This is a follow-up to the previous thread.
Say I put in a TFPList elements of types C C1 C2, where C is a super-class.
When I retrieve an element using
var element : C;
...
element := C(list[index]);
text := element.text;
...
what is the actual type of element? and of its value? Concretely, if the
originally inserted element was of type C1, C1.text is called -- this is the
point discussed & solved in the previous thread.
So, it seems the language knows (at runtime) about the real type of a *value*.
I must declare element, the variable, with type C, since I do not know anything
at coding time. But Pascal correctly calls C1.text, due to "virtual" &
"override" modifiers. So, it means that the language has additional type
information and uses it to make a distinction between variable and value types.
Correct? Else, how can it correctly call C1.text?
But I'm not 100% sure this means that, for Pascal and at runtime, element's
value is of type C1. Is there a way to know value and/or variable's types, such
as a func TypeName()?
(Side-note: I do not understand why this overriding mechanism needs to be
explicitely stated with a modifier keyword: when a programmer writes a
specialised method on a sub-class, obviously s/he intends it to be called, no?)
As you have certainly noted ;-), I have very limited knowledge about static
languages. But I thought there were only variable types (at least in Pascal and
C). While in dynamic languages there are only value types (and in fact there is
no notion of variable like in Pascal, the name:value relation is different).
But the case above contradicts this simplistic POV of mine.
Can I rely on this mechanism to correctly dispatch method calls in any case, by
building a system such as:
* All values are class instances.
* They have a unique ancestor class C, on which all possibly overridable
methods are declared virtual.
* On sub-classes, all methods existing on the ancestor(s) are explicitely
declared override.
Then, whatever the class of a variable or of its value for Pascal, the correct
method will be called?
If yes, as I think now, this is a very constructive point :-)
Another one would be to acually know a value's type. How to get its name or,
much better, a pointer to it? (Since the language obviously has such a pointer.)
Denis
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