Delegate part of an interface to a subclass in C++? [duplicate]

Question

Here is what I am talking about

// some guy wrote this, used as a Policy with templates
struct MyWriter {
  void write(std::vector<char> const& data) {
    // ...
  }
};

In some existing code, the people did not use templates, but interfaces+type-erasure

class IWriter {
public:
  virtual ~IWriter() {}

public:
  virtual void write(std::vector<char> const& data) = 0;
};

Someone else wanted to be usable with both approaches and writes

class MyOwnClass: private MyWriter, public IWriter {
  // other stuff
};

MyOwnClass is implemented-in-terms-of MyWriter. Why doesn't MyOwnClass' inherited member functions implement the interface of IWriter automatically? Instead the user has to write forwarding functions that do nothing but call the base class versions, as in

class MyOwnClass: private MyWriter, public IWriter {
public:
  void write(std::vector<char> const& data) {
    MyWriter::write(data);
  }
};

I know that in Java when you have a class that implements an interface and derives from a class that happens to have suitable methods, that base class automatically implements the interface for the derived class.

Why doesn't C++ do that? It seems like a natural thing to have.

Answer 1

This is multiple inheritance, and there are two inherited functions with the same signature, both of which have implementation. That's where C++ is different from Java.

Calling write on an expression whose static type is MyBigClass would therefore be ambiguous as to which of the inherited functions was desired.

If write is only called through base class pointers, then defining write in the derived class is NOT necessary, contrary to the claim in the question. Now that the question changed to include a pure specifier, implementing that function in the derived class is necessary to make the class concrete and instantiable.

MyWriter::write cannot be used for the virtual call mechanism of MyBigClass, because the virtual call mechanism requires a function that accepts an implicit IWriter* const this, and MyWriter::write accepts an implicit MyWriter* const this. A new function is required, which must take into account the address difference between the IWriter subobject and the MyWriter subobject.

It would be theoretically possible for the compiler to create this new function automatically, but it would be fragile, since a change in a base class could suddenly cause a new function to be chosen for forwarding. It's less fragile in Java, where only single inheritance is possible (there's only one choice for what function to forward to), but in C++, which supports full multiple inheritance, the choice is ambiguous, and we haven't even started on diamond inheritance or virtual inheritance yet.

Actually, this problem (difference between subobject addresses) is solved for virtual inheritance. But it requires additional overhead that's not necessary most of the time, and a C++ guiding principle is "you don't pay for what you don't use".