[C++-sig] wrapping operator=

[Hans Roessler]

Thank you, William.
The __setitem__ emulation is definitely a better idea than the assign function, as the class is actually a container type.

But still I am looking for a way to allow the simple "data=42." syntax with the same result, because it would best copy the C++ syntax.

Hans



----- Ursprüngliche Mail ----
> Von: William Ladwig <wladwig at wdtinc.com>
> An: Development of Python/C++ integration <cplusplus-sig at python.org>
> Gesendet: Montag, den 25. Mai 2009, 18:54:53 Uhr
> Betreff: Re: [C++-sig] wrapping operator=
> 
> Generally how that situation is handled on the Python side is to use the 
> container emulation API, so that assigning all values would look like this to a 
> user of your class:
> 
> data[:] = 42.
> 
> To do this, you just need to write a small c++ wrapper to expose the 
> __setitem__(self, key, value) function, which accepts (or extracts) a slice 
> object for the key and then calls your class operator= function.  Or, you could 
> just stick with the assign function that you created, which may be better if 
> your class isn't supposed to look like a container.  This may actually be less 
> error prone.
> 
> Here is the Python container emulation documentation:
> 
> http://docs.python.org/reference/datamodel.html#emulating-container-types
> 
> Other people may have better suggestions.  
> 
> Regards,
> Bill
> 
> ________________________________________
> From: cplusplus-sig-bounces+wladwig=wdtinc.com at python.org 
> [cplusplus-sig-bounces+wladwig=wdtinc.com at python.org] On Behalf Of Hans Roessler 
> [hansroessler at yahoo.de]
> Sent: Monday, May 25, 2009 9:08 AM
> To: cplusplus-sig at python.org
> Subject: [C++-sig] wrapping operator=
> 
> Hello!
> I want to wrap with boost.python a C++ class "DataCube" which has overloaded the 
> operator=.
> 
> The  constructor DataCube::DataCube(double x) allocates huge amounts of memory 
> and fills it with x's.
> The  DataCube::operator=(double x) just overwrites the already allocated memory 
> with x's.
> 
> Now in C++ these commands first allocate memory, which is filled with 0.'s, and 
> then overwrite the memory with 42.'s:
> >DataCube data(0.)
> >data=42.
> 
> In Python these commands first build the DataCube as desired, but then set 
> data=42. (now data is a float), where the reference to the DataCube is lost:
> >data=DataCube(0.)
> >data=42.
> 
> I have circumvented this by replacing the second line with
> > data.assign(42.)
> with a function assign which is defined appropriately, but I would prefer to use 
> the same assignment as in C++.
> 
> Finally the question:
> Can I define the DataCube class in Python, so that the data variable above will 
> behave as in C++, when I write "data=42." ?
> In other words, is there any possibility in Python that "x=y" does NOT make x a 
> reference to y?
> 
> Thank you
> Hans
> 
> 
> 
> 
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