# Inheritance from c

**URL:** <https://www.ruby-forum.com/t/inheritance-from-c/62501>\
**Category:** Ruby\
**Created:** [June 5, 2006, 5:32pm UTC](https://www.ruby-forum.com/t/inheritance-from-c/62501 "2006-06-05T17:32:27Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![splashlin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/90db22/32.png) [@splashlin](https://www.ruby-forum.com/u/splashlin)\
**Post date:** [June 5, 2006, 5:32pm UTC](https://www.ruby-forum.com/t/inheritance-from-c/62501/1 "2006-06-05T17:32:27Z")

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Hi All,

I’m messing about with inheritance from my C application but can’t get  
it to  
work properly. Here is a quick overview of what I want to achieve:

class Hello should parent Helloworld. Both use Data\_Wrap\_Struct to wrap  
up  
and struct whos memory is allocated with ALLOC. Silly example:

```
struct Hello_struct {
    VALUE name;
}

struct Helloworld_struct {
    VALUE count;
}

```

The trouble is when I try to call a function from a Helloworld instance  
that  
reads/writes to a copy of the Hello\_struct I get a segfault. This  
implies to  
me that memory is not being allocated for Hello\_struct. Do I have to  
tell  
Heloworld that space for Hello needs to be allocated? If so how?

Confused,  
Phil

---

<div class="post-metadata">

**Author:** ![splashlin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/90db22/32.png) [@splashlin](https://www.ruby-forum.com/u/splashlin)\
**Post date:** [June 5, 2006, 9:30pm UTC](https://www.ruby-forum.com/t/inheritance-from-c/62501/2 "2006-06-05T21:30:07Z")

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Phil J. wrote:

> ```
> }
> 
> struct Helloworld_struct {
> VALUE count;
> }
> 
> ```
> 
> The trouble is when I try to call a function from a Helloworld instance that  
> reads/writes to a copy of the Hello\_struct I get a segfault. This implies to  
> me that memory is not being allocated for Hello\_struct. Do I have to tell  
> Heloworld that space for Hello needs to be allocated? If so how?

I’m not sure I understand, but it sounds like you want your Helloworld  
instances to have both a Hello\_struct and a Helloworld\_struct as their  
data? And you want instance methods of the Hello class, when called on a  
Helloworld object, to operate on an inherited Hello\_struct?

This won’t work, unfortunately. Ruby T\_DATA objects have only one data  
pointer. They don’t inherit data pointers from superclasses. You need to  
manage the inheritance of C structs somehow, possibly by embedding the  
Hello\_struct members (or embed the struct itself) inside of  
Helloworld\_struct:

```
struct Helloworld_struct {
    VALUE name;
    VALUE count;
}

```

If you do this, then Hello methods defined in C just have to cast the  
data pointer to a Hello\_struct \*, and can then access the “name” member.

I wrote the cgen/cshadow library (on RAA) to generate C code in this  
way, based on pure-ruby attribute declarations and “in-line” C code.  
This is the examples/inherit-example.rb, which I think is included in  
the download:

=====

require ‘cgen/cshadow’

class Parent  
include CShadow

shadow\_attr\_accessor :ruby\_str =\> String  
shadow\_attr\_accessor :c\_int =\> “int c\_int”  
end

class Child \< Parent  
shadow\_attr\_accessor :obj =\> Object # VALUE type  
end

Parent.commit

# we’re done adding attrs and methods, so make.

x = Child.new  
x.ruby\_str = “foo”  
x.obj = [1,2,3]  
x.c\_int = 3

p x

# ==\> #\<Child:0xb7ba96f4 ruby\_str=“foo”, c\_int=3, obj=[1, 2, 3]\>

=====

The generated code for the Child class ends up looking like this:

typedef struct Child\_Shadow {

```
/* Parent_Shadow members */
VALUE self;
VALUE ruby_str; // String;
int c_int;

VALUE obj; // Object;

```

} Child\_Shadow;

=====

(The “self” member can be very useful–you can get back to the ruby  
object from the shadow struct.)

See the other examples for samples of method definitions using in-line C  
code. A very brief snippet from complex.rb:

=====

class MyComplex \< Numeric  
include CShadow

shadow\_attr\_accessor :re =\> “double re”, :im =\> “double im”

define\_c\_method(:abs) do  
include “\<math.h\>”  
returns “rb\_float\_new(sqrt(pow(shadow-\>re, 2) + pow(shadow-\>im,  
2)))”  
end  
end

=====

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<div class="post-metadata">

**Author:** ![splashlin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/90db22/32.png) [@splashlin](https://www.ruby-forum.com/u/splashlin)\
**Post date:** [June 6, 2006, 11:31am UTC](https://www.ruby-forum.com/t/inheritance-from-c/62501/3 "2006-06-06T11:31:44Z")

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On Tue, Jun 06, 2006 at 04:27:36 +0900, Joel VanderWerf wrote:

> Phil J. wrote:

[…snip…]

> ```
> struct Helloworld_struct {
> VALUE name;
> VALUE count;
> }
> 
> ```

This solved the problem, thanks. I’d made the assumption that Ruby would  
build and manage the parent for me by calling new on it.

> I wrote the cgen/cshadow library (on RAA) to generate C code in this  
> way, based on pure-ruby attribute declarations and “in-line” C code.  
> This is the examples/inherit-example.rb, which I think is included in  
> the download:

I had a quick go with this, an excellent tool, very powerful. I would  
certainly consider using it for guideline code in the future!

Thanks for your help,  
Phil