[Qt-interest] binaries in Ubuntu versions

[Harald Fernengel]

Hi,

On Monday 01 December 2008 16:17:04 Oliver.Knoll at comit.ch wrote:
> Harald Fernengel wrote on Monday, December 01, 2008 1:25 PM:
> >> ...
> >> Generally? I remember one "glitch" where the Trolls accidentally made
> >> a default argument (something like "int foo = 0") non-default. That
> >> broke the binary compatibility in the early 4.x days between two Qt
> >> versions (or was it even in the late 3.3.x days? Can't remember).
> >
> > Default arguments are not part of the ABI.
> >
> > If you know about any ABI changes in the Qt 4 series, please let us
> > know.
>
> I obviously seem to remeber the same "binary incompatibility" as Arnold has
> in mind, see his post:
>
> "Apart from one minor release in the Qt4 cycle, Qt is binary compatible."
>
> And AFAIK it had to to with an argument in some Qt method had a default
> value, as in:
>
>   QFoo::theMethod(int bar = 42);
>
> which was accidentally removed in the next minor release, as in:
>
>   QFoo::theMethod(int bar);
>
> Now older Qt applications which relied on the default value of '42' now
> would provide "random memory garbage" for the argument 'bar' at best, which
> clearly breaks binary compatibility. So I don't quite understand what you
> meant with "Default arguments are not part of the ABI", because they
> clearly are.

the default value is substituted by the compiler at compile time, not by the 
linker at runtime. No "random memory garbage" is possible.

A good reference of what is binary compatible and what not can be found here:

	http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++

Removing default arguments can be source code incompatible, though, but any Qt 
application linked to an older version of Qt will run with the newer version.

Best regards,
Harald