fpcl...@silvermono.co.za wrote:
On Sunday 07 June 2009 22:19:47 Jonas Maebe wrote:
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 understand the difference, but a pointer to a record still suffers the from
the same problem as a pointer to a class i.e. if memory is not allocated then
the code fails at runtime. It's interesting to note though, that Delphi allows
the assignment to the fields of a record type property via the with - do
construct. Is this also the case with FPC 2.3.1?
property ARecord: TSomeRecorType read FTheRecord;
property AClass: TSomeClassType read FTheClass;
must be treated the same as
function GetARecord: TSomeRecorType;
function GetAClass: TSomeClassType;
Why? Because that is what a property is for. A property means you are
free to change the "read" part to use a getter-function. And doing that
change *must* not break any code.
You can not (not in a sensible way) do
GetARecord() := ....
Neither does it make sense to do
GetARecord().X := ....
The last bit may be possible, but it would assign .X to a *temporary*
copy of the record, same as
MyTemp := GetARecord();
MyTemp.X := ....
What is the Point of this assignment, the value is lost, and unless you
actually have a temp variable (like in the expanded example) you can not
even retrieve the assigned value.
However
with GetARecord() do begin
x := 5; // only the temp copy
writeln(x); // using the value from the temp copy
end;
-------------
As for classes
GetAClass()
does not return a temporary copy of your class, it return a temporary
pointer to your class.
so the following makes NO sense
GetAClass() := TClass.Create; // trying to replace the class
but
GetAClass().X := 5;
following the temporary pointer still leads to the real object
-------------
This is because TClass behaves like a pointer.
Record does not behave like a pointer ** (the whole memory occupied by
all values of the record is copied). **
This difference also applies if you pass such values as params to a
procedure or function.
Martin
_______________________________________________
fpc-pascal maillist - fpc-pascal@lists.freepascal.org
http://lists.freepascal.org/mailman/listinfo/fpc-pascal
