[fpc-devel] Reading empty collection from stream

[Michael Van Canneyt]

On Thu, 7 Sep 2006, Marc Weustink wrote:

> Vincent Snijders wrote:
>> Hi,
>> 
>> I am investigating Lazarus issue 7305. The TSynEdit class has a KeyStrokes 
>> property of the type TSynEditKeyStrokes, which is a TCollection descendant. 
>> The TSynEdit constructor fills the collection with some default items. If I 
>> remove the items and stream the TSynEdit, the following line is shown in 
>> the lfm:
>>     Keystrokes = <>
>> 
>> If I read this lfm, I don't get a empty KeyStrokes collection, but one 
>> filled with the default value.
>> 
>> I suspect the following lines in TReader.ReadCollection cause this 
>> behaviour:
>>     if not EndOfList then
>>       Collection.Clear;
>> 
>> Is this a bug or by design? How do I load an empty collection from a 
>> stream, if the collection has a default value?

You can't.

> Delphi has the same construction, which is IMO a design flaw.
> IMO when a collection is "default" is shouldn't be streamed, so there would 
> be nothing written.

There is no way to know whether the collection is default or not, 
since that would imply keeping a copy of the collection as it was 
just after creation.

> I don't know why it is there, since when a "default" collection is written, 
> all elements are written. So it never will be <>.
>
>> Note, if I remove the 'Keystrokes = <>' line from the lfm, I will get the 
>> default collection too, so I wonder why there are two ways to get the 
>> default collection.
>
> I propose to remove the "if not EndOfList then Collection.Clear;" line.

You may not, because then inherited form streaming will be broken: 
a collection of "<>" means: don't touch, keep the collection of
the parent form.

Michael.