Error code “Project ****** raised exception class ‘EReadError with unknown property ‘AutoCommit’

Sent from my iPhone

This is only when I try to compile on Lazarus 2.0.6 on Windows. The project works fine on 2.0.10 on Linux. Any ideas?

Sent from my iPhone

I forgot to add that it compiles correctly, but then I get the runtime error

Sent from my iPhone

Your .lfm file contains a property "autocommit" that exists in 2.0.10 but not in 2.0.6. I think 2.0.10 is based on FPC 3.2.0 and 2.0.6 was still based on 3.0.4.

A lot of new properties appeared between 3.0.4 and 3.2.0.

Michael.

So we lose some backwards portability?

Sent from my iPhone

This problem was brought to light when I couldn’t get 2.0.10 apps to hook up to MySQL on Windows. I have no problem on my Linux box.

Sent from my iPhone

Yes, but that's always so when properties are added to components.

Michael.

Usually we try to use the "default" feature of the newly added properties so that they are streamed only when really set and used. When they are not used, the LFM can still be loaded in a legacy version of Lazarus because the new properties are not streamed.

Ondrej

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On Fri, Nov 13, 2020 at 6:19 PM Larry Dalton via lazarus <

wrote: So we lose some backwards portability?

IIRC it has been called "forward compatibility" as opposed to "backward compatibility".
Backward compatibility means your old code works in new versions of FPC/LCL/Lazarus.
The other direction is not guaranteed. New code cannot always be used in old versions.

Juha
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Ondrej Pokorny wrote

Usually we try to use the "default" feature of the newly added properties so that they are streamed only when really set and used. When they are not used, the LFM can still be loaded in a legacy version of Lazarus because the new properties are not streamed.

I set defaults on all boolean properties of sqlscript, rev. 47412

Strange, I thought that boolean properties were not streamed if they had value 'false'.

Michael.

Michael Van Canneyt via lazarus schrieb am
Fr., 13. Nov. 2020, 22:37:

No, cause their intended default value could be True. Thus no special handling for them.

Regards,
Sven

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The FPC documentation is spot on about the stored/default modifiers: https://www.freepascal.org/docs-html/ref/refsu38.html

Only "/String, floating-point and pointer properties have implicit////default//value of empty string, 0 or nil, respectively. Ordinal and set properties have no implicit//default//value./"

I polished the docs section in r1777 a little bit.

Then I saw the next chapter "Overriding properties". It wrongly described redeclaring of properties as overriding. I fixed that as well in r1778.

Ondrej

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Exactly.

Although in case of LFM streaming we usually try to help with default property values as we discussed before. And yes, I usually forget to add the default values as well :)

Ondrej

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Indeed.

I polished the docs section in r1777 a little bit.

Well done, thank you !

Then I saw the next chapter "Overriding properties". It wrongly described redeclaring of properties as overriding. I fixed that as well in r1778.

I think the difference is rather artificial, but if you think this is better: great.

Thank you for your efforts !

Michael.

You are right. I didn't know one can override also read/write specifiers (I thought one must redeclare the property in that case):

  TAncestor = class(TComponent)
  private
    fPropName: string;
  published
    property PropName: string read fPropName write fPropName;   end;
  TOverride = class(TAncestor)
  private
    fPropName2: string;
  published
    property PropName read fPropName2 write fPropName2; // no type declaration
  end;

But anyway as stated in the Delphi docs
http://docwiki.embarcadero.com/RADStudio/Sydney/en/Properties_(Delphi)#Property_Overrides_and_Redeclarations :

/Whether a property is hidden or overridden in a derived class, property look-up is always static./

So you are right - there is no real difference in property override/reintroduce like there is override/overload/reintroduce difference in object methods.

I'll make that more clear in FPC docs.

Ondrej

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You can of course say the same for all other types for which an implicit 'default' is implemented. The more so because a class is initialized with 0 in which case every enumerated and boolean will be ord(0) unless otherwise initialized in the constructor.

So I don't see why this rule is not applied to all properties, I don't think it is logical.

Michael.

I agree. One would not expect the implicit default values for string/real/pointer properties.

And then people wonder why their empty string properties are not streamed:

TMyClass = class
published
   property MyString: string read FString write FString stored
IsMyStringStored;
end;
constructor TMyClass.Create;
begin
   FMyString := 'abc';
end;
function TMyClass.IsMyStringStored: Boolean;
begin
   Result := FMyString'abc';
end;

What's wrong?
-> The nodefault is missing in order the empty string be streamed:   property MyString: string read FString write FString stored IsMyStringStored nodefault;

Not logical, not intuitive, but that is how it works and is documented :/

Ondrej