[Qgis-developer] Template programming in QGIS

[Tim Sutton]

Hi Christoph

On Fri, May 13, 2011 at 9:32 AM, GOO Creations <goocreations at gmail.com> wrote:
> Hi all
>
> I'm currently writing C++ code that I want to use via Python with SIP. The
> following is a skeleton for my code:
>
> template<class T>
> class MyClass
> {
>     MyClass(QString s);
>     T* calculate();
> }
>
> class B
> {
>     B();
>     void doStuff();
> }
>
> void B::doStuff()
> {
>     MyClass<int> var("");
>     int *var2 = var.calculate();
> }
>
> The only class I've included in my SIP file is class B (and not class A).
> Everything compiles correctly, but when QGIS starts, I get a Python error:
>
>   File "/home/goocreations/apps/share/qgis/python/qgis/utils.py", line 283,
> in _import
>     mod = _builtin_import(name, globals, locals, fromlist, level)
> ImportError: /home/goocreations/apps/lib/libqgis_mylib.so.1.7.0: undefined
> symbol: _ZN15MyClassIiEC1E7QString
>
> This error seems strange, since I've never included MyClass in the SIP files
> and the only place MyClass is instantiated is in my cpp file (I didn't even
> include the MyClass directory in the cmake file that handles the SIP files).
> So I'm a bit confused  of how Python gives an error on code that isn't
> linked to SIP.
>
> So, I've though that creating a SIP binding for MyClass may resolve the
> issue, but I can't figure out how I would use templates in SIP.
>
> By the may: when I remove the template from MyClass and make everything by
> default int, everything compiles and runs perfectly.
>
> Does anyone have an idea of why this is happening, or if I maybe have to
> include something special in the cmake files?
>
> Thanks
>
> Chris
>

I remember some discussions at the lisbon hackfest about templated c++
code and difficulties wrapping it with sip. Martin Dobias probably
know more, but I would add that we generally avoid template
programming in QGIS as it isnt very 'Qt' like, and also isnt very
consistent with the rest of our api.

Regards

Tim

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>
>



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Tim Sutton - QGIS Project Steering Committee Member (Release  Manager)
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