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Building Decorator Chains

, 22 Oct 2010 CPOL
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
#include <iostream>
#include <string>

#include <boost/functional/factory.hpp>
#include <boost/bind.hpp>
#include <boost/bind/placeholders.hpp>

#include "decorator_chain.hpp"



using namespace std;

class abstract_class
{
public:
    virtual ~abstract_class( void ) { }
    virtual void do_something( void ) = 0;
};

class concrete_class : public abstract_class
{
public:
    void do_something( void ) { cout << "concrete_class_a\n"; }
};

class decorator : public abstract_class
{
    abstract_class *m_decorated_class;
public:
    decorator( abstract_class *decorated_class ) : m_decorated_class( decorated_class ) { }
    void do_something( void )
    {
        m_decorated_class->do_something();
        cout << "decorator\n";
    }
};

class int_decorator : public abstract_class
{
    abstract_class *m_decorated_class;
    int m_val;
public:
    int_decorator( abstract_class *decorated_class , int val ) : m_decorated_class( decorated_class ) , m_val( val ) { }
    void do_something( void )
    {
        m_decorated_class->do_something();
        cout << "int_decorator : " << m_val << "\n";
    }
};

class string_decorator : public abstract_class
{
    abstract_class *m_decorated_class;
    string &m_val;
public:
    string_decorator( abstract_class *decorated_class , string &val ) : m_decorated_class( decorated_class ) , m_val( val ) { }
    void do_something( void )
    {
        m_decorated_class->do_something();
        cout << "string_decorator " << m_val << "\n";
    }
};

int main( int argc , char **argv )
{
    concrete_class c;
    decorator d( &c );
    d.do_something();

    decorator_chain< abstract_class > dc;
    dc.create_class( boost::factory< concrete_class* >() );
    dc.decorate( boost::bind( boost::factory< decorator* >() , _1 ) );
    dc.decorate( boost::bind( boost::factory< int_decorator* >() , _1 , 10 ) );
    string str = "Hello world!";
    dc.decorate( boost::bind( boost::factory< string_decorator* >() , _1 , boost::ref( str ) ) );
    dc.get_class_reference().do_something();

    return 0;
}

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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headmyshoulder
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