Re: [Boost-users] 'standard' free functions for operators, etc

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-06-23 06:36:27

Gottlob Frege wrote:

> Now I understand that map is a different situation than
> intrusive_ptr - I'm definitely stretching things a bit.  But I'm
> looking for reasons to use one technique over the other (and also
> instead of, say, virtual functions / runtime polymorphism, but I
> think I understand most of the differences there).  Obviously
> passing in a predicate/policy is the way to go when it is reasonable
> to expect different policies/predicate used *on the same type*, and
> intrusive_ptr is probably a case where different policies don't make
> sense.  Is that enough to make it better? ie:
>
> - a policy based add_ref for intrusive_ptr would allow it to be used
> in those rare cases where there is a 'type-overlap' or a 'lack of
> strong typing'
> HOWEVER,
> - a policy based approach would *also* allow different add_refs for
> a single type (ie 'logical' or 'strong' type), which would be
> senseless and *disasterous* when the type really only has one way of
> doing add_ref,
> THUS
> intrusive_ptr chooses free functions, over a policy-based approach
>
> ie we thought about a policy-based approach, but specifically wanted
> to *enforce* one-add_ref-per-type, or
> we didn't even think about it (at least not consciously), it was
> just the natural and obvious thing to do

Well, it really depends on your goals.

If you want to implement the most flexible intrusive_ptr in the world, you 
would add an AddRef policy and a Release policy.

If you want to implement *the* intrusive_ptr and specify *the* interface to 
implement to make your class X intrusively-countable, you end up with the 
current boost::intrusive_ptr.

The upside of the latter approach is that, as usual, you can pass 
intrusive_ptr's from library A to library B (without using templates and 
putting everything in headers.)

It also gives programmers a standard interface for manipulating the 
reference count, if for some reason they need to do that without using 
intrusive_ptr.

http://boost.org/libs/smart_ptr/sp_techniques.html#intrusive