operator+= in python
"Langer, Stephen A. \(Fed\) via Swig-user" <[email protected]> Fri, 13 Jun 2025 20:18:41 +0000
| Newsgroups | gmane.comp.programming.swig |
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Hi --
What=92s the correct way to use SWIG to create a Python version of C++ comp=
ound assignment operator, like operator+=3D ? The simple thing isn=92t d=
oing what I expect.
For an object of a Python class that defines __iadd__,
pyobject +=3D other_pyobject
doesn=92t change the identity of pyobject. But for a C++ object that defin=
es operator+=3D, SWIG creates an __iadd__ method that returns a new Python =
object.
So
swigobject +=3D other_swigobject
changes the identity of swigobject, although it still refers to the same C+=
+ object. When the redundant Python object is deleted, the C++ destructor =
is called an extra time, which is bad.
The swig file just declares a simple class that defines MyClass& operator+=
=3D(const MyClass&). The generated __iadd__ wrapper function ends with
resultobj =3D SWIG_NewPointerObj(SWIG_as_voidptr(result), SWIGTYPE_p_=
MyClass, SWIG_POINTER_OWN | 0 );
return resultobj;
Changing SWIG_POINTER_OWN|0 to 0 fixes the problem, as far as my simple tes=
ts show. I don=92t know if it introduces other problems.
I=92ve tried SWIG 4.1 to 4.3.1, and they all behave the same.
Thanks.
-- Steve
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<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">Hi --<o:p></o:p></s=
pan></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p> </o:p></=
span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">What=92s the correc=
t way to use SWIG to create a Python version of C++ compound assignment ope=
rator, like operator+=3D ? The simple thing isn=92t doing=
what I expect.<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p></o:p></span><=
/p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">For an object of a =
Python class that defines __iadd__,<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"> &=
nbsp;pyobject +=3D other_pyobject<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">doesn=92t change th=
e identity of pyobject. But for a C++ object that defines operator+=
=3D, SWIG creates an __iadd__ method that returns a new Python object. =
;
<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">So<o:p></o:p></span=
></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"> =
swigobject +=3D other_swigobject<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">changes the identit=
y of swigobject, although it still refers to the same C++ object. Whe=
n the redundant Python object is deleted, the C++ destructor is called an e=
xtra time, which is bad.<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p> </o:p></=
span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">The swig file just =
declares a simple class that defines MyClass& operator+=3D(const MyClas=
s&). The generated __iadd__ wrapper function ends with
<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:10.5pt"> &=
nbsp; resultobj =3D SWIG_NewPointerObj(SWIG_as_voidptr(result), =
SWIGTYPE_p_MyClass, SWIG_POINTER_OWN | 0 );<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:10.5pt"> =
return resultobj;<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">Changing SWIG_POINT=
ER_OWN|0 to 0 fixes the problem, as far as my simple tests show. I do=
n=92t know if it introduces other problems.<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p> </o:p></=
span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">I=92ve tried SWIG 4=
.1 to 4.3.1, and they all behave the same.<o:p></o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p> </o:p></=
span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt">Thanks. <o:p></o:p>=
</span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"> -- Steve<o:p>=
</o:p></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11.0pt"><o:p> </o:p></=
span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:10.5pt"><o:p> </o:p></=
span></p>
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