# C++ extension

**URL:** <https://www.ruby-forum.com/t/c-extension/79034>\
**Category:** Ruby\
**Created:** [March 9, 2006, 4:48pm UTC](https://www.ruby-forum.com/t/c-extension/79034 "2006-03-09T16:48:28Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![reevesg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a6a055/32.png) [@reevesg](https://www.ruby-forum.com/u/reevesg)\
**Post date:** [March 9, 2006, 4:48pm UTC](https://www.ruby-forum.com/t/c-extension/79034/1 "2006-03-09T16:48:28Z")

</div>

I am trying to understand some aspects of writing a ruby extension in  
C++.

Actually what I am trying to do is wrap a C++ library and make it  
accessible from ruby. Since I have been having problems with the library  
that I am trying to wrap, I decided to create a really simple library  
and try wrapping that.

The biggest problem that I am having is understanding scope. When I  
create the wrapper for the class I create a new instance of the class  
that I am wrapping and store it in a ptr. Then when I want to access  
methods of this class the wrapper class just delegates to the ptr.  
Simple stuff.

When I use a regular ptr things mostly work OK except that I can only  
create one instance of the class.

The library I am trying to wrap uses smart pointers, so I created a  
version of my code that uses simple pointers. The problem that I have  
hear is that these objects then fall out of scope and the destructors  
get called immediately after wrapper class method exists. The result of  
course is that I get segmentation faults or weird results.

I have only a little experience in C++ and this is my first extension  
that I have written.

How should I go about wrapping these c++ classes?

Thanks in advance.

Here is the code…  
[//extension.cpp](https://extension.cpp)  
#include \<ruby.h\>  
#include \<test.h\>  
#include \<smartptr.h\>

typedef VALUE (ruby\_method)(…);

auto\_ptr test;  
//test\* test;

extern “C” VALUE t\_TEST\_init(VALUE self, VALUE value )  
{  
Param \* param = new Param( NUM2INT( value ) );  
test = auto\_ptr( new Test( param ) );  
//test = new Test( param );  
return self;  
}

extern “C” VALUE t\_TEST\_value(VALUE self)  
{  
return INT2NUM( test-\>ParamValue() );  
}

auto\_ptr param;  
//Param\* param;

extern “C” VALUE t\_PARAM\_init(VALUE self, VALUE value )  
{  
param = auto\_ptr( new Param( 7 ) );  
//param = new Param( NUM2INT( value ) );  
return self;  
}

extern “C” VALUE t\_PARAM\_value(VALUE self)  
{  
return INT2NUM( param-\>value() );  
}

VALUE cTest;  
VALUE cParam;

extern “C” void Init\_Extension()  
{  
cTest = rb\_define\_class( “Test”, rb\_cObject );  
rb\_define\_method( cTest, “initialize”, (ruby\_method\*)  
&t\_TEST\_init, 1 );  
rb\_define\_method( cTest, “value”, (ruby\_method\*) &t\_TEST\_value,  
0 );

```
    cParam = rb_define_class( "Param", rb_cObject );
    rb_define_method( cParam, "initialize", (ruby_method*) 

```

&t\_PARAM\_init, 1 );  
rb\_define\_method( cParam, “value”, (ruby\_method\*)  
&t\_PARAM\_value, 0 );  
}

[//test.h](https://test.h)  
#include \<param.h\>

class Test {

public:  
Test(Param \*);  
~Test();  
int ParamValue();  
private:  
Param \*param;

};

[//param.h](https://param.h)  
class Param {

public:  
Param(int);  
~Param();  
int value();  
private:  
int internal;

};

---

<div class="post-metadata">

**Author:** ![reevesg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a6a055/32.png) [@reevesg](https://www.ruby-forum.com/u/reevesg)\
**Post date:** [March 10, 2006, 8:46am UTC](https://www.ruby-forum.com/t/c-extension/79034/2 "2006-03-10T08:46:50Z")

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I’ve been a total wimp about accessing C/C++ libraries from Ruby. I  
haven’t done anything particularly hard, but it’s always worked.

I use SWIG ([www.swig.org](http://www.swig.org)). The most I’ve had to do is write a couple  
helper functions and that’s about it. SWIG even comes with some  
helpful Ruby examples, extconf.rb skeletons, and Makefile skeltons.  
The SWIG documentation has a couple of sections on smart pointers.  
While I was learning to use SWIG, their community was helpful with Ruby  
specific suggestions.

Let us know how you do. Good Luck!

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<div class="post-metadata">

**Author:** ![reevesg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/a6a055/32.png) [@reevesg](https://www.ruby-forum.com/u/reevesg)\
**Post date:** [March 10, 2006, 1:04pm UTC](https://www.ruby-forum.com/t/c-extension/79034/3 "2006-03-10T13:04:26Z")

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> How should I go about wrapping these c++ classes?

You need to wrap your class into a Ruby structure in order for this  
to work right.

See: Data\_Wrap\_Struct and Data\_Make\_Struct for details

I’m not sure the implications of using an auto\_ptr in the mix…as  
I’ve never tried it. But, if you use the above methods you don’t  
need it since Ruby will handle the lifetime of your object for you  
(and provide a garbage collection hook for your destructor).