Construction of classes at runtime

[Sami]

Hi,

Is there anyway to construct a object of class with its class name
obtained dynamically? Prototype and Factory method along with Abstract
factory design pattern can be used to do it. But it involves using of
if-else statement at the factory level. To avoid using the if-else
statements (there would be plenty in my case), is there any way to
construct the object based on the value of a string variable ? So that
when the class name is assigned to the string variable, the string
value (class name) can be used in construction of the object.

Your response is appreciated.

Thanks,
Sami

[Sami]

On Aug 20, 2:41*pm, Sami <[e-mail]:

Hi,

Is there anyway to construct a object of class with its class name
obtained dynamically? Prototype and Factory method along with Abstract
factory design pattern can be used to do it. But it involves using of
if-else statement at the factory level. To avoid using the if-else
statements (there would be plenty in my case), is there any way to
construct the object based on the value of a string variable ? So that
when the class name is assigned to the string variable, the string
value (class name) can be used in construction of the object.

Your response is appreciated.

Thanks,
Sami

The implementation needed is in C++, hence posted in this group.
Thanks.

[Ian Collins]

Sami wrote:

Hi,

Is there anyway to construct a object of class with its class name
obtained dynamically? Prototype and Factory method along with Abstract
factory design pattern can be used to do it. But it involves using of
if-else statement at the factory level. To avoid using the if-else
statements (there would be plenty in my case), is there any way to
construct the object based on the value of a string variable ?

Use a map of strings (names) and factory objects to create the required
class.

You can't create a class 'on the fly' from its name in C++, you have to
know the classes at compile time.

--
Ian Collins.

[Harsh Puri]

On Aug 20, 10:41*am, Sami <[e-mail]:

Hi,

Is there anyway to construct a object of class with its class name
obtained dynamically? Prototype and Factory method along with Abstract
factory design pattern can be used to do it. But it involves using of
if-else statement at the factory level. To avoid using the if-else
statements (there would be plenty in my case), is there any way to
construct the object based on the value of a string variable ? So that
when the class name is assigned to the string variable, the string
value (class name) can be used in construction of the object.

Your response is appreciated.

Thanks,
Sami

A virtual copy constructor, or a clone function might help you?

[AnonMail2005]

On Aug 20, 5:41*am, Sami <[e-mail]:

Hi,

Is there anyway to construct a object of class with its class name
obtained dynamically? Prototype and Factory method along with Abstract
factory design pattern can be used to do it. But it involves using of
if-else statement at the factory level. To avoid using the if-else
statements (there would be plenty in my case), is there any way to
construct the object based on the value of a string variable ? So that
when the class name is assigned to the string variable, the string
value (class name) can be used in construction of the object.

Your response is appreciated.

Thanks,
Sami

Take a look at Modern C++ Design. It's factory implementation uses no
if-else statements. Essentially, it uses a map of std::strings to
function pointers.

HTH

[Sami]

On Aug 20, 6:22*pm, "[e-mail]" <[e-mail]>
wrote:

On Aug 20, 5:41*am, Sami <[e-mail]:

Hi,

Is there anyway to construct a object of class with its class name
obtained dynamically? Prototype and Factory method along with Abstract
factory design pattern can be used to do it. But it involves using of
if-else statement at the factory level. To avoid using the if-else
statements (there would be plenty in my case), is there any way to
construct the object based on the value of a string variable ? So that
when the class name is assigned to the string variable, the string
value (class name) can be used in construction of the object.

Your response is appreciated.

Thanks,
Sami

Take a look at Modern C++ Design. *It's factory implementation uses no
if-else statements. *Essentially, it uses a map of std::strings to
function pointers.

HTH

Thanks Ian, Harsh Puri for the responses.

Thanks HTH, it is a nice idea to have a map.

Also is there any way to implement reflection concept of java in C++.
I know there is no support in native C++ language. Is there any
pattern similar to that in C++?

Thank you
Sami

[r.grimm]

Hallo,

>
Also is there any way to implement reflection concept of java in C++.
I know there is no support in native C++ language. Is there any
pattern similar to that in C++?

have a look at Pattern-Oriented Software Architecture: A System of
Patterns.
There must be a description of implementing reflection. The mapping
from
strings to Functions Pointers is known as dispatch table.

