Wrapping a MEX function that has an "ans" output in some cases

I have a MEX function that returns an "ans" output in some cases, but in others not. It looks something like this:

   /* Listing: fooMEX.c */
   #include <mex.h>
   void mexFunction(int resc, mxArray *resv[], int argc, const mxArray *argv[]) {
       mxAssert(argc==1 && mxIsLogicalScalar(argv[0]), "nope");
       bool with_ans = mxIsLogicalScalarTrue(argv[0]);
       if (with_ans) {
         resv[0] = mxCreateDoubleScalar(42);
       }
   }

Usage from MATLAB:

fooMEX(false) % no ans
fooMEX(true) % ans = 42

Now I want wrap this function in a MATLAB function, but I don't see how it can be done. This doesn't work:

function varargout = foo(with_ans)
  /* ERROR: "One or more output arguments not assigned during call to "fooMEX" */
  varargout{1:nargout} = fooMEX(with_ans);
end

I can achieve it with a try-catch, but that's an ugly hack:

function varargout = foo(with_ans)
  try
    varargout{1:nargout} = fooMEX(with_ans);
  catch
    fooMEX(with_ans);
  end
end

Is there a better way?

Joel

P.S: This is a simplified context. In the real context, "with_ans" is actually an index corresponding to one out of thousands of wrapped C++ functions. The wrapping functions, with different indices, are member functions of a "classdef" style MATLAB class.

Answer 1

[James Tursa]

I am not on a machine with MATLAB at the moment to verify this, but I think the syntax for passing variable number of arguments up the chain of function calls is to use square brackets on the lhs. E.g.,

[varargout{1:nargout}] = fooMEX(with_ans);

[Joel Andersson]

Oh, your right! Simply adding the square brackets solved it.

Answer 2

[Walter Roberson]

If you need to you can test nargout and only do the assignment when it is at least 1.

However, my recollection is that the presence or absence of an output is carried "upstream". Test this:

fooz = @(x)find(x+nargout);
bar = @(x) fooz(x);
bar(-2:2)
z = bar(-2:2)
[z,q] = bar(-2:2)

The changes in output reflect that fooz sees different nargout even though the nargout is being inherited from its caller bar.

I would thus suspect that you can possibly code with a real variable name rather than varargout, and let the nargout be inherited from the caller.