# Sub-classing a block defined in an \_impl class

**URL:** <https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461>\
**Category:** GNU Radio\
**Created:** [October 10, 2014, 12:05pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461 "2014-10-10T12:05:09Z")\
**Posts on this page:** 10\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [October 10, 2014, 12:05pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/1 "2014-10-10T12:05:09Z")

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Hello,

this question my arise from my ignorance about C++, but I don’t  
understand how, if it is possible, to sub-class a GNURadio block that  
has moat of his code in an \_impl class.

To my understanding I would need to subclass both the class defining the  
block and his \_impl class. However, it seems that the \_impl class is not  
part of the exposed API, therefore there is no way to subclass it. There  
is way to achieve this without copying around significant portions of  
code?

The case at hand is subclassing gr::analog::pll\_carriertracking\_cc  
changing the phase\_detector() method to use the regular atan2() instead  
of the fast version.

Thanks!

Cheers,  
Daniele

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<div class="post-metadata">

**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 12:18pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/2 "2014-10-10T12:18:00Z")

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You’re right – that’s one of the reasons why we try and keep the  
block’s guts (their kernels) elsewhere than the actual block definition.

Not all of our blocks do this, though.

M

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**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 12:23pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/3 "2014-10-10T12:23:15Z")

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Hello Martin,

I’m not sure I understand. Are you saying that there is no way of doing  
what I want without copying over to my class the implementation of the  
block that is in the \_impl class?

Why is this a good thing?

Cheers,  
Daniele

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<div class="post-metadata">

**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 12:26pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/4 "2014-10-10T12:26:11Z")

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Hi Daniele,

\_impl classes are not meant to be subclassed usually.  
GR distinguishes between interface or public headers, which are  
located in the include directory and private headers which are located  
in the lib directory.  
You can just add new classes to your lib folder and use them within  
this folder. But if you want to export these classes and make them  
accessable from Python, you need a public header for it.

On 10.10.2014 12:03, Daniele N. wrote:

> around significant portions of code?  
> The public description of a block is in the corresponding include  
> folder. It is pure virtual to make sure it remains as an interface file.

> The case at hand is subclassing gr::analog::pll\_carriertracking\_cc  
> changing the phase\_detector() method to use the regular atan2()  
> instead of the fast version.  
> You may just change this in GR directly. You have a personal copy of  
> the code so there is nothing that should contradict this approach.  
> Unless you want to compare the two but even then you should be able to  
> accomplish that.

I hope that gives you an idea of the structure and may help you solve  
your problem.

Cheers  
Johannes  
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<div class="post-metadata">

**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 12:32pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/5 "2014-10-10T12:32:01Z")

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Hi,

> There  
> is way to achieve this without copying around significant portions of code?

No, you’ll need to copy the \_impl

> The case at hand is subclassing gr::analog::pll\_carriertracking\_cc  
> changing the phase\_detector() method to use the regular atan2() instead  
> of the fast version.

Actually this is a private non-virtual method, so even if all that was  
in the public API you wouldn’t be able to do it.

Cheers,

```
Sylvain

```

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**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 12:37pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/6 "2014-10-10T12:37:52Z")

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On 10/10/14 12:25, Johannes D. wrote:

> \_impl classes are not meant to be subclassed usually.  
> GR distinguishes between interface or public headers, which are  
> located in the include directory and private headers which are located  
> in the lib directory.  
> You can just add new classes to your lib folder and use them within  
> this folder. But if you want to export these classes and make them  
> accessable from Python, you need a public header for it.

That was my understanding. What I still don’t understand is why things  
are done in this way.

> On 10.10.2014 12:03, Daniele N. wrote:
> 
> > The case at hand is subclassing gr::analog::pll\_carriertracking\_cc  
> > changing the phase\_detector() method to use the regular atan2()  
> > instead of the fast version.
> 
> You may just change this in GR directly. You have a personal copy of  
> the code so there is nothing that should contradict this approach.  
> Unless you want to compare the two but even then you should be able to  
> accomplish that.

Changing GNURadio code directly makes is very hard to redistribute my  
code, and I would like to retain the possibility to use the original  
GNURadio block.

I don’t have a problem copying over the code and changing the methods I  
want to change, it just does not feel as the most elegant solution.

Cheers,  
Daniele

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**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 12:39pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/7 "2014-10-10T12:39:02Z")

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The good part is the separation of the API from the implementation. The  
not so good thing is that the implementations themselves often contain  
all the logic, usually because they are fairly simple. So, your choices  
are to subclass or copy/edit the implementation files. [Just saw  
Sylvain’s not about phase\_detector() being privite - OK].

