问使用飞权模式在位图对象之间共享位图

EN

#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>
#include <map>
#include <string>
#include <iostream>

// represents the bitmap data
class Bitmap
{
public :
    Bitmap( std::string const& name ) : name( name )
    {
        std::cout << "Bitmap " << name << std::endl ;
    }

    ~Bitmap()
    {
        std::cout << "~Bitmap " << name << std::endl ;
    }

    std::string name ;
};

// the flyweight pool
class Factory
{
public :

    typedef std::map< std::string , boost::weak_ptr< Bitmap > > Map ;

    boost::shared_ptr< Bitmap > get( std::string const& what )
    {
        Map::iterator x = map.find( what );

        // retrieve existing object from map's weak pointers

        if( x != map.end() )
        {
            if( boost::shared_ptr< Bitmap > shared = x->second.lock() )
            {
                return shared ;
            }
        }

        // populate or update the map

        boost::shared_ptr< Bitmap > shared( new Bitmap( what ) );
        boost::weak_ptr< Bitmap > weak( shared );
        map.insert( std::make_pair( what , weak ) );
        return shared ;
    }

private :
    Map map ;
};


int main(int argc, char** argv)
{
    Factory f ;

    // we try our flyweight bitmap factory ...

    boost::shared_ptr< Bitmap > a = f.get( "a" );
    boost::shared_ptr< Bitmap > b = f.get( "b" );

    // a is not made again
    boost::shared_ptr< Bitmap > a2 = f.get( "a" );

    a.reset();
    a2.reset();

    // a is destroyed before ------

    std::cout << "------" << std::endl ;
}