On 07 Jun 2009, at 10:35, fpcl...@silvermono.co.za wrote:
A high level, a class is like a record that has been modified to
include
functions and procedures. I know that I'm over simplifying thing
here, please
bare with me.
The difference you skip over is the fundamental reason why one works
and the other doesn't: a class is a pointer, while a record is a value.
I'm trying to understand the logic employed by the creators of
Delphi where
they don't allow to write to the fields of a record type property,
but if the
property points to a class type, then anything goes.
In case of a class, the property returns a pointer (namely the pointer
to the class instance data), and then you (implicitly) dereference the
pointer and write data where it points to. In case of a record, the
property returns a record's value, and then (semantically) you change
the value of this returned record (not the value of the element of the
record that the property referred to).
This worked in previous FPC versions because rather than treating the
result of the property like a function result (which it has to,
because that's the semantical meaning of a property, so you can
transparently change them into getters/setters without breaking any
code), it treated it like a direct field access in case no getter/
setter existed. So rather than returning a record's value, the
property returned "a reference to a record".
In the example bellow,
where a property is of a class type, both Delphi and FPC compile the
code,
but there is no guarantee that the object referenced to by the
property has
been instantiated before the property is used (The programmer must
instantiate the TTestProp class within TTestClass prior to any call
made to xx
the property). IMO, it would be a nice feature if the compiler could
be
modified to issue a warning in such a case.Again, I'm over
simplifying, to
the compiler, it would be similar to checking for a variable
declaration
before the variable is used.
Except that it's about dynamically allocated memory and depends on
interprocedural control flow graph analysis, which makes it immensely
more complex.
I thing that the "error" in the way that FPC allows record
properties to
access the record fields could be handy if retained. Perhaps this
feature
could be reserved for objfpc mode. What are your thoughts on the
matter?
Things will remain the way they are for the reasons explained above.
Jonas
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