# Subclassing SWIG generated c++ class

**URL:** <https://rubytalk.org/t/subclassing-swig-generated-c-class/141>\
**Category:** ruby-talk\
**Created:** [7 June 2002 07:34 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141 "2002-06-07T07:34:34Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![Martin\_Man](https://avatars.discourse-cdn.com/v4/letter/m/41988e/32.png) [@Martin\_Man](https://rubytalk.org/u/Martin_Man)\
**Post date:** [7 June 2002 07:34 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141/1 "2002-06-07T07:34:34Z")

</div>

hi all,

I’ve created simle c++ class

— class.cxx —  
#if SWIG  
%module myext  
#endif

class Foo {  
Foo () {};  
hello\_world () {};  
};

and then generated ruby extension module ‘Myext’ using SWIG for it…  
no problem so far, I’m able to require the extension,  
create instances of Foo, invoke methods, etc.

but when I try to subclass such class in ruby, the original class gets  
always created, am I missing something, e.g.

— test.rb —  
require ‘./myext.so’

module Myext  
class Bar \< Foo  
end  
end

k = Myext::Foo.new  
puts k  
k1 = Myext::Bar.new  
puts k1

both .new statements create instance of Myext::Foo…

ruby 1.6.7 (2002-03-19) [i386-linux]  
swig 1.3.11

I’m now trying the cvs versions of SWIG and ruby, but if anyone knows what’s  
happening…

greets,  
martin

> **···**
>
> –  
> Dennis: Help! Help! I’m being repressed! Come see the violence  
> inherent in the system! Violence inherent in the system!  
> – Monty Python and the Holy Grail

---

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**Author:** ![Pierre\_Brengard1](https://avatars.discourse-cdn.com/v4/letter/p/76d3ee/32.png) [@Pierre\_Brengard1](https://rubytalk.org/u/Pierre_Brengard1)\
**Post date:** [7 June 2002 08:13 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141/2 "2002-06-07T08:13:59Z")

</div>

maybe, the generated code lacks the call ‘rb\_obj\_call\_init’ in the code  
of the C functions that implement the ‘new’ Ruby method.

Pierre Brengard

Martin Man wrote:

> **···**
>
> > hi all,
> > 
> > I’ve created simle c++ class
> > 
> > — class.cxx —  
> > #if SWIG  
> > %module myext  
> > #endif
> > 
> > class Foo {  
> > Foo () {};  
> > hello\_world () {};  
> > };
> > 
> > and then generated ruby extension module ‘Myext’ using SWIG for it…  
> > no problem so far, I’m able to require the extension,  
> > create instances of Foo, invoke methods, etc.
> > 
> > but when I try to subclass such class in ruby, the original class gets  
> > always created, am I missing something, e.g.
> > 
> > — test.rb —  
> > require ‘./myext.so’
> > 
> > module Myext  
> > class Bar \< Foo  
> > end  
> > end
> > 
> > k = Myext::Foo.new  
> > puts k  
> > k1 = Myext::Bar.new  
> > puts k1
> > 
> > both .new statements create instance of Myext::Foo…
> > 
> > ruby 1.6.7 (2002-03-19) [i386-linux]  
> > swig 1.3.11
> > 
> > I’m now trying the cvs versions of SWIG and ruby, but if anyone knows what’s  
> > happening…
> > 
> > greets,  
> > martin

---

<div class="post-metadata">

**Author:** ![Christian\_Szegedy](https://avatars.discourse-cdn.com/v4/letter/c/ebca7d/32.png) [@Christian\_Szegedy](https://rubytalk.org/u/Christian_Szegedy)\
**Post date:** [7 June 2002 08:34 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141/3 "2002-06-07T08:34:22Z")

</div>

Martin Man wrote:

> k = Myext::Foo.new  
> puts k  
> k1 = Myext::Bar.new  
> puts k1
> 
> both .new statements create instance of Myext::Foo…

… and this is completely correct and intended. The object in  
the subclass contains data for all base classes. What’s the  
problem?

