On 2/3/02 11:54 PM, "Jon Wang" <webmaster@boost.100ok.com> wrote:
1. Mr Stroustrup said that there wasn't a need for auto_array, for "a better solution is to use a vector". So, should we use vector or scoped_array/shared_array?
You should normally use std::vector unless you have some special reason to use boost:: scoped_array and boost::shared_array. Those classes are provided both for completeness, and because occasionally there are such special reasons. But not often.
2. In the document, four implementations are said to have been discussed: Direct detached, Indirect detached, Embedded detached and Placement detached. I've tried to describle them (Are I right? See the attached file), but I don't understand the Placement detached. Could someone please tell me what it's like?
The concept here is that the object is allocated in a special way, which require use of placement, so the count is in a header before the object. This is a form of the "embedded attached" scheme that does not require changes to the original object's class, but it does require a special form of new, so you couldn't just call the normal "new" and then give the object to such a smart pointer. [Note: There was no attached file.]
3. In the file smart_ptr.hpp, we can see the private function shared_ptr::share.
I'm sorry that I don't understand that reason why ++*rpn should be done before dispose(). Would someone please give me an example? Or is it like this?
This function is gone from the latest implementation of shared_ptr slated for the next boost release, so it's probably not worthwhile to discuss this particular aspect of the old implementation in detail. The code you enclosed looks something like the kind of code that caused trouble -- the key is that deleting the old object pointed to can end up deleting the new shared pointer in some cases, and this must not happen before incrementing the reference count. The new implementation avoids this issue by using the well-known "copy and swap" idiom for implementing assignment, and is easier to understand.
4. Why not use policies? I'm afraid that the word "parameterization will discourage users" might not be always true, for we can use default parameters.
We'll probably be using policies in future version, if we can work out all the issues. Andrei Alexandrescu is considering submitting his smart pointer class template from his Loki library to Boost for formal review, which is the most likely candidate for a Boost policy-based smart pointer. If it turns out well enough, it may replace all the existing smart pointer class templates. -- Darin