I have been asking that for some time now, i am afraid you won't get much
of an audience.
You can get rid of both additional allocation and indirection but it is
not pretty. We could definitely use some help/sugar on this.
http://www.artima.com/cppsource/backyard3.html
On Thu, 19 Jan 2012 22:48:39 +0200, Roberto Caravani <[email protected]>
wrote:
Qt for example uses the pimpl idiom for achieving ABI compatibility
between releases. The problem is additional heap allocations, additional
indirection and you pay for it, whether needed or not. (For example even
derived classes in the same library pay for it.)
I wondered whether this would still be necessary in D and I think not.
In D, as interface files are automatically generated, it could be
possible to have ones created with let's say a special "@private_impl"
property or something. For these classes the object size would have to
be stored as hidden static member in the library. The new operator could
then simply read the size and allocate the needed space. Derived class
methods can also use the size to calculate the offset of the derived
class data members.
So you would lose some optimizations, e.g. initializing of base members
can't be inlined and stuff. But this is not possible with pimpl either
and you gain the following:
- You only pay for it if you want it. You can also use the standard .di
file and lose the ABI compatiblity between versions if you so want and
derived classes in the same library do not need to pay any additional
overhead either.
- It is completely transparent: If you later on decide you need ABI
compatibility between releases, it's just a matter of a compiler switch
and differend .di files.
- I think it will also be more efficient than pimpl in all regards.
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?
Best regards,
Robert
