type cast using a string in c++

Question

1.00/5 (1 vote)

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Is it possible to type cast a base class to a child class using a string?

Something like this maybe? Please

std::map<cstring,baseclass> types;
types["childClass1"] = childClass1;
.
.
.
CString nameOfClass = getStringFromCLI();

types[nameOfClass] * aChildClass = (types[nameOfClass] *) new baseClass();
//Now aChildClass is of type childClass1 (NOT baseClass)

Right now I have hundreds of thousands of child classes, and needless to say, I use a script to generate if statements and type cast the baseClass accordingly.

All ideas are welcome! but you don't have to mention that my code is worthless cause I already know that.

Here is how you can do this in Java (from http://www.knowledgesutra.com/forums/topic/32520-can-i-cast-a-string-as-a-class-object/[^]

import java.lang.reflect.*;
import java.awt.*;
......
//Your class in string
 String s="<your class>";
//Get class definition
 Class cls = Class.forName(s);
//Instantiate
 Object obj=cls.newInstance();

Posted 12-Jan-11 10:20am

Ribhi Kamal

Updated 12-Jan-11 10:56am

v2

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dasblinkenlight 12-Jan-11 16:47pm

Suppose you manage to upcast the pointer to the child class. I still do not see how is it useful, since the static type of *aChildClass remains baseClass. Your script-generated code can make good use of the new type because the pointer inside the 'if' block is statically typed to the child type, but your pseudocode wouldn't be able to do the same, because it does not have thousands of 'if' blocks. A good way to see what I mean would be to try providing an example of what happens after the last line of your pseudocode: you wouldn't be able to call aChildClass->aChildMethod(), unless aChildMethod is in the base class.

3 solutions

Solution 2

No, not even close.

You need to know better what classes are, and only then what the cast is supposed to do. Hm... I don't even know where to start. Perhaps read and try to code something very basic first.

Posted 12-Jan-11 10:40am

Sergey Alexandrovich Kryukov

Comments

Ribhi Kamal 12-Jan-11 16:44pm

I know that is not even close...lol I just used the code above as a way to make the question clear.

Sergey Alexandrovich Kryukov 12-Jan-11 17:23pm

Hey, Shakazzolo, you did not get it... it was not about your pseudo-code.
You have a full right to use pseudo-code.
That was about your idea -- it is based on deep misunderstanding what a type and class are; you also probably don't really know difference between type cast and conversion.

I assume that was your vote, huh?

Sergey Alexandrovich Kryukov 12-Jan-11 18:18pm

Oh, "3" changed to "1"? Naturally. Ignorant, coward anonymous should be also aggressive -- those traits always come together. I am not afraid to hurt someone's feeling -- why? -- if I don't know who am I talking to...

Nuri Ismail 12-Jan-11 18:49pm

I'll try to compensate this ignorant vote with my 5! :)

Sergey Alexandrovich Kryukov 12-Jan-11 18:51pm

Thank you, Nuri. I knew you would understand things properly, as I saw your own posts.

fjdiewornncalwe 12-Jan-11 23:07pm

+5 because I had a good chuckle reading this... (That I agree with you is not a factor, of course)

Sergey Alexandrovich Kryukov 12-Jan-11 23:36pm

Sure, Marcus, thank you. He-he, it's getting funny.

Sergey Alexandrovich Kryukov 12-Jan-11 23:56pm

By the way, for some fun, I've recently written a whole assay in comments, it so happened:
http://www.codeproject.com/Answers/144147/which-is-the-better-career-option-sap-trainee-or-s.aspx?cmt=41490#answer1
(nowhere to vote, of course :-)

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Espen Harlinn · Accepted Answer · 2011-01-12T10:38:00

Once upon a time there was something called taligent, some code ended up in the java world, some ended up as part of IBM's C++ class library. I havent looked at that library for many years, but if I remember correcty there was something there that might have done something akin to what I think you want. This was in the days before static_, dynamic_, cast became a part of the c++ language, but the functionality of dynamic_cast was needed.

If I remember correctly each class definition contained a macro that added an entry to a global map for each class and it's parent class. This defined the inheritance tree for the entire library in an extensible manner.

As I said - if I remember correctly ...

The functionality was used to simulute dynamic_cast, as in validate that it was "right" to cast from one type to another, and to retrieve the name of the type for a given object.

This method only supported single inheritance, and a cast was considered valid if it was from a base class to a descandant class. The object had to be of, or descend from, the target type of the cast.

Not that I would advice you to do anything like that today.
C++ Primer[^], by Stanley B. Lippman makes for an excellent reading.

Update:
Java supports reflection

Take look at this article:
Metaclasses and Reflection in C++[^]

The map mentioned earlier was a very simplistic metadata container ...

Regards
Espen Harlinn

Arman S. · Accepted Answer · 2011-01-12T11:50:00

The short answer is NO! There is no support for reflections in C++, so you cannot achieve it in the Java-way (or any other language-way that supports reflections. Class, Method and other classes are a part of Java reflections).
Consider redesigning your architecture; you may want to consider C++ meta-programming, or a more robust object-oriented design.
Personally, I think one should never need to use any kind of reflection in C++ unless there are serious flaws in his/her design.

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type cast using a string in c++

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