Troubleshooting sip assignHelper != NULL assertion

[Scott Talbert]

On Thu, 28 May 2020, Phil Thompson wrote:

> On 28/05/2020 16:30, 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.
>> 
>> That is probably how I would have designed the API, but alas it wasn't
>> done that way.  :)
>> 
>> Shouldn't a (default) copy constructor be sufficient to copy the
>> wxPGWindowList in this case?
>
> SIP may be being too conservative when determining if there is an implied 
> copy ctor. Try adding an explicit copy ctor to the wxPGWindowList .sip file.

I don't think that will be enough, though.  SIP seems to only set the 
assignment helper if there are *both* a public default constructor and a 
public copy constructor, see:

https://www.riverbankcomputing.com/hg/sip/file/d85e9957e726/sipgen/transform.c#l596

Scott