Finalized proposal for i18n in KF5, second iteration

[Chusslove Illich]

> [: Oswald Buddenhagen :]
> as for the TRANSLATION_DOMAIN define, that really should come from the
> build system to start with.

I have nothing against that, but a there must be a way which is independent
of the build system. It would seem bizare to me that one could not, for
example, write a single-file program and build it directly with single
compiler invocation, and have i18n working too.

And I actually don't have an idea how one would do this with a build system
in a cleaner way.

> it's always a bad idea to use defines which configure headers in the
> sources themselves (you run into include order problems really quickly).

Maybe I should strenghten that point in the doc, but the idea (talking about
a public header) is that TRANSLATION_DOMAIN plus inclusion is done only
where necessary, and cancelled as soon as not necessary. For example, what
would be the problem with:

  #ifndef SOMEHEADER_H
  #define SOMEHEADER_H

  #include <foo.h>
  #include <bar.h>
  #include <klocalizedstring.h> // for whatever ordinary functionality
  #include <qwyx.h>

  // some code

  #define TRANSLATION_DOMAIN "blurb"
  #include <klocalizedstring.h>

  // some code with i18n calls, no includes in between

  #undef TRANSLATION_DOMAIN
  #include <klocalizedstring.h>

  // some code

  #endif

Of course, the assumption is that foo.h, bar.h, etc. also did this where
necessary, leaving a clean macro environment behind themselves.

> fwiw, this also means that the trick suggested at the bottom of
> klocalizedstring.h is not such a great idea (and to start with, to avoid
> that the preprocessor floods you with redefinition warnings, the #undefs
> have to come first, unconditionally).

I think that redefinition warnings are the good thing to have here, as
otherwise a wrongly placed include would silently clobber the outer
environment:

  #ifndef SOMEHEADER_H
  #define SOMEHEADER_H

  #define TRANSLATION_DOMAIN "blurb"
  #include <klocalizedstring.h>
  #include <foo.h>
  #include <bar.h> // also does TRANSLATION_DOMAIN thingy
  #include <qwyx.h>

  // some code, i18n calls here silently clobbered

  #undef TRANSLATION_DOMAIN
  #include <klocalizedstring.h>

  #endif

-- 
Chusslove Illich (Часлав Илић)
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