[PyQt] Empty and template classes

[Phil Thompson]

On 10 Apr 2016, at 5:52 pm, Shaheed Haque <srhaque at theiet.org> wrote:
> 
> Hi Phil,
> 
> On 10 April 2016 at 16:40, Phil Thompson <phil at riverbankcomputing.com> wrote:
>> On 10 Apr 2016, at 2:53 pm, Shaheed Haque <srhaque at theiet.org> wrote:
>>> 
>>> Hi Phil,
>>> 
>>> On 10 April 2016 at 13:32, Phil Thompson <phil at riverbankcomputing.com> wrote:
>>>> On 10 Apr 2016, at 1:21 pm, Shaheed Haque <srhaque at theiet.org> wrote:
>>>>> 
>>>>> Hi,
>>>>> 
>>>>> One enhancement request, and a question. First, SIP 4.16.9 does not
>>>>> like empty classes like this:
>>>>> 
>>>>> class KUserOrGroupId
>>>>> {
>>>>> };
>>>>> 
>>>>> The above results in a syntax error. Adding anything makes it work, for example:
>>>>> 
>>>>> class KUserOrGroupId
>>>>> {
>>>>> %TypeHeaderCode
>>>>> #include <KCoreAddons/kuser.h>
>>>>> %End
>>>>> };
>>>>> 
>>>>> is accepted. It would be nice to have support for this corner case.
>>>>> Second, the syntax for template classes does not seem to work as I
>>>>> expected. This example results in a syntax error, as does every
>>>>> variation I could think of (including trying to match PyQt):
>>>>> 
>>>>> template <typename T>
>>>>> class KUserOrGroupId<T>
>>>>> {
>>>>> %TypeHeaderCode
>>>>> #include <KCoreAddons/kuser.h>
>>>>> %End
>>>>> };
>>>>> 
>>>>> Like the function case, I realise the implementation for this is not
>>>>> trivial (templated methods and the like), but I cannot seem to make
>>>>> the basic syntax work. It should, right?
>>>> 
>>>> See...
>>>> 
>>>> http://pyqt.sourceforge.net/Docs/sip4/specification_files.html#syntax-definition
>>>> 
>>>> ...also look at the PyQt .sip files.
>>>> 
>>>> In this case, omit 'typename'.
>>> 
>>> I tried (having reviewed the syntax page and parser.y) several
>>> variations such as this:
>>> 
>>> ====
>>> %Module test_template
>>> 
>>> template <T>
>>> class KUserOrGroupId<T>  <<< line 4
>>> {
>>> %TypeHeaderCode
>>> #include <KCoreAddons/kuser.h>
>>> %End
>>> };
>>> ====
>>> 
>>> which does not work. However, it *does* work if I discard the "<T>"
>>> from line 4, or use %MappedType. What is eluding me are the rules I
>>> need to follow for references to T within the class. For example,
>>> let's say the class contained a method with the signature:
>>> 
>>>   void foo(T *bar);
>>> 
>>> If I understand correctly, the semantics of passing T in are knowable,
>>> but SIP does not like the "T", and I have not been able to work out
>>> how the signature for foo should be written to get it past the syntax
>>> errors. Is a template class able to support things like this (once I
>>> implement the %MappedType)?
>> 
>> I honestly can't remember if it's supposed to work or not. What's the error when SIP 'does not like the "T"'?
>> 
>> I would actually approach it from the other end - first of all how do you want to represent KUserOrGroupId in Python?
> 
> The error from SIP is simply "syntax error" [1].

Methods in a MappedType must be static.

> But you ask an apposite question and in truth, I don't yet know, yet.
> The point is that SIP for KF5 (ad KDE generally) is a substantial
> undertaking for which I am attempting to develop an automated approach
> [2]. At this point, I'm trying to limit how deep I go in favour of
> breadth and trying to get *some* bindings working.
> 
> Trying to get enough of the generated SIP to run through the syntax
> phase at least seems important to me to ensure the overall automation
> approach is sound (this is not something I consider proved yet). This
> seems critical to me because the inability to sustain the maintenance
> effort on PyKDE4 seems to me to be the root cause that KDE 5 has no
> bindings.

I completely agree.

> It may be that this ends up being unrealistic, and I end up having to
> go deep and perhaps generate manual code for some of the base classes
> before anything useful can be made to work, but I'm not quite there
> yet.

The way I started to do it (many, many years ago) was to pick a simple, complete C++ example a do enough to be able to run a Python version of it. I then expanded the depth and the breadth as required.

Phil