Re: [Boost-users] Any update on a form of boost::threads getting into the C++ 0x standard?

From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2007-07-25 14:48:52

On Jul 25, 2007, at 12:16 PM, Anthony Williams wrote:

> "Graham Reitz" <graham.cpp_at_[hidden]> writes:
>
>> Our group has had a lot of success with boost::threads and grown  
>> comfortable
>> with its use.
>
> The latest proposal is N2320
> (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/ 
> n2320.html), which is
> largely based on boost.thread, but with movable threads and  
> cancellation. The
> C++ committee actively discussed this at the recent meeting in  
> Toronto, but
> there were some areas of disagreement so it has not yet been  
> approved for
> C++0x. Howard might be able to give more information as he was there.

In a nutshell, cancellation is the elephant in the room, and causing  
no end of grief for everyone involved.  Everyone sees at as something  
slightly different and incompatible.  It is getting renamed, probably  
to interruption (that is not a committee decision at this point, there  
are no committee decisions at this point).

N2320 proposes cancellation is just another exception that one thread  
can ask another to throw.  There is nothing that special about it.

Some would like to see the cancellation exception to be uncatchable.   
Some would like to see it catchable, but there is an implicit rethrow  
at the end of the catch clause that can not be turned off.

Some would like to see cancellation, once acknowledged by the  
receiving thread, to be "sticky".  Like a subpoena; once you've been  
served, you have to go.  Others prefer to give more control to the  
thread (allow it to acknowledge then ignore the cancellation).

Some would like to see guaranteed interoperability between C  
cancellation (i.e. pthread_cancel) and C++ cancellation.  Others fear  
this is not implementable on some major platforms (without breaking C  
(not C++) language ABI).

Some would like to see cancellation pulled from the proposal because  
it is causing entirely too much grief and taking up too much committee  
time from other issues.

There is no issue involving cancellation that has wide spread  
support.  There is no resolution to these issues that does not have  
vocal objections.

If you're into strange entertainment, bring a lawn chair and a martini  
to a standards meeting, walk into a crowded room and shout  
"cancellation", set up shop and quietly watch the show. :-)

Another major issue is ~thread():  what should it do:

boost::thread::~thread() does: if (joinable()) detach().

N2320 proposes std::thread::~thread():  if (joinable())  
{request_cancellation(); detach();}

Some would like to see:  if (joinable()) {request_cancellation();  
join();}

As long as we're at it, the 4th reasonable solution is:  if  
(joinable()) join().

-Howard