and invariant checks, use [CRTP](http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern) for static polymorphism instead, and something like this becomes completely trivial. — ildjarn, Sep 28 '12 at 03:08

score 4 · Accepted Answer · answered Sep 28 '12 at 02:32

4

Unfortunately there is just no way to make this happen in C++. You can't force a non-abstract method to be overridden in child classes. However, I might note that concrete base classes should be fairly rare in C++ and you might want to reconsider your design. With more information about your overall aims we might be able to provide a better answer for your precise needs.

answered Sep 28 '12 at 02:32

Mark B

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Ah, I was afraid of that. Well, I am in the middle of a redesign anyway, so I'll muddle through somehow. Thank you very much. – OmnipotentEntity Sep 28 '12 at 02:34

score 1 · Answer 2 · answered Sep 28 '12 at 02:56

1

It has been some time I touched C++, but I do remember you can have pure virtual method with body.

// in header
class YourBase {
public:
  virtual Foo bar() = 0;
};

// in source
Foo YourBase::bar() {
  // a default impl
}

That should force the child class to override bar(), while leaving a usable impl of bar() in YourBase

answered Sep 28 '12 at 02:56

Adrian Shum

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the implementation doesn't help, since it's no use in using a cloning implementation that's cloning only the abstract class part. the only effect is that `YourBase` cannot be separately instantiated, since it's abstract. – Cheers and hth. - Alf Sep 28 '12 at 03:01
maybe your "ability to use my base class" is a bit too vague. I thought you mean ability to use the impl in base class in derived class. – Adrian Shum Sep 28 '12 at 03:50

score 1 · Answer 3 · answered Sep 28 '12 at 02:59

Unfortunately you can't enforce at compile time that a class overrides a method of a concrete base class, but you can simply assert in each clone function implementation that the type is the type of the class where that implementation resides,

assert( typeid( *this ) == typeid( ThisClass ) );

and then run a test that exercises the cloning functionality of every class.

Forcing a derived class to overload a virtual method in a non-abstract base class

3 Answers3

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