Class private data is Lost after typecast to more specific class

[JJZhang]

To simplifying my situation in hope to get a confirmation for this is expected behavior for class inheritance.

I have class A that has a couple private data, I have class B that inherits class A. The software performances following operation:

1. Initialize class A private data

2. Typecast to class B with "To More Specific Class function"

3. It seems like private data from step 1 is reset to default value after performing step 2. Is this expected outcome? I was expecting the private data to not change after to More Specific Class function class.

[crossrulz]

You are getting an error when you use the To More Specific Class.  You cannot promise that A is actually B.  But if you initialized as class B, you would be fine.

[JJZhang]

No error message since class B inherits from class A. In this case class A is part of class B. I was expecting functions takes new implementation and private data retains original value after to more specific function call.

[StevenD]

To more specific class changes the data type of the control, documentation is somewhat ambiguous on what happens to the data, but I am guessing a new control is created (with default data). Maybe not what you are wanting to do, but why not just have a constant of the type B class and bundle the A class data into it?

[crossrulz · Accepted solution]

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@JJZhang wrote:

No error message since class B inherits from class A. In this case class A is part of class B. I was expecting functions takes new implementation and private data retains original value after to more specific function call.

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Are you even checking for the error?  And you cannot just take A and then say it is B.  A square (B) is a rectangle (A), but a rectangle is not necessarily a square.  So unless your initially stated at creation of the object that it is B, you cannot use a To More Generic Class to cast it to B.

Perhaps you should post an example so we can figure out exactly what you are doing.

[JJZhang]

A general error handling function automatically took care of it. Just wrote a dump down version and it did indeed provide an error. Thank you.

I actually was thinking a 4 sides shape (A) can be a square (B), or a rectangle (C). I'm loading 4 sides length and object identification from a database. Then typecast to B or C, or stay as A based on object identification from the database. But I guess you are right, it usually is going to more general, and not to more specific.