Also, the implementations are free to change without affecting the API.  
Guaranteeing stability of implementations (even the \_impl.h) makes the  
implementations part of the API, which would be bad. If there were a  
family of carrier tracking algorithms, it would make sense to have them  
subclass from a common abstract class.

In more complicated blocks, like some in the filter area, a good portion  
of the logic is separated out and reusable.

The

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**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 2:30pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/8 "2014-10-10T14:30:35Z")

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On 10/10/14 12:30, Sylvain M. wrote:

> Hi,
> 
> > There  
> > is way to achieve this without copying around significant portions of code?
> 
> No, you’ll need to copy the \_impl

Ok. Thanks for confirming my understanding.

I copied the \_impl definition, however, I would prefer to do not copy  
the interface definition, therefore I defined my block as follows:

namespace gr {  
namespace baz {

```
class BAZ_API pll_carriertracking_cc
  : public gr::analog::pll_carriertracking_cc
{
public:
  typedef boost::shared_ptr<pll_carriertracking_cc> sptr;
  static sptr make(float loop_bw, float max_freq, float min_freq);
};

```

} // namespace baz  
} // namespace gr

Then I copied the definition of the pll\_carriertracking\_cc\_impl class  
implementation into my project and changed it where I wanted.

However, in this way, swig is not happy:

src/gr-baz/swig/baz\_swigPYTHON\_wrap.cxx: In function ‘PyObject\*  
\_wrap\_new\_pll\_carriertracking\_cc(PyObject\*, PyObject\*)’:  
/home/manip/src/gr-baz/swig/baz\_swigPYTHON\_wrap.cxx:11071:87: error:  
cannot allocate an object of abstract type  
‘gr::baz::pll\_carriertracking\_cc’  
result = (gr::baz::pll\_carriertracking\_cc \*)new  
gr::baz::pll\_carriertracking\_cc();

But if I copy the definition of my block from the GNURadio definition,  
swig is happy, the compilation goes fine, and I can correctly use the  
block from Python.

What am I doing wrong? Sorry if this is a trivial C++ error, but my C++  
is definitely rusty…

Thanks!

Cheers,  
Daniele

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**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 3:25pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/9 "2014-10-10T15:25:29Z")

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On 10/10/14 14:29, Daniele N. wrote:

> ```
> typedef boost::shared_ptr<pll_carriertracking_cc> sptr;
> static sptr make(float loop_bw, float max_freq, float min_freq);
> };
> 
> ```
> 
> } // namespace baz  
> } // namespace gr
> 
> Then I copied the definition of the pll\_carriertracking\_cc\_impl class  
> implementation into my project and changed it where I wanted.
> 
> However, in this way, swig is not happy:

It seems to be a limitation of SWIG (which I don’t know much, so I may  
still be doing something wrong). If I define a virtual method in the  
definition of my block interface, SWIG generates the correct C++ code.

I ended up adding a definition for the work() virtual method to fix the  
issue. Again, not the most elegant thing to do, but it works…

Cheers,  
Daniele

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**Author:** ![Daniele\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/74df32/32.png) [@Daniele\_N](https://www.ruby-forum.com/u/Daniele_N)\
**Post date:** [October 10, 2014, 4:55pm UTC](https://www.ruby-forum.com/t/sub-classing-a-block-defined-in-an-impl-class/232461/10 "2014-10-10T16:55:27Z")

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I think it might be more of a C+±inherent thing:  
If you declare a method in a class:

class daniele\_s\_class {  
public:  
void my\_method(int argument);  
};

Then you’ll have to either implement that method, or can’t instantiate  
the class – which is kind of correct, if you think about it, because  
that class is simply incomplete. But it’s not too bad, either, because  
noone ever should instantiate daniele\_s\_class, it’s just meant to  
define the interface. That’s why we have the “make”, which will return  
a new instance of the \_impl subclass. So, schematically, the GNU Radio  
way of doing this is:

class daniele\_s\_class {  
public:  
virtual void my\_method(int argument) = 0; //this tells the compiler  
that there is a method that a instantiable subclass _must_ implement  
static sptr make();  
};

daniele\_s\_class::sptr daniele\_s\_class::make(){  
return sptr(new daniele\_s\_class\_impl());  
}

class daniele\_s\_class\_impl{  
public:  
virtual void my\_method(int argument);  
};

void daniel\_s\_class\_impl::my\_method(int argument){  
//do whatever here 🙂  
}

Greetings,  
Marcus

On 10.10.2014 15:24, Daniele N. wrote:

> > make(float loop\_bw, float max\_freq, float min\_freq); };  
> > may still be doing something wrong). If I define a virtual method  
> > \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Discuss-gnuradio  
> > mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
> > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)

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