Re: [Boost-users] Why is boost::thread::mutex noncopyable?

From: Phil Endecott (spam_from_boost_users_at_[hidden])
Date: 2006-01-01 13:05:55

>>Can someone help me to understand why boost::thread::mutex is 
>>noncopyable?  I expect that there is a good reason, but I can't see it 
>>at the moment.
> 
> http://www.boost.org/doc/html/threads/faq.html#id2786688

Thanks for the link.  Here is the text:

Why do Mutexes  have noncopyable semantics?

To ensure that deadlocks don't occur. The only logical form of copy 
would be to use some sort of shallow copy semantics in which multiple 
mutex objects could refer to the same mutex state. This means that if 
ObjA has a mutex object as part of its state and ObjB is copy 
constructed from it, then when ObjB::foo() locks the mutex it has 
effectively locked ObjA as well. This behavior can result in deadlock. 
Other copy semantics result in similar problems (if you think you can 
prove this to be wrong then supply us with an alternative and we'll 
reconsider).

I disagree with the first comment: "The only logical form of copy would 
be to use some sort of shallow copy semantics in which multiple mutex 
objects could refer to the same mutex state."  What's wrong with the 
copy of a mutex being a new independent mutex?  (There's a problem if 
you copy a locked mutex, but I don't propose to do that.)

--Phil.