Re: [Boost-users] [smart_ptr] comparison operators: shared_ptr ==weak_ptr?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-09-25 12:49:10

Matt.Sullivan_at_[hidden] wrote:
> Hi,
>
> Probably a newbie question, but what's the best way to test if a
> shared_ptr and weak_ptr point to the same thing?
>
> My current code uses: if (shared_ptr != weak_ptr.lock())
> but it feels wrong to create a temporary with some small locking
> overhead - even though I realise it will make absolutely no
> difference to performance.
>
> So, my question is really why is there no operator==(shared_ptr,
> weak_ptr)?

I can think of three reasons, none of them authoritative:

1. If this operator== ends up doing shared_ptr != weak_ptr.lock() under the 
covers, an argument could be made that the explicit version is better since 
it doesn't hide the overhead from the reader;

2. There are two sensible meanings for such an operator (as explained in 
N1590), "points to the same address" and "shares ownership with"; it might 
be better to leave the choice to the user and, again, to keep the choice 
explicit to benefit the reader;

and last but not least

3. Nobody suggested the need for such an operator, so it wasn't included.

> Google provided little explanation other than this:
> http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2004/n1590.html which
> approaches the subject but isn't exactly authoritative.

;-)