How to compile Matlab class into C lib?

Question

The origin of this question is from here How to use "global static" variable in matlab function called in c.

I\'m trying to encapsulate the \"global variab

Answer 1

Following on from the thread in the previous post, the suggestion wasn't to wrap your functions in a class, but rather to use a class to pass about the global variable which compiling leaves you unable to use.

classdef Foo < handle
  properties
    value
  end

  methods
    function obj = Foo(value)
      obj.value = value;
    end
  end
end

Note: the class Foo extends the handle class in order to make it pass by reference, rather than pass by value. See: the comparison between handle and value classes.

function foo = matlabA()
  foo = new Foo(1);
end

function matlabB(foo)
  foo.value
end

As far as I know, the matlab compiler doesn't compile the code as such, but rather packages it with a copy of the MATLAB Component Runtime and writes some wrapper functions to handle invoking said runtime on the code from c/c++.

I would recommend avoiding jumping back and forth between matlab and c/c++ too much; there is bound to be some overhead to converting the datatypes and invoking the MCR. All I really use it for is wrapping up a complex but self-contained matlab script (i.e.: doesn't need to interact with the c/c++ code mid way through said script) as a function, or packaging up code for deployment to environments which don't have a full copy of matlab.

As an interesting side note: if you are calling C++ from within Matlab, and that C++ code needs access to a global variable, things are much easier. You can simply do this by wrapping your C++ code into a mexFunction and compiling that. In the places you need to access a variable which is in the Matlab workspace, you can do so using the mexGetVariablePtr which will return a read-only pointer to the data. The variable you are accessing can be in either the global workspace, or that of the function which called your mexFunction.

With this approach I would suggest liberally comment the variable that you are getting in both the C++ and Matlab code, as the link between them may not be obvious from the Matlab side; you wouldn't want someone to come along later, edit the script and wonder why it had broken.

In this case it seems that the C++ side doesn't really need access to the data, so you could refactor it to have matlab do the calling by wrapping the "get current position of fingers" code into a mexFunction, then have matlab do the loop:

data = loadData();
while(~stop) {
  position = getFingerPositionMex();
  soundByCoef(position, data);
}

Assuming you don't modify the data within soundByCoef Matlab will use pass by reference, so there will be no copying of the large dataset.