C++ pimpl

[Walter Bright]

On 1/20/2012 2:12 AM, Robert Caravani wrote:
> First of all thanks for this fast answer!
>
>> On 1/19/2012 12:48 PM, Roberto Caravani wrote:
>>> I think this would be a real neat and very important feature, when it
>>> comes to shared libraries. Is there any plan to implement something
>>> like that in the future? Do I miss something?
>>
>> The pimpl pros and cons are the same for C++ and D.
>
> Well, yes they are currently. But as D has pure reference semantics for
> classes, it would be easier to handle the case when the size of an object is
> not known at compile time. It basically comes down to having operator new read
> the size from the library and having derived classes members do some offset
> calculation. (Member initialization would have to be done by a function) Is
> there something missing?
>
> Is there a flaw in my proposal? Isn't it worth the effort? What are the
> problems?
>
> In Java pimpl for ABI compatibility seems not to be neccessary, according to:
> http://java.sun.com/docs/books/jls/second_edition/html/binaryComp.doc.html
> (It states that you can freely add and remove private fields)
> but ok, Java is not a system programming language, they might cheat somehow.

Java can recompile things on the fly.


> Such a feature seems really cool to me, especially for library developers,
> because they have to care less that they are implementing a dynamic library.
> And you can easily avoid the costs if you don't need it, no rewrite necessary,
> just a recompilation of the program. (Not even the library, if you don't care
> about the hidden static size member and the initialization methods)
>
> Maybe I am completely wrong, but if so I want to know
>
> This is not really a feature request, as there are far more important things
> to do at the moment. This is more a: "Is it possible?" "Would you like to have
> it, if someone else implemented it?"

What you're asking for is dynamic typing. You can achieve something equivalent 
using opDispatch and std.variant.