[C++-sig] Instance ownership transfer in constructor.
Hi,
I'm new to python.boost library and I'm trying to use it to wrap a third
party library. Everything else I've managed to get working - thanks to
excellent library and lot of examples I've found around a net.
Only thing I can't get working properly is instance ownership transfer
in constructor.
So classes in library are defined like follows:
class Owner {
...
}
class Data {
Data(Owner *owner); // <-- Here happens ownership transfer.
}
Python code is like follows:
owner = Owner()
data_1 = Data(owner)
data_2 = Data(owner)
So when Python script stops runnning it causes crash due the fact that
I've data objects are automatically destroyed by Owner-class but Python
deletes them by using Data-class destructor (which shouldn't happen ever).
--
Jani Tiainen
- Well planned is half done and a half done has been sufficient before...
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Re: [C++-sig] Instance ownership transfer in constructor.
I want to do it another way around:
Instance of Owner should hold reference to data_1 and data_2 as long as
owner is alive.
Now following happens:
owner = Owner()
data_1 = Data(owner) # data_1 ownership is transferred to owner object
data_2 = Data(owner) # data_2 ownership is transferred to owner object
print owner.get_data_objects()
,
del data_1
del data_2
print owner.get_data_objects()
# Crash because data_1 and data_2 are deleted even owner should still hold
the reference.
I tried to do it like
http://www.boost.org/doc/libs/1_49_0/libs/python/test/injected.cpp but it
didn't worked for me.
On Thu, Jul 5, 2012 at 7:00 PM, John Reid wrote:
> On 05/07/12 11:49, Jani Tiainen wrote:
> > Hi,
> >
> > I'm new to python.boost library and I'm trying to use it to wrap a third
> > party library. Everything else I've managed to get working - thanks to
> > excellent library and lot of examples I've found around a net.
> >
> > Only thing I can't get working properly is instance ownership transfer
> > in constructor.
> >
> > So classes in library are defined like follows:
> >
> > class Owner {
> > ...
> > }
> >
> > class Data {
> > Data(Owner *owner); // <-- Here happens ownership transfer.
> > }
> >
> >
> > Python code is like follows:
> >
> > owner = Owner()
> > data_1 = Data(owner)
> > data_2 = Data(owner)
> >
> > So when Python script stops runnning it causes crash due the fact that
> > I've data objects are automatically destroyed by Owner-class but Python
> > deletes them by using Data-class destructor (which shouldn't happen
> ever).
> >
>
> If I understand you correctly then you want the owner object to remain
> alive at least as long as data_1 and data_2? If so you could use
> with_custodian_and_ward:
>
> http://www.boost.org/doc/libs/1_49_0/libs/python/doc/v2/with_custodian_and_ward.html
>
> For example something like the following should work:
>
> namespace py = boost::python;
> py::class_<
> ...
> > my_class(
> "Data",
> "docstring.",
> py::init< Owner * >( "Constructor." )[
> py::with_custodian_and_ward< 1, 2 >()
> ]
> );
>
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--
Jani Tiainen
- Well planned is half done, and a half done has been sufficient before...
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Re: [C++-sig] Instance ownership transfer in constructor.
Hi,
I'm still strugling with this whole thingy so I came up with more
concrete example I'm trying to achieve:
http://pastebin.com/dVRfT56x
And in python code I do like this:
owner = Owner()
data = Data(owner)
After script is finished I get warning message that Data destructor
shouldn't be called directly. And then python crashes due memory corruption.
I've tried to make usage of shared_ptr but no luck and I didn't
understood how to apply shared_ptr_for_this.
(NOTE: in my case all classes are from 3rd party library that I have no
control over)
5.7.2012 20:44, Holger Brandsmeier kirjoitti:
Jani,
ok what you want to do is quite a lot more intrusive, so you need some
more involved methods, but it should be possible. I would do it by
noting the following
- data_1 is published to python as a boost::shared_ptr (default
holder type)
- usually it is difficult for a class member function, to obtain a
shared_ptr<> to itself, i.e. something like `this` but not of type
Data* but of boost::shared_ptr.
- fortunately there is something supported by boost itself, called
`enable_shared_from_this`, see
http://stackoverflow.com/questions/142391/getting-a-boostshared-ptr-for-this
- Now you want `owner` to hold on to this `shared_ptr` which
ensures that `data_1` does not get deleted before `owner`, you can do
this by
owner->setData( shared_from_this() )
in any member function of `Data`.
- Note that I _think_ that you are not allowed to use
`shared_from_this()` in the constructor itself, so you might have to
use boost::pythons `make_constructor` to make an ordinary member /
non-member function behave as the constructor in python.
Maybe someone has a quicker way of doing this, but remember that this
is exactly the use case that `shared_ptr` are made for,
Holger
On Thu, Jul 5, 2012 at 6:47 PM, Jani Tiainen wrote:
I want to do it another way around:
Instance of Owner should hold reference to data_1 and data_2 as long as
owner is alive.
Now following happens:
owner = Owner()
data_1 = Data(owner) # data_1 ownership is transferred to owner object
data_2 = Data(owner) # data_2 ownership is transferred to owner object
print owner.get_data_objects()
,
del data_1
del data_2
print owner.get_data_objects()
# Crash because data_1 and data_2 are deleted even owner should still hold
the reference.
I tried to do it like
http://www.boost.org/doc/libs/1_49_0/libs/python/test/injected.cpp but it
didn't worked for me.
On Thu, Jul 5, 2012 at 7:00 PM, John Reid
wrote:
On 05/07/12 11:49, Jani Tiainen wrote:
Hi,
I'm new to python.boost library and I'm trying to use it to wrap a third
party library. Everything else I've managed to get working - thanks to
excellent library and lot of examples I've found around a net.
