casting from a string object (doing like Class.forName in java)

[jean-francois]

In java you can do it with
Class.forName("myClass").newInstance();
this will return you a new instance of the class myClass, provided that
you defined it elsewhere. How can I get this in c++ (linux)?
For exemple I'm fetching elements from a vector, they are all
subclasses of a base class Field, but the data type within each
subclass can vary according to a template that was used (exemple
(SubField<int>, Subfield<long>,. Subfield<myclass>, etc..) The objects
were stored/retrieved with the base class pointer Field since they are
taken from a vector
I can still define a common method "void * getData()" in the base class
field but I need to know how to recast once I fetch the values from the
vector if I'm doing some processing with the data.
I'm at least able to store the data name in a member variable, i.e.

SubField<T>: public Field
<template typename T>
setData( T data) {
m_data = data;
m_dataType = typeid(data).name();
}
But once I extract back this object from the vector the string
m_dataType that I can fetch from the Field *object is useless unless I
use a big switch ( I don't like it)
So how can one instanciate an object or define a cast type with only a
string descriptor?

[Victor Bazarov]

jean-francois wrote:

In java you can do it with
Class.forName("myClass").newInstance();
this will return you a new instance of the class myClass, provided
that you defined it elsewhere. How can I get this in c++ (linux)?

Yes, you can, if you implement it yourself. Basically, you need
a bunch of functions (factories) which would create your objects,
and then introduce a factory manager which will have every factory
named. Then you can call your manager's 'forNameNewInstance' and
get back a pointer (void*, most likely) to the newly created object.

For exemple I'm fetching elements from a vector, they are all
subclasses of a base class Field, but the data type within each
subclass can vary according to a template that was used (exemple
(SubField<int>, Subfield<long>,. Subfield<myclass>, etc..) The objects
were stored/retrieved with the base class pointer Field since they are
taken from a vector
I can still define a common method "void * getData()" in the base
class field but I need to know how to recast once I fetch the values
from the vector if I'm doing some processing with the data.
I'm at least able to store the data name in a member variable, i.e.

SubField<T>: public Field
<template typename T>
setData( T data) {
m_data = data;
m_dataType = typeid(data).name();
}
But once I extract back this object from the vector the string
m_dataType that I can fetch from the Field *object is useless unless I
use a big switch ( I don't like it)
So how can one instanciate an object or define a cast type with only a
string descriptor?

See above.

V
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[I V]

On Fri, 19 Jan 2007 09:36:17 -0800, jean-francois wrote:

For exemple I'm fetching elements from a vector, they are all
subclasses of a base class Field, but the data type within each
subclass can vary according to a template that was used (exemple
(SubField<int>, Subfield<long>,. Subfield<myclass>, etc..) The objects
were stored/retrieved with the base class pointer Field since they are
taken from a vector
I can still define a common method "void * getData()" in the base class
field but I need to know how to recast once I fetch the values from the
vector if I'm doing some processing with the data.

You might find that Boost.Variant gives you a different (and possibly
better) way to solve your problem. See
http://www.boost.org/doc/html/variant.html