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


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