Ross Levine wrote:
I was wondering if boost has considered a smart pointer that is similar to scoped_ptr but supports deep copy semantics. This would be useful in classes which use the pimpl idiom, as a common way to program these classes is with a scoped_ptr or auto_ptr, but then a copy constructor and copy assignment operator needs to be defined. Herb Sutter worked on an implementation at http://www.gotw.ca/gotw/062.htm, so there's precedent. Is there any interest for this?
I believe the reason why there is no such thing in Boost is that there is no agreement about how the copying would work, and also about how operator= would work either. Indeed, one could argue the most natural way to enhance those is to layer on top of virtual functions, but virtual operator= has limitations and the language provides no virtual copy constructor facility (and if it did, it would need to change its allocation model anyway). Some methods nevertheless exist, with allocation a real problem there since it is often too tightly coupled with construction. Another approach would be to ditch the virtual function model altogether and use boost.any-like techniques instead, but that means additional space overhead as well as possibly decayed type information. I think none ever managed to satisfy everyone.