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

From: Christian Engström (christian.engstrom_at_[hidden])
Date: 2003-04-24 09:57:46

HEATH DAVIS wrote:
> I have experienced some difficulty with sorting lists of type:
> 
> list < shared_ptr < foo > >
> 

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

sortable_ptr<myclass> aptr(new myclass(/* Something */));
sortable_ptr<myclass> bptr(new myclass(/* Something else*/));
assert((aptr < bptr) == (*aptr < *bptr));

This is different from boost::shared_ptr, which does only define the < 
operator to do a direct comparison of the pointers themselves (or 
something to that effect).

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.  If it is true this will mean that both aptr and 
bptr refer to the same myclass object, which of course is neither less 
than or greater than itself.  If the expression aptr == bptr is false, 
it will mean that aptr and bptr point at different objects that are 
equivalent as to their sort order.

What is the advantage of this?

Before answering that question, we note that we have at leas not done 
anything illegal by introducing these definitions.  Neither 
boost::shared_ptr nor sortable_ptr support the notion of pointer 
arithmetic, so we do not need the < operator to be defined in any 
particular way for that purpose.

We can still store sortable_ptr pointers in any of the sequence 
containers in the Standard Template Library, since these only demand 
that the == operator tests for proper identity between the objects that 
are being stored in the list or deque or vector, which it does.
The sort algorithms in STL use the < operator of the objects in the 
container by default, so if we sort a container of sortable_ptr 
pointers, they will appear in the same order as the objects themselves 
would have been sorted.  For vectors and deques it can be considerably 
faster to sort pointers rather than the objects themselves, since the 
generic sort algorithms work by swapping the items in the container, and 
it can be much faster to swap two small pointers than to swap two big 
and complex objects.

For the associative containers set, multiset, map, and multimap, the 
default is to use the < operator of the objects that are stored in the 
container to define equivalence and sort order, which is exactly what we 
want.  When we store sortable_ptr pointers in an associative container, 
the pointers will be sorted in the same order as the actual objects 
would have been if we had stored them directly in a corresponding 
associative container for objects.  The definition of operator< ensures 
that any two sortable_ptr pointers are equivalent if, and only if, the 
two underlying objects are equivalent, so for sets and maps, which 
cannot hold more than one object with equivalent keys, it will be the 
same objects that are represented in the container whether the objects 
are stored directly or via sortable_ptr pointers.

- - - -

The draft implementation looks like this at the moment:

//======================================
//  'sortable_ptr'
//--------------------------------------
template <class T, class Compare = std::less<T> >
class sortable_ptr : public boost::shared_ptr<T>
{
private:
   typedef sortable_ptr self;
   typedef boost::shared_ptr<T> base;

public:
   // From 'boost::shared_ptr'
   //   typedef T element_type;
   //   typedef T value_type;
   //   typedef T * pointer;
   //   typedef typename detail::shared_ptr_traits<T>::reference reference;
   //
   typedef Compare element_compare;

   element_compare element_comp() const {return element_compare();}

   bool operator<  (const self& other) const {return 
element_compare()(**this, *other);}
   bool operator>  (const self& other) const {return other < *this;}
   bool operator<= (const self& other) const {return !(other < *this);}
   bool operator>= (const self& other) const {return !(other > *this);}

// Constructors
   sortable_ptr() {}

   template<class Y>
   sortable_ptr(const sortable_ptr<Y>& other) : base(other) {}

   template<class Y>
   explicit sortable_ptr(const boost::shared_ptr<Y>& other) : base(other) {}

   template<class Y>
   explicit sortable_ptr(const boost::weak_ptr<Y>& other) : base(other) {}

   template<class Y>
   explicit sortable_ptr(std::auto_ptr<Y>& other) : base(other) {}

   template<class Y>
   explicit sortable_ptr(Y* other) : base(other) {}

// Assignment

};

- - - -

Does this represent a sensible idea, or am I barking up a totally 
pointless tree?

Christian Engström