H5L.iterate exception and behavior of native matlab functions() method

Hello,

I'm developing a wrapper to use hdf5 files via set of low-level [+H5(?)] packages, provided my Matlab. As a part of this project I need to use H5L.iterate method, that has the following signature:

[status idx_out opdata_out] = H5L.iterate(group_id,index_type,order,idx_in,iter_func,opdata_in)

where iter_func is a pointer to a callback function that is supposed to be used on every step of iteration.

The H5L.iterate method is called inside a general class (let's say hdf5Adapter), so I would like to use a handle to another method of hdf5Adapter as [iter_func] argument for H5L.iterate.

However when doing so I receive an exception thrown by H5L.iterate caused by the following validation inside H5L.iterate (%matlabroot\toolbox\matlab\imagesci\+H5L\iterate.m) :

f = functions(iter_func);
if isempty(f.file) 
    error(message('MATLAB:imagesci:H5:badIterateFunction'));
end

because f.file appears to be empty.

I have tested the behavior of functions() method when applied to class methods (example below) and it seems to return empty 'file' field in the return data structure. See the test class definition in the attachment.

The way I test it:

foo = tstClass();

disp('instance method, as object');
ifhi = @foo.testFuncInstance;           % instance method, as object
disp(functions(ifhi));

disp('instance method, as class');
cfhi = @tstClass.testFuncInstance;      % instance method, as class
disp(functions(cfhi));

disp('static method, as object');
ifhs = @foo.testFuncInstance;           % static method, as object
disp(functions(ifhs));

disp('static method, as class');
cfhs = @tstClass.testFuncInstance;      % static method, as class
disp(functions(cfhs));

My questions are the following:

1. Is there any way to make the result struc of functions(handle_to_class_method) to have a non-trivial 'file' value?
2. What is the purpose of the check 'if isempty(f.file)' in H5L.iterate?

Thank you, Maxim Semyonov

[Francesco Zucchi]

I will just put the tstClass here as well

%%this is my test class
classdef tstClass % < handle  % uncomment to test a handle class
    % test class to be used to check native Matlab functions() behaviour

      properties ( SetAccess = public, GetAccess = public )
         state = ''; 
      end

      methods ( Access = public )
          function obj = tstClass()
              obj.state = 'new object';
          end
      end

      methods ( Access = public )
          function val = testFuncInstance(obj)
             disp('in the testFuncInstance'); 
             val = 0;
          end
      end    

      methods ( Static, Access = public )
          function val = testFuncStatic()
             disp('in the testFuncStatic'); 
             val = 0;
          end  
      end
  end

Answer 1

[Philip Borghesani]

I do not know why H5L.iterate needs function handles with a file defined. I suggest you file an enhancement request to remove the requirement.

I can give you a workaround. You will need to make two changes.

1. Create your function handles from a function not a script.
2. The instance method as class and static method as class function handles must be created in the form of an anonymous function:

cfhi = @() tstClass.testFuncInstance;      % instance method, as class
cfhs = @() tstClass.testFuncStatic;      % static method, as class

If you look at the function line the parser is doing this for you but because it is generating the code the file is not being filled in. In the case of a script the owning file is the calling function or the base workspace.

Just a note here you can't call an instance method in the form classname.method not sure why you are testing that. My recomended syntax for a call to an instance method for iterating would be:

fh=@(loc_id,attr_name) myIterateMethod(obj, loc_id, attr_name);

[Francesco Zucchi]

Hi Philip, I'm investigating on your reply, I'll be back once I can confirm that solution you've proposed does the job. Thank you!

[Francesco Zucchi]

Hi Philip, I can confirm that your solution works in order to have non-empty 'file' field in the output of struct function(). I managed to make it work for static method as well without any special modifications, just defining the method as Static and referring to it as className.methodName.

However, next obstacle I'm facing is that when submitting the anonymous function to H5L.iterate I get an exception because of the following check:

if (nargin(iter_func) ~= 3) || (nargout(iter_func) ~= 2)
   error(message('MATLAB:imagesci:H5:invalidIterationFunctionSignature'));  
end

It turns out that nargout(iter_func), where iter_func is my anonymous funtion is equal to -1.

Do you have any idea to make a workaround for this?

Many thanks in advance! Maxim

[Philip Borghesani]

The only way to make both checks pass is to create a local or nested helper function with the correct signature. Anonymous functions can't be made to return a defined number of outputs. If a static method is used than a local function will work otherwise it will need to be nested.

Incomplete psudocode for nested function use:

function demo
   obj=makeobj;
   fh=@nestedHelper
   % use fh here
   %...
   function [status, opdata_out] = nestedHelper(group_id,name,opdata_in)
       [status, opdata_out] =obj.method(group_id,name,opdata_in)   ;
   end
end

I believe that iterate is being much to restrictive about the types of function handle it will accept.

[Francesco Zucchi]

That definitely works, though non-intuitive for me and probably requires the knowledge of internal parser logic of Matlab...

Thank you very much Philip, your guidance is highly appreciated