Re: Fwd: memory

[Chris Vine <chris cvine freeserve co uk>]

On Tuesday 16 May 2006 11:16, Ramashish Baranwal wrote:
> boost::shared_ptr is thread safe.

"Thread safe" can mean many different things.  boost::shared_ptr<> is thread 
safe in the sense in which a bare pointer is thread safe.

It means that you can safely reference an object with shared pointers in one 
thread and the same object with different instances of shared pointer in 
another thread.  But you cannot safely modify a shared pointer (ie apply a 
non-const operation to the shared pointer) in one thread and read from or 
modify the same instance of shared pointer in another thread.  (And of course 
you cannot safely modify the referenced object in one thread and read or 
modify it in another thread, unless the object does its own locking.)

What this means is that you can have the same object referenced by a container 
of shared pointers in one thread and a container of shared pointers in 
another thread provided that you arrange separate locking of the referenced 
object.  The circumstances in which you might want to do that are quite 
limited.

Chris