Boost C++ Libraries: Ticket #2049: extracting c++ pointer from python object with multiple parents

Dave Abrahams — Wed, 25 Jun 2008 16:38:24 GMT

Does this fail even when the dtors are non-virtual?

bloodyfanatic@… — Wed, 25 Jun 2008 16:48:15 GMT

Replying to dave:

Does this fail even when the dtors are non-virtual?

yes it does, with the same error as always:

Traceback (most recent call last):
  File "test.py", line 15, in <module>
    extractBase2(obj1)  # doesn't work
TypeError: No registered converter was able to extract a C++ pointer to type class Base2 from this Python object of type both1

status changed; cc, resolution set

Dave Abrahams — Thu, 26 Jun 2008 14:27:58 GMT

cc Stefan Seefeld added
status new → closed
resolution → invalid

Replying to bloodyfanatic@gmx.de:

Replying to dave:

Does this fail even when the dtors are non-virtual?

yes it does, with the same error as always:

In that case, please post the fully reduced test case

Also, you need an init method in both1 and both2 that initializes *both* bases or this can never work, because a wrapped class' init function is what actually constructs the C++ base object. Without it, there is no base object to which a pointer can be returned from extract:

  def __init__(self):
     Base1.__init__(self)
     Base2.__init__(self)

I don't know how to do this properly with super (the technique Stefan was suggesting), but his suggestion as written didn't take care of both bases, as the following demonstrates:

>>> class X(object):
...    def __init__(self):
...        print 'X'
...
>>> class Y(object):
...     def __init__(self):
...         print 'Y'
...
>>> class Z(X,Y): pass  # essentially what you posted
...
>>> z = Z()
X
>>> class Z(X,Y):
...     def __init__(self):
...         super(Z,self).__init__()  # Stefan's suggestion
...
>>> z = Z()
X
>>> class Z(X,Y):
...     def __init__(self):
...         X.__init__(self)
...         Y.__init__(self)
...
>>> z = Z()
X
Y
>>>

I'm pretty sure this solves your problem so I'm closing the ticket, but if it doesn't, please re-open it.

anonymous — Thu, 26 Jun 2008 14:56:34 GMT

Replying to dave:

Replying to bloodyfanatic@gmx.de:

Replying to dave:

Does this fail even when the dtors are non-virtual?

yes it does, with the same error as always:

In that case, please post the fully reduced test case

Also, you need an init method in both1 and both2 that initializes *both* bases or this can never work, because a wrapped class' init function is what actually constructs the C++ base object. Without it, there is no base object to which a pointer can be returned from extract:
  def __init__(self):
     Base1.__init__(self)
     Base2.__init__(self)
I don't know how to do this properly with super (the technique Stefan was suggesting), but his suggestion as written didn't take care of both bases, as the following demonstrates:
>>> class X(object):
...    def __init__(self):
...        print 'X'
...
>>> class Y(object):
...     def __init__(self):
...         print 'Y'
...
>>> class Z(X,Y): pass  # essentially what you posted
...
>>> z = Z()
X
>>> class Z(X,Y):
...     def __init__(self):
...         super(Z,self).__init__()  # Stefan's suggestion
...
>>> z = Z()
X
>>> class Z(X,Y):
...     def __init__(self):
...         X.__init__(self)
...         Y.__init__(self)
...
>>> z = Z()
X
Y
>>>
I'm pretty sure this solves your problem so I'm closing the ticket, but if it doesn't, please re-open it.

just tested that and it works. Thank you very much :)

Stefan Seefeld — Thu, 26 Jun 2008 15:02:03 GMT

Dave,

The use of 'super' with init does in fact work, as long as all classes use it:

class A(object):
    def __init__(self):
        super(A, self).__init__()
        print 'A'
class B(object):
    def __init__(self):
        super(B, self).__init__()
        print 'B'
class C(A, B):
    def __init__(self):
        super(C, self).__init__()
        print 'C'
c = C() # prints A B C

The reason for this is pretty well explained in http://fuhm.net/super-harmful/. In a nutshell: super() doesn't (necessarily) invoke the superclass method itself, but the next one in the MRO chain. If you omit super in one case, you break out of this chain.

FWIW.

Dave Abrahams — Thu, 26 Jun 2008 16:27:52 GMT

I knew that once ;-) thanks for the reminder.

milestone deleted

Mon, 03 Nov 2008 14:20:34 GMT

milestone Boost 1.35.1

Milestone Boost 1.35.1 deleted