Re: how to get an object instance from its class name?

James Kanze <>
Mon, 17 Mar 2008 14:15:44 -0700 (PDT)
On 14 mar, 17:23, dotNeter <> wrote:

On Mar 14, 4:51 pm, Michael DOUBEZ <> wrote:

dotNeter a =E9crit :

Is there any way for this?

If I get a string representing a class name, how do I new
an instance upon this name?

There is no native way. You have to use a factory i.e. an
object that knows how to build an object from a parameter.
That also means that all such object have a common ancestor.

There are many way to implement such a factory (builder, prototype ...).=

   But you have to know the types in advance or have a way to register
them into the factory.

An alternative non-portable way is to put those classes in a
dynamic library and define for each a factory function
(make_<Class>() by example) and then use the dlopen(),
dlsym() calls to open the library and call the factory

This is very fascinating. I've spent some time on factory
pattern, but I don't like to write so many switch-case in my
system. One further question is that if it truly fit my
future requirement since the classes may get enriched someday.

The usual solution is to use some sort of dynamic registration
with a map. And you don't need to use dynamic linking for it to
work. Basically, for each type, you define a static factory
object for that specific type, whose constructor registers it
with the factory map. Roughly speaking:

    class AbstractFactory
        virtual ~AbstractFactory() {}
        Base* create() const = 0 ;

                        AbstractFactory( std::string const&
typename ) ;
    } ;

    class FactoryMap
        typedef std::map< std::string, AbstractFactory const* >
                        Map ;

        static FactoryMap&
                        instance() ;
        void enrol( std::string const& typename,
                               AbstractFactory const* factory ) ;
        Base* create( std::string const& typename ) const ;
        Map myMap ;
        // plus the usual singleton stuff...
    } ;

        std::string const& typename )
        FactoryMap::instance().enrol( typename, this ) ;

        std::string const& typename,
        AbstractFactory const*
                            factory )
        assert( myMap.find( typename ) == myMap.end() ) ;
        myMap.insert( Map::value_type( typename, factory ) ) ;

        std::string const& typename ) const
        Map::const_iterator entry
            = myMap.find( typename ) ;
        return entry == myMap.end()
            ? NULL
            : entry->second->create() ;

James Kanze (GABI Software)
Conseils en informatique orient=E9e objet/
                   Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34

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