Re: [boost] Re: Sorting list < shared_ptr < foo > > using std::list::sort()

From: Matthew Towler (towler_at_[hidden])
Date: 2003-04-24 10:37:31

> For a while I have been toying with the idea of a smart pointer called 
> sortable_ptr, which would behave in the way that [I believe that] the 
> original poster erroneously presumed that boost::shared_ptr already 
> did.  I was originally planning to do some more practical testing of 
> the concept before possibly submitting it as a proposal to Boost, but 
> since the topic has now been raised, I will take the opportunity to 
> get some feedback as to whether the idea is sound, or if I have missed 
> something crucial.
>
> The following is an extract from the draft documentation I have been 
> writing.
>
>
> The class sortable_ptr is publicly derived from boost::shared_ptr, but 
> defines the comparison operator < (and its siblings >, <=, and >= ) to 
> make a comparison of the actual objects.  This means that if the class 
> myclass has operator< defined, we can write
>
>
> The == and != operators retain the semantics that they inherited from 
> boost::shared_ptr, which is to test the pointers themselves for 
> identity.  This means that even if neither aptr < bptr nor aptr > bptr 
> is true for two sortable_ptr pointers, the expression aptr == bptr may 
> or may not be true.  

I can see users getting confused by the fact that == and != compare the 
pointers (so you can use them in loops), but the other operators compare 
the objects.

I can also see a problem in that some objects will define just < and 
your pointer will need to derive all the others from it, whereas others 
will define the whole set, and your pointer should forward to them.  I 
am not sure there is a way of simply coding "implement it like this if 
the function exists, otherwise do it like that".

Therefore I think this is a bad idea

Matt