Re: thread & containers

From: bill_kempf (williamkempf_at_[hidden])
Date: 2002-07-19 13:43:21

--- In Boost-Users_at_y..., Herve Blanc <blanc_at_s...> wrote:
> Is there anything available or coming in boost::thread to wrap STL 
containers
> into thread safe containers?

No.

> If not, any advice for coding those wrappers efficiently/elegantly 
or not doing
> that?

Trying to do this is generally a bad idea.  Using an internal mutex 
to protect the class will only allow you to synchronize at the method 
call level, and most user code will need synchronization around 
multiple such calls not a single call.  Example:

my_thread_safe_vector[0] += some_value;
if (my_thread_safe_vector[0] > 100)
   my_thread_safe_vector[0] = 100;

The above code is full of race conditions, even if 
my_thread_safe_vector uses a mutex internally.

> If not again, why and how do you deal with the problem then?

Use an external mutex to achieve the proper lock granularity.  
Example:

boost::mutex::scoped_lock lock(my_mutex);
my_vector[0] += some_value;
if (my_vector[0] > 100)
   my_vector[0] = 100;

There's no race in the above since the external mutex and lock insure 
the proper granularity for the three calls to the container methods.

Bill Kempf