Only thing I can't get working properly is instance ownership transfer
in constructor.
So classes in library are defined like follows:
class Owner {
...
}
class Data {
Data(Owner *owner); // <-- Here happens ownership transfer.
}
Python code is like follows:
owner = Owner()
data_1 = Data(owner)
data_2 = Data(owner)
So when Python script stops runnning it causes crash due the fact that
I've data objects are automatically destroyed by Owner-class but Python
deletes them by using Data-class destructor (which shouldn't happen
ever).
If I understand you correctly then you want the owner object to remain
alive at least as long as data_1 and data_2? If so you could use
with_custodian_and_ward:
http://www.boost.org/doc/libs/1_49_0/libs/python/doc/v2/with_custodian_and_ward.html
For example something like the following should work:
namespace py = boost::python;
py::class_<
...
> my_class(
"Data",
"docstring.",
py::init< Owner * >( "Constructor." )[
py::with_custodian_and_ward< 1, 2 >()
]
);
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--
Jani Tiainen
- Well planned is half done, and a half done has been sufficient before...
___
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--
Jani Tiainen
- Well planned is half done and a half done has been sufficient before...
___
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Re: [C++-sig] Instance ownership transfer in constructor.
Ok, last suggestions were closest I could get: Now I see that data is
deleted within owner destructor but still I get error messages that data
objects are being deleted directly second time by Python.
So now I need some way to say to python that "data" classes are not
meant to be deleted by python ever.
6.7.2012 12:57, Holger Brandsmeier kirjoitti:
Jani,
can you try what John Reid proposed but instead of
py::with_custodian_and_ward< 1, 2 >()
please try to use
py::with_custodian_and_ward< 2, 1 >()
or (more likely to work)
py::with_custodian_and_ward_postcall< 2, 1 >()
This should bind the lifetime of `self` to its first argument. I just
never tried if this is a legal thing to do in the constructor.
-Holger
On Fri, Jul 6, 2012 at 8:20 AM, Jani Tiainen wrote:
Hi,
I'm still strugling with this whole thingy so I came up with more concrete
example I'm trying to achieve:
http://pastebin.com/dVRfT56x
And in python code I do like this:
owner = Owner()
data = Data(owner)
After script is finished I get warning message that Data destructor
shouldn't be called directly. And then python crashes due memory corruption.
I've tried to make usage of shared_ptr but no luck and I didn't understood
how to apply shared_ptr_for_this.
(NOTE: in my case all classes are from 3rd party library that I have no
control over)
5.7.2012 20:44, Holger Brandsmeier kirjoitti:
Jani,
ok what you want to do is quite a lot more intrusive, so you need some
more involved methods, but it should be possible. I would do it by
noting the following
- data_1 is published to python as a boost::shared_ptr (default
holder type)
- usually it is difficult for a class member function, to obtain a
shared_ptr<> to itself, i.e. something like `this` but not of type
Data* but of boost::shared_ptr.
- fortunately there is something supported by boost itself, called
`enable_shared_from_this`, see
http://stackoverflow.com/questions/142391/getting-a-boostshared-ptr-for-this
- Now you want `owner` to hold on to this `shared_ptr` which
ensures that `data_1` does not get deleted before `owner`, you can do
this by
owner->setData( shared_from_this() )
in any member function of `Data`.
- Note that I _think_ that you are not allowed to use
`shared_from_this()` in the constructor itself, so you might have to
use boost::pythons `make_constructor` to make an ordinary member /
non-member function behave as the constructor in python.
Maybe someone has a quicker way of doing this, but remember that this
is exactly the use case that `shared_ptr` are made for,
Holger
On Thu, Jul 5, 2012 at 6:47 PM, Jani Tiainen wrote:
I want to do it another way around:
Instance of Owner should hold reference to data_1 and data_2 as long as
owner is alive.
Now following happens:
owner = Owner()
data_1 = Data(owner) # data_1 ownership is transferred to owner object
data_2 = Data(owner) # data_2 ownership is transferred to owner object
print owner.get_data_objects()
,
del data_1
del data_2
print owner.get_data_objects()
# Crash because data_1 and data_2 are deleted even owner should still
hold
the reference.
I tried to do it like
http://www.boost.org/doc/libs/1_49_0/libs/python/test/injected.cpp but
it
didn't worked for me.
On Thu, Jul 5, 2012 at 7:00 PM, John Reid
wrote:
On 05/07/12 11:49, Jani Tiainen wrote:
Hi,
I'm new to python.boost library and I'm trying to use it to wrap a
third
party library. Everything else I've managed to get working - thanks to
excellent library and lot of examples I've found around a net.
Only thing I can't get working properly is instance ownership transfer
in constructor.
So classes in library are defined like follows:
class Owner {
...
}
class Data {
Data(Owner *owner); // <-- Here happens ownership transfer.
}
Python code is like follows:
owner = Owner()
data_1 = Data(owner)
data_2 = Data(owner)
So when Python script stops runnning it causes crash due the fact that
I've data objects are automatically destroyed by Owner-class but Python
deletes them by using Data-class destructor (which shouldn't happen
ever).
If I understand you correctly then you want the owner object to remain
alive at least as long as data_1 and data_2? If so you could use
with_custodian_and_ward:
http://www.boost.org/doc/libs/1_49_0/libs/python/doc/v2/with_custodian_and_ward.html
For example something like the following should work:
namespace py = boost::python;
py::class_<
...
> my_class(
"Data",
"docstring.",
py::init< Owner * >( "Constructor." )[
py::with_custodian_and_ward< 1, 2 >()
]
);
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