Troubleshooting sip assignHelper != NULL assertion

[Phil Thompson]

On 28/05/2020 15:58, Scott Talbert wrote:
> On Thu, 28 May 2020, Phil Thompson wrote:
> 
>>> Hi,
>>> 
>>> I'm running into the following assertion in wxPython that I don't
>>> quite understand:
>>> 
>>> python3: ../../../../sip/siplib/siplib.c:3444: parseResult: Assertion
>>> `assign_helper != NULL' failed.
>>> 
>>> This is the relevant C++ class:
>>> 
>>> class wxPGWindowList
>>> {
>>> public:
>>>     wxPGWindowList(wxWindow* primary, wxWindow* secondary = NULL)
>>>         : m_primary(primary)
>>>         , m_secondary(secondary)
>>>     {
>>>     }
>>> 
>>>     void SetSecondary(wxWindow* secondary) { m_secondary = secondary; 
>>> }
>>> 
>>>     wxWindow* GetPrimary() const { return m_primary; }
>>>     wxWindow* GetSecondary() const { return m_secondary; }
>>> 
>>>     wxWindow*   m_primary;
>>>     wxWindow*   m_secondary;
>>> };
>> 
>> It doesn't matter what the C++ class looks like, it's the 
>> corresponding .sip that would be of interest.
> 
> My bad, here it is:
> class wxPGWindowList
> {
>     %Docstring
>         PGWindowList(primary, secondary=None)
> 
>         Contains a list of editor windows returned by CreateControls.
>     %End
>     %TypeHeaderCode
>         #include <wx/propgrid/editors.h>
>     %End
> 
> public:
>     wxPGWindowList(
>         wxWindow * primary,
>         wxWindow * secondary = NULL
>     );
> 
>     void SetSecondary(
>         wxWindow * secondary
>     );
>     %Docstring
>         SetSecondary(secondary)
>     %End
> 
>     wxWindow * GetPrimary() const;
>     %Docstring
>         GetPrimary() -> wx.Window
> 
>         Gets window of primary editor.
>     %End
> 
>     wxWindow * GetSecondary() const;
>     %Docstring
>         GetSecondary() -> wx.Window
> 
>         Gets window of secondary editor.
>     %End
> 
>     public:
> 
> 
>     %Property(name=Primary, get=GetPrimary)
>     %Property(name=Secondary, get=GetSecondary, set=SetSecondary)
> };  // end of class wxPGWindowList
> 
> 
>>> The assertion occurs when trying to return an instance of
>>> wxPGWindowList in a Python method, e.g.:
>>> 
>>> def foo():
>>>     return wxpg.PGWindowList(a, b)
>>> 
>>> From what I can tell, there is no assignment helper assigned by sip
>>> because there is no default constructor?  I may be missing something,
>>> but I can't see why a default constructor would be needed.
>> 
>> You don't say what version of the sip module you are using but I'm 
>> guessing that that assertion is when it's parsing the Python object 
>> returned by a re-implementation of a C++ virtual. That doesn't seem to 
>> be happening in the above which suggests things aren't happening where 
>> you think they are.
> 
> sip module version is 4.19.19.  You are correct, I oversimplified what
> is actually happening.  This is really what is happening:
> 
> class LargeImageEditor(wxpg.PGEditor):
>     def CreateControls(self, propgrid, property, pos, sz):
>         ...
>         return wxpg.PGWindowList(self.tc, btn)
> 
> Where CreateControls is a C++ virtual, relevant .sip snippet:
> 
>     virtual
>     wxPGWindowList CreateControls(
>         wxPropertyGrid * propgrid,
>         wxPGProperty * property,
>         const wxPoint & pos,
>         const wxSize & size
>     ) const = 0;
>     %Docstring
>         CreateControls(propgrid, property, pos, size) -> PGWindowList
> 
>         Instantiates editor controls.
>     %End

So SIP need to copy the wxPGWindowList from the stack to the heap. 
Shouldn't CreateControls return a pointer to the wxPGWindowList?

Of course SIP should detect this when generating the code rather than 
rely on a runtime assertion.

Phil