return empty array in c++ mexFunction in the out-of-process mexHost

I want to use the out-of-process functionnality and to return an empty array in my C++ mexFunction.

It is not exactly the same question as :

https://fr.mathworks.com/matlabcentral/answers/234467-c-mex-file-i-want-to-return-an-empty-array-does-anyone-know-how

since it is in C++ and with feval.

Here the problem:

void MexFunction::operator()(mm::ArgumentList outputs, mm::ArgumentList inputs)
{
	outputs[0] = f.createEmptyArray();
}

that i called with

>> mh = mexhost; res = feval(mh,'example');
Error using matlab.mex.MexHost/feval
Too many output arguments.

but directly called via the Command window I have:

>> example
ans =
     []

[David Gregoratti]

I'm facing the same problem, any update?

Answer 1

[Samay Sagar]

Hi @Vincent Huber, @David Gregoratti,

I have also encountered this issue in the past while returning an empty array from a C++ MEX function. I have verified that this issue was fixed in MATLAB R2021a.

As a workaround, you can return a non-empty value like a string literal (say "empty_array") instead of an empty array, which can be later handled in MATLAB to replace the value with [ ].

Another workaround is to catch the error in MATLAB and handle it there to use [ ].

mh = matlab.mex.MexHost();
try
    res = feval(mh, "example");
catch ME
    switch ME.identifier
        case 'MATLAB:maxlhs' % The error when empty array is returned
            res = [];
        case 'MATLAB:mex:MexHostCrashed' % A general MEX error
            clear mex;
        otherwise
            rethrow(ME)
    end
end