Re: Calling methods based on derived type

James Kanze <>
27 Apr 2007 01:09:36 -0700
flopbucket wrote:

Say I have a baseclass B that has many derived classes, let's say
C..Z, for example:

class B

class C : public B {};
class D : public B {};
class E : public B {};

Also assume that these classes are provided as is, that is, I can not
make any changes to them (the real classes contain many data members,

Now, there is a factory method that creates the correct class based on
input from some external source. Something like:

B *b = SomeFactoryMethod();

Of course SomeFactoryMethod will actually return any of the dervied
classes depending on the external information.

Now here is my question. There are a set of methods such as:

processObject(C *);
processObject(D *);
processObject(E *);
... for all derived types.

Given the base clas pointer returned from the factory, how can I call
the correct method? Obviously I can have a bunch of dynamic_cast<>'s
but that does not seem the best way. I was thinking I could do
something with templates and typeinfo?

Any ideas? Please remember that I can not change the existing classes.

If you cannot change the existing classes nor the factory
function, then you'll have to maintain a parallel hierarchy,
something like:

    class AbstractObjectProcessor
        virtual ~AbstractObjectProcessor() {}
        virtual void process( B* pObj ) const = 0 ;
    } ;

    template< typename T >
    class ObjectProcessor : public AbstractObjectProcessor
        virtual void process( B* pObj ) const
            assert( dynamic_cast< T* >( pObj ) != NULL ) ;
            processObject( static_cast< T* >( pObj ) ) ;
    } ;

Then, a static object for each type, and to find it:

    struct TypeInfoCmp
        bool operator()(
                                std::type_info const* lhs,
                                std::type_info const& rhs ) const
            return lhs->before( *rhs ) ;

    } ;
    std::map< std::type_info const*,
              AbstractObjectProcessor const*,
              TypeInfoCmp > map ;

In such cases, I'll often add a constructor to the template:

    template< typename T >
        map[ &typeid( T ) ] = this ;

Be careful about the order of initialization if you do this;
either the map must be wrapped in a singleton, or you'll have to
put all of the static instances in the same file, after the map.

To call the function, of course:

    B* pB = SomeFactoryMethod() ;
    map[ &typeid( *pB ) ]->process( pB ) ;

Note too that anytime someone adds a derived class, you'll have
to add a static ObjectProcessor object.

James Kanze (GABI Software)
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