[fpc-pascal] casting back a pointer to original type

Michael Van Canneyt michael at freepascal.org
Sat May 29 18:11:38 CEST 2010

On Sat, 29 May 2010, spir ☣ wrote:

> Hello,
> Say I store class instances of various types in a TFPList. All happen to be descendants of same superclass C and each one has a specific method "text". When retrieving and writing an element, if they were all direct instances of C, I would do something like:
>    element := C(list[index]);   // casting back
>    text := element.text;
> I cannot do that, even if elements all are *indirect* instances of C, because this calls C.text instead of the element's own proper text method. Which is wrong, indeed.
> So, I must cast elements back to their own type; but I have no idea how to do this. Doesn't the compiler store this information? After all, when an element is put into the list, its type is known. Is there a reason why retrieving an element does not cast it back to its original type automatically? Should I store element types manually in a parallel list?
> There must be a way to solve this issue, I guess...

There are 2
1. Make all descendents descendent of a super class that has a text
2. Make the "text" property published, and use RTTI to get/set it.

uses typinfo;




> Side-question: it seems, when subclassing, that methods with same name and same parameters do not need any modifier keyword (such as "override" or "overload"). Eg I can have
>    function text;
> in both a superclass C0 and a subclass C1 work as expected. Correct?

It depends on what you want to achieve. If you make it a static method, then
the compiler will emit a warning "function Text hides method of parent
class" (or something like it)

If you make it virtual, then you must explicitly add  "override" to 
override the implementation of the method.


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