on Wed Nov 02 2011, greened-AT-obbligato.org (David A. Greene) wrote:
Dave Abrahams <dave@boostpro.com> writes:
on Mon Oct 24 2011, Allan Johns <allan.johns-AT-drdstudios.com> wrote:
Just gauging initial interest in a boost.deepcopy library
<pet peeve alert>
Sorry to slam this idea out of the gate, but the whole notion of a "deep copy" is broken and wrong (in my humble opinion). When you say you're going to "deep copy" an object it shows you don't understand the boundaries of that object's value. The object's value is copied by its copy constructor, and compared by its operator== (assuming it has one). If your "deep copy" extends beyond the boundaries of the value, there's no way of knowing how far it should extend.
Fascinating discussion. I'm curious about your statement here. Do you object to "deep copy" as implemented in a copy constructor or the notion of "deep copy" of some aggregate data structure in general?
I object to the terms "shallow copy" and "deep copy," because they're imprecise, and because they tend to confuse the notion of copying (full stop), which is fundamental and well-defined. I want to be able to talk about "copying this list of pointers" without someone saying "wait, do you mean a deep or a shallow copy?" And when they do say that, what do you suppose "shallow" and "deep" mean? It isn't clear: * shallow might mean that the two lists share storage, so that changing the first pointer in the original list changes the first pointer in the copy. * shallow might mean merely that no effort is made to clone the objects being pointed to by the list elements (i.e. ordinary std::list<T*> semantics). So then I need to ask for a definition of shallow/deep. The whole thing is a mess that arises all the time in languages with mutation but without an intrinsic notion of value semantics, but there's no reason we should go into that territory in C++.
For example, I have often needed to clone some branch of a tree data structure where nodes contain pointers to other nodes. For me this is in the context of a compiler and subtree cloning is a convenient way to perform code duplication.
Awesome. Cloning a subtree is a well-defined operation. If nodes in the tree happen to contain pointers that aren't part of the tree's parent/child/sibling link structure, I know they get copied bitwise and that's the end of it.
In the past I have implemented a virtual clone() member for each kind of tree node in order to get polymorphic behavior. clone() clones child nodes and all that.
If your objection covers cases like the above, I am very interested in alternative solutions.
It might make sense to create a class that encapsulates a subtree and whose copy ctor implements the clone operation. Then again, it might not; depends on your application. -- Dave Abrahams BoostPro Computing http://www.boostpro.com