CODEWITHSUNDEEP
c++
jmasterx
jmasterx

Reputation: 54173

Can a base class implementation stand in for a pure virtual method of an interface?

Say I have CFooer and it implements method void foo().

I then have ISomething that has virtual void foo() = 0.

I now create a class:

class CSuperFoo : public CBase, public CFooer, public ISomething
{
};

When I instance CSuperFoo, will I get a cannot instance abstract class error, or would this use CFooer's implementation to satisfy ISomething. If this does not work, why doesn't it?

Thanks

Upvotes: 3

Views: 179

Answers (5)

David Rodríguez - dribeas
David Rodríguez - dribeas

Reputation: 208436

There are different cases to consider here. The first thing is what is the exact meaning of your sentence: CFooer implements method void foo(). If the meaning of that sentence is that there is a function with the same signature of the pure abstract function in ISomething, then that is not implementing the function on the interface.

If CFooer, on the other hand derives from ISomething interface, then CFooer::foo does implement the function and provides a final overrider. The problem is that because CSuperFoo derives from both CFooer and ISomething, it contains two different subobjects of type ISomething, and CFooer is only providing a final-overrider for one of them, and CSuperFooer is still abstract.

If that is the case, changing the inheritance both in CFooer and CSuperFooer from ISomething to virtual will solve the issue (virtual inheritance is designed so that even if the type is mentioned in different levels or branches of the hierarchy tree, all of those that are marked as virtual represent a single unique object. CSuperFooer will have a single ISomething subobject, and the final overrider can be that of CFooer.

Upvotes: 0

ritesh
ritesh

Reputation: 992

As per the C++ FAQ this is "legal, but not moral".

http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.8

Essentially, you will "hide" the CFooer::foo() by implementing a virtual or non-virtual method with the same name foo in the derived class.

Upvotes: 0

dchhetri
dchhetri

Reputation: 7136

Short answer: No you cannot instantiate it.

Explanation: First you cannot instantiate a abstract class. An abstract class is a class that has a pure virtual function. You know that. Now any class that derives from this abstract class is still abstract until it implements those pure virtual functions. So if I SuperFoo inherits from ISomething, then until it implements the pure virtual functions declared in ISomething, SuperFoo will stay abstract. Now you may think that well, CFooer implements that pure virtual function declared in ISomething, and hence SuperFoo implements it. But if you think that then your forgetting something, namely function hiding, see, now SuperFoo definition of the pure virtual function gets hidden from the function prototype in ISomething, hence the pure virtual function needs to be defined. All of this assumes CFooer inherits from ISomething, since it has the same function definition and if not then your doing it wrong, actually you probably should even be in this situation.

One more thing to add is that if CFooer does inherit from ISomething, as it probably should, then there is no need for ISomething to be part of CSuperFoo. You can simply declare the defined function in CFooer as virtual and then again implement it in SuperFoo. There are also more problems that are possible depending on the minor detail of your code, such are the need for virtual inheritance.

Upvotes: 0

Alok Save
Alok Save

Reputation: 206596

You will have to provide an implementation in the Derived class CSuperFoo to be able to create objects of it, otherwise it will be an Abstract class.

The method foo() from the Base class ISomething needs to be overridden in the deriving class CSuperFoo explicitly because compiler cannot see any relation between method from class CFooer and ISomething::foo() as there is no relation between CFooer & ISomething, SO it demands the specific method being implemented in the derived class so that it is not an Abstract class anymore.

Does'nt work on Ideone.

Upvotes: 3

Mark B
Mark B

Reputation: 96291

If CFooer virtually inherits from ISomething I believe it should work ok (I didn't test it). If it doesn't inherit virtually then it will be treated as two distinct methods, and the abstract one will need to be overridden.

Typically in C++ one doesn't use inheritance to implement an interface though and see this code seems like a code smell at first glance. There might be a better, more C++ idiomatic approach if you gave us more details.

Upvotes: 3

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