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From: Greg Colvin (gcolvin_at_[hidden])
Date: 2001-11-17 14:24:25

You can't let the counter type be user-specified because on
most systems not just any type can be made atomic efficiently.

And 32 bit integers are not guarantees by Win32 to be atomic,
except on single-CPU systems.

From: Mattias Flodin <flodin_at_[hidden]>
On Sat, 17 Nov 2001, Peter Dimov wrote:

> In order to make shared_ptr thread safe (w.r.t. reference counting) we need
> an atomic_counter type, with the following operations defined:
> atomic_counter a(n); // n is the initial value, convertible to 'long'
> ++a; // atomic increment, returns the new value (by value, type 'long')
> --a; // atomic decrement, returns the new value (by value, type 'long')
> a; // convertible to long, atomic read (to support use_count() and unique())
> See below for a proposed implementation.
> Comments are welcome.

Pretty nice. Two questions: Firstly is there a rationale for not
allowing / implementing postfix increment and decrement? Secondly, I'm
just curious, why are you adding zero to _MSC_VER?

Since the required range of a counter may vary, perhaps it is not
unreasonable to parameterize this class with respect to the counter type.
I realize that this is not necessary with atomic_counter being in Thread's
detail namespace, but I don't see why a type such as this shouldn't be
exposed for the library user to benefit from.

Speaking of atomic access, I've been thinking about the sig_atomic_t type
discussed earlier. Wouldn't it be useful with an portable atomic-access
integer type? In systems where sig_atomic_t exists as an atomic type, this
could be simply a typedef. In win32, accesses to 32-bit aligned integers
are guaranteed atomic, so there it could be just typedef int atomic_int.
In other systems where no atomic type is available this would need to fall
back on a mutex. The type could perhaps be declared somewhere in
Boost.Threads, or in Boost.Integer.


Mattias Flodin <flodin_at_[hidden]>   "A good thing about C++ is that only
Room D418                           friends can access your private parts"
Department of Computing Science
Umeå University
S-901 87 Umeå, Sweden
Note: Any opinions expressed in this mail are personal, and do not
necessarily reflect an official standpoint of Umeå University.
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