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From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-04-25 05:52:31

From: "Howard Hinnant"
> On Apr 24, 2004, at 2:04 PM, Peter Dimov wrote:
> >> why are we trying to built thread-safety into shared_ptr? There is
> >> good
> >> saying that drove C++ to the point where we are right now: "do not pay
> >> for what you do not use". If I'm willing to provide own
> >> synchronization
> >> for shared pointers,
> >
> > You can't. Not unless you control the entire program and you can
> > determine
> > with certainty which shared_ptr instances share ownership at every
> > particular moment.
> Actually I'm not convinced about this. I have the same concerns as
> Bronek. They are only concerns. I am not sure of myself. But here's
> a data point:
> In the Metrowerks std::tr1::shared_ptr I give the option to the client
> whether or not shared_ptr contains a mutex (via a #define flag). This
> decision can be made independent of whether the entire C++ lib is
> compiled in multithread mode or not. And at the same time I use
> shared_ptr in the implementation of std::locale.
> In the std::locale implementation I am careful to wrap each use of the
> locale implementation with a mutex lock. I believe I would have to do
> this whether or not shared_ptr protected its count. And yet
> std::locale is just a library, not an application.

I am not sure whether you can skip the count synchronization here. Even
though you do not expose a shared_ptr directly, the user can still copy a
std::locale at will. When two threads copy the same std::locale at the same
time, a non-synchronized count leads to undefined behavior.

Note that there's no mutex requirement. But you need the atomic updates.

> Perhaps a more correct statement is that if you expose a shared_ptr as
> part of your interface, and advertise that multiple threads can operate
> on copies of that exposed shared_ptr, then shared_ptr must be
> internally protected.

It doesn't matter whether the user sees the shared_ptr or not. What matters
is whether it can be copied by two threads (or copied by one thread and
destroyed in another) at the same time. Which I believe is the case in your
std::locale example. But I may be wrong because std::locale isn't one of my
areas of expertise.

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