[C++-sig] express pointer ownership

2015-08-13 Thread MM 
I have the following class:

class T {
};
// T has been exposed to python with class_

and free function:

void add_T( T* );

Ownership of the T* is taken by this C++ function.


If I create an instance of the python version of T, how do I "def" the
add_T function?

def("add_T", add_T)

fails to compile.
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Re: [C++-sig] express pointer ownership

2015-08-13 Thread MM 
On 13 August 2015 at 10:24, MM  wrote:

> I have the following class:
>
> class T {
> };
> // T has been exposed to python with class_
>
> and free function:
>
> void add_T( T* );
>
> Ownership of the T* is taken by this C++ function.
>
>
> If I create an instance of the python version of T, how do I "def" the
> add_T function?
>
> def("add_T", add_T)
>
> fails to compile.
>

Apologies. This compiled correctly.

This function:

const T* get_T( const std::string& name );

failed to compile.

so the T pointer is owner by a container in the c++ world, it gets stored
there by add_T,
then the get_T  returns a raw pointer to it. I want to tell python to let
c++ manage it.
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Re: [C++-sig] express pointer ownership

2015-08-13 Thread Stefan Seefeld 
On 13/08/15 06:58 AM, MM wrote:
> On 13 August 2015 at 10:24, MM  > wrote:
>
> I have the following class:
>
> class T {
> };
> // T has been exposed to python with class_
>
> and free function:
>
> void add_T( T* );
>
> Ownership of the T* is taken by this C++ function.
>
>
> If I create an instance of the python version of T, how do I "def"
> the add_T function?
>
> def("add_T", add_T)
>
> fails to compile.
>
>
> Apologies. This compiled correctly.
>
> This function: 
>
> const T* get_T( const std::string& name );
>
> failed to compile.
>
> so the T pointer is owner by a container in the c++ world, it gets
> stored there by add_T,
> then the get_T  returns a raw pointer to it. I want to tell python to
> let c++ manage it.

Sounds like you want to use the "return_internal_reference" call policy
(see
http://boostorg.github.io/python/doc/html/tutorial/tutorial/functions.html#tutorial.functions.call_policies).

Stefan

-- 

  ...ich hab' noch einen Koffer in Berlin...

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Re: [C++-sig] express pointer ownership

2015-08-13 Thread MM 
On 13 August 2015 at 12:01, Stefan Seefeld  wrote:

> On 13/08/15 06:58 AM, MM wrote:
> > On 13 August 2015 at 10:24, MM  > > wrote:
> >
> > I have the following class:
> >
> > class T {
> > };
> > // T has been exposed to python with class_
> >
> > and free function:
> >
> > void add_T( T* );
> >
> > Ownership of the T* is taken by this C++ function.
> >
> >
> > If I create an instance of the python version of T, how do I "def"
> > the add_T function?
> >
> > def("add_T", add_T)
> >
> > fails to compile.
> >
> >
> > Apologies. This compiled correctly.
> >
> > This function:
> >
> > const T* get_T( const std::string& name );
> >
> > failed to compile.
> >
> > so the T pointer is owner by a container in the c++ world, it gets
> > stored there by add_T,
> > then the get_T  returns a raw pointer to it. I want to tell python to
> > let c++ manage it.
>
> Sounds like you want to use the "return_internal_reference" call policy
> (see
>
> http://boostorg.github.io/python/doc/html/tutorial/tutorial/functions.html#tutorial.functions.call_policies
> ).
>
> Stefan
>
>
That policy says:

"Ties lifetime of one argument to that of result"

The argument of my function is just the string Really its lifetime
doesn't matter
In the context of a call from python:

t = get_T( 'name1' )

At some point, a std::string temporary must be constructed, holding
'name1'?  and then get_T uses it for lookup.
Once get_T returns   the const T*, it doesn't care about it anymore.

What I want to express is:

The t returned by the python function should refer to object T held in c++
memory, and for instance

del t

should not delete the actual T object in c++ memory

Should I still use "return_internal_reference"  ?
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Re: [C++-sig] express pointer ownership

2015-08-13 Thread Alex Mohr 

On 8/13/2015 4:26 AM, MM wrote:

What I want to express is:

The t returned by the python function should refer to object T held in
c++ memory, and for instance

del t

should not delete the actual T object in c++ memory

Should I still use "return_internal_reference"  ?


You can use reference_existing_object 
(http://www.boost.org/doc/libs/1_59_0/libs/python/doc/v2/reference_existing_object.html)


But as the docs say, that can be dangerous since if I do 't = get_T()' 
in python and then sometime later C++ deletes the object that t refers 
to, now I have a dangling pointer.  If I try to use 't' in python now I 
have undefined behavior (and likely crash).


Alex

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