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Mailing lists / fpc-pascal / 2005-04

Property with no default value

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Michalis Kamburelis

Hi

While converting some Delphi project to Lazarus I noticed that in FPC RTTI there is no way to mark a property as having *no* default value. This particular Delphi project depends on it (it relies on the fact that when saving some object to the stream, some of the properties will *always* be saved in the stream (this way it knows that "Set" methods of these properties will *always* be called when this object will be read from the stream)).

Looking at FPC typinfo unit, and implementation of TWriter class, and reaction on 'nodefault' directive in compiler/pdecvar.pas, there is no equivalent possibility in FPC. Every property has "implicit" default of false / 0 / [] / #0 etc. This means that I can never guarantee that when TWriter writes an object with e.g. some boolean property, it will *always* write that boolean property value, regardless of it's value (whether it's true or false).

The question is: did FPC decided to not implement this ? Or is it just not done yet ? In case it's planned to do this in FPC in a Delphi-compatible way, below is 1-paragraph description how it's done in Delphi (I don't describe here Delphi's implementation, only visible interface, so I guess that FPC devels may read this without any legal concerns :)

Delphi uses special value for TPropInfo.Default field: $80000000. It's used only when TPropInfo.Kind is one of tkInteger, tkChar, tkEnumeration, tkSet (in case of FPC this should also include tkBool). Then when TPropInfo.Default of this property is $80000000, the TWriter will always write this property value to the stream. The funny consequence is that an integer property with a "default $80000000" will always be saved to a stream, even if it doesn't have to be. So this is not very elegant solution from Borland (some field in TPropInfo like IsDefault would be better), but it's probably too late to change such things now, even for Borland, not to mention FPC if it tries to keep typinfo unit absolutely compatible.

Michalis

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Michael Van Canneyt
to Michalis Kamburelis
Michalis Kamburelis wrote 18 lines
Hi While converting some Delphi project to Lazarus I noticed that in FPC RTTI there is no way to mark a property as having *no* default value. This particular Delphi project depends on it (it relies on the fact that when saving some object to the stream, some of the properties will *always* be saved in the stream (this way it knows that "Set" methods of these properties will *always* be called when this object will be read from the stream)). Looking at FPC typinfo unit, and implementation of TWriter class, and reaction on 'nodefault' directive in compiler/pdecvar.pas, there is no equivalent possibility in FPC. Every property has "implicit" default of false / 0 / [] / #0 etc. This means that I can never guarantee that when TWriter writes an object with e.g. some boolean property, it will *always* write that boolean property value, regardless of it's value (whether it's true or false). The question is: did FPC decided to not implement this ?

No, just there wasn't time yet.

Show quoted text 16 lines
Or is it just not done yet ? In case it's planned to do this in FPC in a Delphi-compatible way, below is 1-paragraph description how it's done in Delphi (I don't describe here Delphi's implementation, only visible interface, so I guess that FPC devels may read this without any legal concerns :) Delphi uses special value for TPropInfo.Default field: $80000000. It's used only when TPropInfo.Kind is one of tkInteger, tkChar, tkEnumeration, tkSet (in case of FPC this should also include tkBool). Then when TPropInfo.Default of this property is $80000000, the TWriter will always write this property value to the stream. The funny consequence is that an integer property with a "default $80000000" will always be saved to a stream, even if it doesn't have to be. So this is not very elegant solution from Borland (some field in TPropInfo like IsDefault would be better), but it's probably too late to change such things now, even for Borland, not to mention FPC if it tries to keep typinfo unit absolutely compatible.

I think we'll change the propinfo, because that is the proper solution. The Borland way is very messy indeed.

But this will be after 2.0, unless you can send a patch.

Michael.

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