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Building Decorator Chains

, 22 Oct 2010
The article explains a method, how flexible and extendible decorator chains can be built in a generic way. Its power is best seen if it is used with boost::factory and boost::bind.
decorator_chain.zip
decorator_chain
#ifndef DECORATORCHAIN_H_
#define DECORATORCHAIN_H_

#include <vector>
#include <stdexcept>

// Compile-time switch for decorators decorating references or pointers
struct reference_decorator_category { };
struct pointer_decorator_category { };

template< class Class , class DecoratorCategory = pointer_decorator_category >
class decorator_chain
{
public:

    typedef Class class_type;
    typedef DecoratorCategory category_type;

    decorator_chain( bool owner_of_class = false , bool owner_of_decorators = false )
        : m_class_instance( 0 ) , m_decorators() , m_current( 0 ) ,
          m_owner_of_class( owner_of_class ) , m_owner_of_decorators( owner_of_decorators )
    {
    }

    ~decorator_chain( void )
     {
         if( m_owner_of_class )
         {
             if( m_class_instance != 0 )
             {
                 delete m_class_instance;
                 m_class_instance = 0;
             }
         }
         if( m_owner_of_decorators )
         {
             for( size_t i=0 ; i<m_decorators.size() ; ++i )
             {
                 delete m_decorators[i];
                 m_decorators[i] = 0;
             }
         }
     }

    template< class Factory >
    void create_class( Factory make_class = Factory() )
    {
        if( m_class_instance != 0 ) throw std::logic_error( "Object already created!" );
        m_class_instance = make_class();
        m_current = m_class_instance;
    }

    template< class Factory >
    void decorate( Factory make_class = Factory() )
    {
        if( m_current == 0 ) throw std::logic_error( "Object has not been created!" );
        class_type *decorator = create_by_category( make_class , category_type() );
        m_decorators.push_back( decorator );
        m_current = decorator;
    }

    class_type& get_class_reference( void ) { return *m_current; }
    const class_type& get_class_reference( void ) const { return *m_current; }

    class_type* get_class_pointer( void ) { return m_current; }
    const class_type* get_class_pointer( void ) const { return m_current; }

private:

    class_type *m_class_instance;
    std::vector< class_type* > m_decorators;
    class_type *m_current;
    bool m_owner_of_class;
    bool m_owner_of_decorators;
    
    template< class Factory >
    class_type* create_by_category( Factory factory , reference_decorator_category )
    {
        return factory( *m_current );
    }

    template< class Factory >
    class_type* create_by_category( Factory factory , pointer_decorator_category )
    {
        return factory( m_current );
    }
};

#endif /* DECORATORCHAIN_H_ */

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headmyshoulder

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