[PyQt] Clarifications on new versions of SIP/PyQt

[Phil Thompson]

On Mon, 18 Jan 2010 12:11:20 +0100, Giovanni Bajo <rasky at develer.com>
wrote:
> Hi Phil,
> 
> I read in the SIP 4.10 changelog:
> 
> Added the -P command line option to build modules with "protected"
> redefined to "public" if supported by the platform. This can result in
> significantly smaller modules.
> 
> Can you please elaborate on:
> 
>  * What consequences on user's code the -P flag has. Does it break
> source code compatibility? What exactly is allowed when the -P flag is
> specified, and forbidden otherwise?
>  * Why does it affect code size so much?
> 
> Also, in PyQt changelog:
> 
> Significantly reduced the size of the modules on Linux and MacOS/X.
> 
> Is this related to the -P flag above? If so, does it mean that it is
> active by default when compiling PyQt on Linux/Mac? If not, how was the
> result achieved? Why it does not apply to Windows as well?

Yes it's the same thing.

Using the preprocessor to redefine "protected" as "public" is a common hack
that allows C++ code to call protected methods from outside of a sub-class.
It can only work if the name mangling scheme doesn't embed the access in
the name of the method. In other words, the actual name in the shared
library of the public void foo() is identical to the protected void foo().
On Linux and MacOS this is the case - but it isn't on Windows.

SIP has to be able to call protected methods from outside of a sub-class.
It does this my generating extra code in the form of a sub-class and
wrapper methods. By exploiting the hack SIP doesn't need to generate that
extra code - which can be a significant saving.

All this should be completely transparent to PyQt code.

Phil