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

James Kanze <>
Mon, 17 Mar 2008 14:18:11 -0700 (PDT)
On 14 mar, 17:29, red floyd <no.s...@here.dude> wrote:

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

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.

Don't use a switch-case. Use a std::map.


class base { };
class derived1 : public base { };
class derived2 : public base { };
class derivedn : public base { };

typedef base (*createfunc )(/*params if needed*/);
std::map<std::string, createfunc> factory_map;

then look up in the map instead of using a case statement.

If you use polymorphic static objects, then you can arrange for
the map to be initialized automatically. (Of course, you can do
that using a map of functions as well, but it's a little bit
more obscure, and IMHO, easier to forget one.)

James Kanze (GABI Software)
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