Re: inheritance headache....

James Kanze <>
Fri, 1 Feb 2008 04:57:58 -0800 (PST)
On Feb 1, 5:03 am, Sean Hunt <> wrote:


// semi pseudo with
abstractTarget* createTarget(abstractDescription* desc)

// shortcut pseudo code for dynamic cast< > ()
   if ( desc is concreteDesc1)

Don't use dynamic_cast to check whether a class is of a
specific type. Only use it if you need the actual dervied
object - otherwise, use typeid:

if (typeid(desc) == typeid(concreteDesc1))

typeid will return an std::type_info object corresponding to
the most derived type. Remember to include <typeinfo> before
using typeid!

The semantics are different. Use typeid if you want to know the
exact most derived type, dynamic_cast if you only want to know
whether the object is a, or not. (In this case, I suspect that
the desired semantics are in fact those of typeid, and not


As far as I know, there's no way to properly reproduce the virtual
call mechanism. However, a map of typeids will work:

bool operator <

Two gotcha's for mapping type_info's to anything:

 1. A type_info is not copiable, so you can't put them in a
    container. You need to use type_info const*.

 2. type_info, per se, doesn't support operator<. You need to
    provide your own, based on type_info::before(). (Hopefully,
    the next version of the standard will correct this, and also
    provide functions for == and getting a hash code of a

For these reasons, I tend to use a wrapper class, something

    class Type
        /* implicit */ Type( std::type_info const& type )
            : myType( &type )

        bool operator<( Type const& other ) const
            return myType->before( *other.myType ) ;

        std::type_info const*myType ;
    } ;

    typedef std::map< Type, whatever >

James Kanze (GABI Software)
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                        TypeMap ;

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