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From: David Abrahams (david.abrahams_at_[hidden])
Date: 2001-09-04 07:11:43

----- Original Message -----
From: "Greg Colvin" <gcolvin_at_[hidden]>

> > Taking a step back, we can come up with any number of designs, but
> > we talk about what the designs should accomplish first?
> We have been, but further discussion is welcome.
> What I'm not trying to accomplish is to parameterize shared_ptr to
> generate a large family of related classes. That is a bigger project
> that should probably start fresh, and which will indeed require a
> clear delineation of a family of Pointer concepts.

Agreed. That's what I think we should do. I don't think that mutating
shared_ptr a little bit just to satisfy one person's needs is the way to go.
It is a relatively simple class, and should remain so.

> What I am trying to do is pull together the discussions and experimental
> code and feature requests built up over the years into a new version of
> shared_ptr (and shared_array). The desiderata for me are:
> 1) Shared_ptr remains one class, with one interface and one semantics.
> Libraries should be able to use shared_ptr as a common means of
> communication without needing to templatize their interfaces by
> (smart) pointer type.

I don't understand the 2nd sentence above.

> 2) It should be possible for users to easily customize the deletion
> function for objects that are not created with operator new. For
> example the many, many C interfaces that manage resouces with a pair
> of pointer-returning functions for acquistion and release. Peter's
> code has an implementation of this idea.


> 3) It should be possible to use shared_ptr with no risk of undefined
> behavior, although for historical reasons some unsafe functions may
> remain, such as construction and reset from raw pointers. These
> functions proven to be a source of difficulty and error. Right
> now I am leaning towards a static create() function with varying
> numbers of parameters, initially faked with a large number of
> create functions, but perhaps done right in the future with a
> language extension.


> 4) It should be possible for the skilled user to modify the underlying
> implementation to make some performance and safety trade-offs
> with out affecting the interface. The current idea is to expose
> the underlying counter object for customization, but it isn't
> clear whether that will prove useful enough to be worth the price.

My only problem with all this is that while you're trying to stretch what is
currently a very simple class to cover many more bases, it's clear that it's
not going to cover /my/ base the way a policy-based smart pointer would. I
suspect that's going to be true of quite a few people.

IMO, simple things should be kept simple. To handle more complicated jobs, a
new design is in order. This is especially true if you envision that
"skilled" users will actually have to modify the source code of shared_ptr
for their own needs. I consider myself somewhat skilled, and whatever skill
I have contributes to this fact: I would rather write a new policy-based
smart pointer than generate even one redundant copy of boost's shared_ptr

> I am also aware of the need, over the next year, to get a version of
> shared_ptr ready for the C++ committee. So it is important to keep
> the interface clean, and the design such that the more advanced
> features can be dropped or modified by the committee without disturbing
> the core functionality. In particular, doing (4) will probably reveal
> more implementation detail than is appropriate for a standard class.

I think you should seriously consider floating the current design without
modification, or possibly with the addition of static create functions. It


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