Re: [Boost-users] Thread safety of smart pointers?

From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2006-03-24 11:28:34

divick kishore wrote:

>Hi,
>   I have several questions related to boost.
>
>1. Are boost::smart pointers thread safe especially
>shared_ptr ? Although it seems but I still want to be
>sure of that.
>  
>
Yes, in that you can safely copy pointers in multiple threads at the 
same time, i.e. access to the reference count is thread-safe.
No, in that you cannot use a single smart pointer object simultaneously 
from several threads.

>2. Why isn't there two flavours of every smart
>pointers, one with multithread safety and one without.
>If one is using some smart pointers in non threaded
>mode, then still does one need to pay the penalty for
>exclusive access ?
>
First, because the synchronization overhead of the current shared_ptr is 
small enough that it's not a reason to worry in most situations. Second, 
because experience has shown that having different types with different 
thread safety rules leads to chaos. Code that is single-threaded now 
might not remain so in the future. How many uses of the single-threaded 
versions are you going to forget to change? What if you refactor the 
code into a library? Will all code that calls this library be 
single-threaded?

It's a tradeoff, and Boost chose to always be thread-safe.

Sebastian Redl