[linux-audio-dev] (OT) C++ flame war

[David Olofson]

On Thursday 06 February 2003 16.17, Martijn Sipkema wrote:
[...classes with private stuff hidden...]
> It doesn't work that way. You cannot create an class instance
> without its full declaration. I don't see the problem of having the
> private part of a class in the header. If you want to be seperated
> from the implementation, use an abstract class. But then you cannot
> create a class instance, you can only use the provided interface on
> an already created instance (, but you can delete it when there is
> a virtual destructor in the interface).

BTW, notice the similarity with C? If you don't know *every* detail of 
a struct, you can't create an instance of one, because you don't know 
it's *size*. The 'new' operator of C++ (and "static" instances for 
that matter) have exactly the same problem, which is why things are 
the way they are.

So, the basic problem is that it's not the constructor that allocates 
memory for the instance; it's the code generated by the 'new' 
operator.

There are ways around this, of course. For example, you can "wrap" the 
constructor of a class in a static member of the class, that performs 
the instantiation and returns a pointer. The "fake" constructor would 
actually give you an instance of a "secret" derived class that 
contains the implementation.

BTW, I believe this is about the way normal constructors work in 
Delpih/Object Pascal - which is why you cannot have static instances 
at all. No matter what you type, what you get is always a pointer.


> > note that the private stuff would be in a file that looked just
> > like the class declaration, but had no public (and protected?)
> > declarations. in other words, the class declaration it includes
> > is implicitly 100% private, but it can include header files that
> > are necessary for the private declarations.
>
> I don't see what this would solve and I don't think this is even
> possible. Changing the private part will break binary compatibility
> (without changing the public header).

Exactly. I've explained the reason why it's impossible above.


//David Olofson - Programmer, Composer, Open Source Advocate

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