MATLAB: How to use object arrays in MEX

Question

Hello everybody!!

I would appreciate any help on my questions below.

I have simple user-defined class, which is actually a wrapper around MEX library. To support vectorized operations with objects of the class effectively I need to work with arrays of objects on C/C++ level in MEX. In relation with this I have several questions:

1. How can I create array of objects of the class inside MEX library?

2. How can I access each individual object of array of objects supplied by user to MEX-function.

I stuck on these issues and I would be grateful for any advice.

Class is NOT handle based, it looks like (bare minimum):

classdef abc

    properties (SetAccess = public)
        a
    end % properties
        function obj = abc(x)
           if nargin == 0     
             obj.a = 0;            
           elseif nargin == 1 
             obj.a = x;
           end 
        end % constructor

end % classdef

Answer 1

You can do this with a helper m-file:

% CreateClassArray.m helper file for mxCreateClassArray mex function
function class_array = CreateClassArray(dims,classname)
estring = 'class_array(';
for k=1:numel(dims);
    estring = [estring '1:' num2str(dims(k)) ','];
end
estring(end) = ')';
estring = [estring '=' classname ';'];
eval(estring);
end

And then you can use this function in your mex routine:

/* Create a class array with default class element entries */
mxArray *mxCreateClassArray(mwSize ndim, const mwSize *dims, const char *classname)
{
    mxArray *rhs[2];
    mxArray *mx, *mxtrap;
    mwSize i;
    double *pr;
      rhs[0] = mxCreateDoubleMatrix(1, ndim, mxREAL);
      pr = mxGetPr(rhs[0]);
      for( i=0; i<ndim; i++ ) {
          pr[i] = dims[i];
      }
      rhs[1] = mxCreateString(classname);
      if( mxtrap = mexCallMATLABWithTrap(1,&mx,2,rhs,"CreateClassArray") ) {
          mxDestroyArray(mxtrap);
          mx = NULL;
      }
      mxDestroyArray(rhs[1]);
      mxDestroyArray(rhs[0]);
      return mx;
  }

Caution: The above is bare bones with NO ARGUMENT CHECKING. I will clean this up with more comments and put in argument checking and post to the FEX in the near future. Also, the above assumes R2008b or later. For R2008a there is no mexCallMATLABWithTrap function in the API. In that case download the FEX submission noted below for mxGetPropertyPtr and there is a replacement mexCallMATLABWithTrap function included.

You might also be interested in this FEX submission for getting the mxArray pointers to class element properties:

http://www.mathworks.com/matlabcentral/fileexchange/30672-mxgetpropertyptr-c-mex-function