---

<div class="post-metadata">

**Author:** ![Lyle\_Johnson1](https://avatars.discourse-cdn.com/v4/letter/l/bcef8e/32.png) [@Lyle\_Johnson1](https://rubytalk.org/u/Lyle_Johnson1)\
**Post date:** [7 June 2002 13:55 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141/4 "2002-06-07T13:55:05Z")

</div>

Martin,

I will take your word for it that this was broken in SWIG-1.3.11 (i.e. I’m  
not going to go back and check ;). But I can tell you that we fixed a lot of  
bugs for the Ruby module in version 1.3.12 (released a few days back) and  
that I cannot reproduce this problem under SWIG-1.3.12.

HTH,

Lyle

---

<div class="post-metadata">

**Author:** ![Martin\_Man](https://avatars.discourse-cdn.com/v4/letter/m/41988e/32.png) [@Martin\_Man](https://rubytalk.org/u/Martin_Man)\
**Post date:** [7 June 2002 08:19 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141/5 "2002-06-07T08:19:01Z")

</div>

> maybe, the generated code lacks the call ‘rb\_obj\_call\_init’ in the code  
> of the C functions that implement the ‘new’ Ruby method.

well that could be, I can see by walking thru ObjectSpace that my class has  
actually been registered within ObjectSpace, but instantiation yields the  
original c++ class, btw: any chance I can get this call included automatically  
by swig instead of hacking the generated wrapper sources ???

> Pierre Brengard

thanx,  
martin

> **···**
>
> ## On Fri, Jun 07, 2002 at 05:13:59PM +0900, Pierre Brengard wrote:
> 
> Peasant 1: Who’s that there?  
> Peasant 2: I don’t know… Must be a king…  
> Peasant 1: Why?  
> Peasant 2: He hasn’t got shit all over him.  
> – Monty Python and the Holy Grail

---

<div class="post-metadata">

**Author:** ![Martin\_Man](https://avatars.discourse-cdn.com/v4/letter/m/41988e/32.png) [@Martin\_Man](https://rubytalk.org/u/Martin_Man)\
**Post date:** [7 June 2002 08:51 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141/6 "2002-06-07T08:51:33Z")

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well the problem is that you’d expect my program to output something like

#Myext::Foo:0x401ae050  
#Myext::Bar:0x401ae430

but it’s outputting

#Myext::Foo:0x401ae050  
#Myext::Foo:0x401ae430

the problem was exactly what Pierre Brengard [pbrengard@bct.de](mailto:pbrengard@bct.de) suggested,  
missing call to rb\_obj\_call\_init inside of wrapped new Ruby method, I’ve now  
tried cvs version of SWIG which splits c++ constructors into two methods,

- singleton method ‘alloc’
- and instance method ‘initialize’

and now I can happily wrap c++ classes inside ruby…

thanx,  
martin

> **···**
>
> On Fri, Jun 07, 2002 at 05:34:22PM +0900, Christian Szegedy wrote:
> 
> > Martin Man wrote:
> > 
> > > k = Myext::Foo.new  
> > > puts k  
> > > k1 = Myext::Bar.new  
> > > puts k1
> > > 
> > > both .new statements create instance of Myext::Foo…
> > 
> > … and this is completely correct and intended. The object in  
> > the subclass contains data for all base classes. What’s the  
> > problem?
> 
> –  
> God: What are you doing now?  
> Arthur: Averting our eyes, oh Lord.  
> God: Well, don’t. It’s just like those miserable psalms, always so depressing.  
> – Monty Python and the Holy Grail

---

<div class="post-metadata">

**Author:** ![Lyle\_Johnson6](https://avatars.discourse-cdn.com/v4/letter/l/bb73d2/32.png) [@Lyle\_Johnson6](https://rubytalk.org/u/Lyle_Johnson6)\
**Post date:** [7 June 2002 15:36 UTC](https://rubytalk.org/t/subclassing-swig-generated-c-class/141/7 "2002-06-07T15:36:00Z")

</div>

> … I’ve now tried cvs version of SWIG which splits c++ constructors into two methods,
> 
> - singleton method ‘alloc’
> - and instance method ‘initialize’

Just to be clear for anyone else following this thread; the Ruby method  
names are still Foo.new and Foo#initialize. I am aware of the changes  
for the Ruby allocation framework in 1.7 and am trying to decide exactly  
what impact (if any) it should have on SWIG’s code generation. It does  
seem to be the case that Matz has made the new allocation framework  
back-compatible, i.e. SWIG-generated code that provides Foo.new instead  
of Foo.allocate seems to do the right thing.