Re: Built-in Functions inside method of a class

[Ben Abbott]

On Oct 10, 2011, at 6:14 PM, Carnë Draug wrote:

> On 10 October 2011 20:55, John W. Eaton <address@hidden> wrote:
>> On  9-Oct-2011, Juan Pablo Carbajal wrote:
>> 
>> | Are built in functions called inside a method of a class considered as
>> | friends (as in C++) of that class?
>> | In other words: Will the function be evaluated in the context of the
>> | caller or in the context of the method.
>> |
>> | I have a class with a field 'Path' that is a structure with many
>> | fields inside. I have a method that is meant to return a cell
>> | containing information inside those subfields. The following code
>> | works correctly
>> |
>> | wanted_ids = {ids{tf}};
>> | for i = 1: numel (wanted_ids)
>> |      paths{i} = obj.Path.(ids{i}).data;
>> | end
>> |
>> | by returning the cell with the contents of the subfields of Path
>> | described in 'wanted_ids'.
>> |
>> | However any of the following doesn't work
>> | % Variation
>> | paths = cellfun (@(s) obj.Path.(s).data,wanted_ids,'UniformOutput',false);
>> | % Another variation
>> | paths = cellfun (@(s) getfield (obj,'Path').(s).data,
>> | wanted_ids,'UniformOutput',false);
>> | % Yet another
>> | paths = cellfun (@(s) getfield (obj.Path,s).data,
>> | wanted_ids,'UniformOutput',false);
>> | % Yet yet another
>> | dummy = @(s) obj.Path.(s).data;
>> | paths = cellfun (dummy, wanted_ids,'UniformOutput',false);
>> |
>> | The error says that the class cannot by indexed with 'Path'. If one
>> | manually gives this interface to the class, by defining a case in
>> | subref 'Path' the problem is solved. However, this breaks the privacy
>> | of the class field 'Path', that is not meant to be accessed directly.
>> 
>> I don't know whether this is a bug or not.  Can you please post a
>> complete example that can be used to reproduce the problem?
> 
> I have attached an example that repoduces this. I believe that inside
> methods the built-in subsref and subsasgn should be used. However, if
> cellfun is used inside a method, the class subsref will be used
> instead. The following code has a simple method that runs the same
> code inside a for loop and then using cellfun. In the end shows that
> the results will also be different
> 
> octave-3.4.2:16> a = testClass;
> octave-3.4.2:17> testMethod(a);
> The following will use built-in susbref because it's inside a method
> The following will use class susbref because it's in a function
> I am in the class subsref
> I am in the class subsref
> loop_result =
> {
>  [1,1] =
>  {
>    [1,1] =  1
>    [1,2] =  2
>    [1,3] =  3
>  }
>  [1,2] =
>  {
>    [1,1] =  1
>    [1,2] =  2
>    [1,3] =  3
>  }
> }
> cellfun_result =
> {
>  [1,1] = <class testClass>
>  [1,2] = <class testClass>
> }
> <@testClass.tar.gz>_

I haven't studied this problem, but I thought I'd try out this example to see 
what ML does.

a = testClass

a = 

        testClass object: 1-by-1

testMethod(a)
The following will use built-in susbref because it's inside a method
Argument to dynamic structure reference must evaluate to a valid field name.

Error in testClass/testMethod (line 5)
    loop_result{i} = obj.(ids(i)).path;
 
Ben