Re: Usable flyweight template class

"Tom Widmer [VC++ MVP]" <>
Thu, 22 Nov 2007 14:21:29 +0000
Daniel Lidstr?m wrote:


I have recently created a cool flyweight template class. It is intended to
make it very easy to implement the flyweight design pattern. Let me show
some examples of its usage:

[snip usage example]

Everytime a flyweight<State> object is created that existed previously, the
only memory overhead will be a pointer. The requirements of the fly object
is that it is EqualityComparable, CopyConstructible, and "Hashable". If you
use streaming you also have DefaultConstructible requirement.

Let's go straight to the code from here. This code should compile (with
some small modifications that you can probably do) with eVC4+STLport,
VC6+STLport, VC7.1, and gcc-4.1 (needs tr1/unordered_multimap). As you will
see I have used boost::hash, but you should be able to replace that very
easily. There is a more advanced flyweight library getting ready to enter
boost (which likely won't compile with eVC4), and I have stolen some of the
ideas for this class.

namespace GFL
   //! Used to hash and compare flyweights.

Are you sure you don't want:

template <class Fly>

   struct flyweight_traits
      //! Compare two flyweights.
      template<class T>
      static bool equal(const T& left, const T& right)
         return left == right;

since that allow you to do this:

typedef std::tr1::shared_ptr<Fly const> pointer;

//factory::insert must return type convertible to pointer
typedef default_factory_type<Fly> factory;
static factory& get_factory()
   static factory f;
   return f;


   //! @author Daniel Lidstrom <>
   //! @date 2007-11-16 11:20
   //! @ingroup GFL
   //! flyweight design pattern. Allows you to easily create flyweight
   //! classes.
   template<class fl, class tr=flyweight_traits>

template<class fl, class tr=flyweight_traits<f1> >

   class flyweight

      typedef fl fly;
      typedef tr traits;

typedef typename tr::pointer pointer;

[snip good stuff]

      //! Internal class used to allow changing of
      //! a flyweight using the -> operator.
      class fly_proxy
         flyweight& mflyweight;
         fly mfly;


         fly_proxy(flyweight& f)
            : mflyweight(f),

         // Exceptions are not allowed to propagate from destructors,
         // is a requirement for using the standard library containers.
         // That is very likely *not* a requirement for fly_proxy.

What if the non-const member function of fly that you are calling
throws? If the assignment in the destructor below also throws, won't you
get an immediate abort?

            mflyweight = mfly;

         fly* operator->()
            return &mfly;
      friend class fly_proxy;

      //! Provides an easy way to access member functions of the fly.
      const fly* operator->() const
         return mfly;

      //! Non-const version.
      fly_proxy operator->()
         return fly_proxy(*this);

Hmm, interesting, and I imagine it will usually work.

      //! Automatic conversion.
      operator const fly&() const
         return get();


        static typename traits::factory& get_factory()
          return traits::get_factory();

Alternatively, you could call traits::get_factory directly from code
needing the factory, obviating the need for the factory typedef.

        pointer mfly;

I would like to have constructive criticism, if possible. For example:
* How do I separate the internal factory into a template parameter, so that
for example the Hashable requirement can be changed.

See above.

* How can I create a policy so that entries in the internal factory can be
erased when no flyweight refers to them any longer. It is ok to use
weak_ptr and shared_ptr for this implementation, I just don't have a good
idea of how.

The factory type you use just needs to have a variable of type:
std::unordered_multimap<std::size_t, std::tr1::weak_ptr<const fly> > mMap;
pointer == shared_ptr<const fly>

Additionally, you could at certain times erase entries that have an
empty weak_ptr, or have a specific call to let you do it.

* Any other ideas for improvements?

You could write a policy based factory to go with it, offering policies
for thread safety (e.g. insert is synchronised), etc.


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