Re: [boost] change to weak_ptr.hpp to compile under Builder C++ 5 , etc

From: Thomas Witt (witt_at_[hidden])
Date: 2002-05-15 06:21:59

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On Wednesday 15 May 2002 12:40, Bjorn.Karlsson_at_[hidden] wrote:
> make_shared returns a shared_ptr that may contain a null pointer (thus, the
> resulting shared_ptr should be tested before use). However, if the returned
> shared_ptr contains a valid resource, then that shared_ptr can be safely
> used because the reference count has been increased.

Is it right to say that this is only an issue, if pointers are shared between 
threads? AFAICT in a single threaded environment you can safely use a 
weak_ptr after testing it.

> That's not true for
> weak_ptr, and that's why make_shared (and the constructor for shared_ptr
> taking a weak_ptr as an argument) adds safety.
>
> Bjorn Karlsson

The problem I have with this change is that it not only breaks my code but it 
breaks my design.

Up to now I have used shared <-> weak ptr to communicate ownership. Shared ptr 
implies shared ownership whereas a weak_ptr implies non ownership by the 
entity holding the weak_ptr. Think of an object holding a shared_ptr<smth.> 
internally and only providing access via weak_ptr. Clients can still stow 
away the weak ptr. But the usage of weak_ptr clearly communicates: "Take care 
it might go away." and in advantage to bald pointers the client can take care 
as he can query the weak ptr about this.

Providing weak -> shared conversion completely breaks this design as clients 
can take ownership whenever they like.

Thomas

- -- 
Dipl.-Ing. Thomas Witt
Institut fuer Verkehrswesen, Eisenbahnbau und -betrieb, Universitaet Hannover
voice: [phone] - 4273, fax: [phone]
http://www.ive.uni-hannover.de
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