Greetings Rainer

[Sami]

On Aug 22, 11:31*am, [e-mail] wrote:

Hallo,

Also is there any way to implement reflection concept of java in C++.
I know there is no support in native C++ language. Is there any
pattern similar to that in C++?

have a look at Pattern-Oriented Software Architecture: A System of
Patterns.
There must be a description of implementing reflection. The mapping
from
strings to Functions Pointers is known as dispatch table.

Greetings Rainer

Thanks Rainer for the info. It helped.

[James Kanze]

On Aug 20, 11:47*am, Ian Collins <[e-mail]:

Sami wrote:
You can't create a class 'on the fly' from its name in C++,
you have to know the classes at compile time.

That's true for pretty much every language, isn't it. It's not
sufficient to just have a name; you need more information from
somewhere. And you can make things pretty dynamic in C++; I
have one case where I look for a dynamically linked object for
the class if it isn't already loaded.

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Ian Collins]

James Kanze wrote:

On Aug 20, 11:47 am, Ian Collins <[e-mail]:

>Sami wrote:
You can't create a class 'on the fly' from its name in C++,
you have to know the classes at compile time.

That's true for pretty much every language, isn't it. It's not
sufficient to just have a name; you need more information from
somewhere. And you can make things pretty dynamic in C++; I
have one case where I look for a dynamically linked object for
the class if it isn't already loaded.

True, but a compiled language will never match the runtime flexibility
of an interpreted one. For example in PHP to process an RPC request I
can write

call_user_func_array( array($object,$method), $parameters );

Where $object is the name of a class, $method is the method to call and
$parameters an array of parameters. The interpreter will find the
class, construct it and call the method. I could write the same
function in C++, but the tables of classes and methods would have to
explicitly constructed.

--
Ian Collins.

[James Kanze]

On Oct 9, 3:59 am, Ian Collins <[e-mail]:

James Kanze wrote:

On Aug 20, 11:47 am, Ian Collins <[e-mail]:

Sami wrote:
You can't create a class 'on the fly' from its name in C++,
you have to know the classes at compile time.

That's true for pretty much every language, isn't it. It's not
sufficient to just have a name; you need more information from
somewhere. And you can make things pretty dynamic in C++; I
have one case where I look for a dynamically linked object for
the class if it isn't already loaded.

True, but a compiled language will never match the runtime
flexibility of an interpreted one.

Yes. There are, in fact, several different levels of
flexibility possible (with, generally, more flexibility implying
less robustness). In totally dynamic languages, with fully
dynamic typing, a "struct" or a "class" is really nothing more
than an associative array; an array whose elements are indexed
by the name of the field. In such cases, you can dynamically
read a set of attribute value pairs, and use the results as a
struct. In most compiled languages, and some interpreted ones
as well, I think, the program can only deal with structs known
to it. With dynamic linking, however, it is possible to make
the set of such structs open, to add to it at runtime.

For example in PHP to process an RPC request I can write

call_user_func_array( array($object,$method), $parameters );

Where $object is the name of a class, $method is the method to
call and $parameters an array of parameters. The interpreter
will find the class, construct it and call the method. I
could write the same function in C++, but the tables of
classes and methods would have to explicitly constructed.

Or automatically generated by some other program:-). I'm not
familiar with PHP, but what you are describing doesn't sound too
different from Java's java.lang.Class.forName( "className" )
..newInstance(). As I said, I've actually implemented this once
in C++, with the programming looking for a corresponding
..so/.dll in a path if the desired class wasn't already present.
(It's interesting to note that when I did more or less the same
thing in Java, it was almost as many lines of code. Since, of
course, not just any class would do; the class had to meet
certain requirements, and I needed code to test those. In C++,
the .so/.dll wouldn't link if the class didn't meet my
requirements.)

Implementing total flexibility is also more or less possible;
your actual instance is basically an std::map< std::string,
boost::any >. Of course, total flexibility often results in
total chaos, but like everything else, with a good enough
process and some programmer discipline, it can be made to work.
If you really, really need it.

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]