Re: [Boost-users] flyweight: lazy initialization

Subject: Re: [Boost-users] flyweight: lazy initialization
From: joaquin_at_[hidden]
Date: 2010-02-23 10:58:26

Johannes Brunen escribió:
> Hello,
>
> Is it possible to initialize a flyweight lazy or better on user request?
> I have the problem that I have a custom allocator that is not ready to
> run before main. Currently, the flyweight initialization is performed on
> init of the fleiweight_core static_initilaizer member. This scheme does
> outrule the usage of the flyweight utility in my application.
>
> Any help would be appreciated.
>   

Hi Johannes,

You can write a custom lazy factory, just as the example attached.
Hope this helps,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo <cid:part1.03080702.04040303_at_[hidden]>


#include <boost/flyweight/factory_tag.hpp>
#include <boost/flyweight/detail/not_placeholder_expr.hpp>
#include <boost/mpl/apply.hpp>
#include <string>

template<typename BaseFactory,typename Entry>
class lazy_factory_class:public boost::flyweights::factory_marker
{
public:
  typedef typename BaseFactory::handle_type handle_type;

  lazy_factory_class():factory_ptr(0){}
  ~lazy_factory_class(){delete factory_ptr;}

  handle_type insert(const Entry& x){return factory().insert(x);}
  void erase(handle_type h){factory().erase(h);}
  const Entry& entry(handle_type h){return factory().entry(h);}

private:
  BaseFactory* factory_ptr;

  BaseFactory& factory()
  {
    if(!factory_ptr)factory_ptr=new BaseFactory;
    return *factory_ptr;
  }
};

template<
  typename BaseFactorySpecifier
  BOOST_FLYWEIGHT_NOT_A_PLACEHOLDER_EXPRESSION
>
struct lazy_factory:public boost::flyweights::factory_marker
{
  template<typename Entry,typename Key>
  struct apply
  {
    typedef lazy_factory_class<
      typename boost::mpl::apply2<BaseFactorySpecifier,Entry,Key>::type,
      Entry
    > type;
  };
};

/* testing */

#include <boost/flyweight.hpp>

using namespace boost::flyweights;

typedef flyweight<
  std::string,
  lazy_factory<hashed_factory<> >
> fw_string;

int main()
{
  fw_string str("boost");
}