Interacting with C++ objects through Mex files

Question

I'm a bit stuck on if my current goal is feasible, and if so, how to do this. I'm hoping to interact with some C++ classes through a Mex file, but I would need the instances of the objects I am accessing to be persistent across calls from different Mex functions. For example, suppose I do the following within an initialization Mex file:

void mexFunction (int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    size_t nCats = (size_t) *mxGetPr(prhs[0]);

    std::vector<Cat> cats;
    for(size_t i = 0; i <nCats; i++){
         cats[i] = Cat(/* arguments to constructor*/);
    }
}

So I've initialized my Cat objects from my external C++ code. Now, later down the line, I need to update info about my Cat objects, so in a different Mex file I have

void mexFunction (int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{

     for(size_t i = 0; i < nCats; i++){
          cats[i].feed(/*parameters to feed method*/);
     }
}

Here are my questions:

1. How would I make this std::vector persist across calls to different Mex files? There isn't a way to return non-Matlab types from Mex files (that I'm aware of), and Mathworks says that local variables within Mex functions are generally garbage collected when the function returns, which I do not want. How can I call the same std::vector with the stored objects I am interested in across different functions? Or even calls to the same function?

2. Is there a better way to do this with Matlab? Essentially I'm trying to use Matlab to drive some C++ code, which does the heavy lifting, and then brings it all back to Matlab for analysis. The trouble is that the C++ code is already written, and I need to try to bend Matlab to fit those classes.

Answer 1

Not sure if possible between Mex functions but you can make some things persistent between calls to the same mex routine.

See documentation for:

• mexMakeMemoryPersistent

• mexMakeArrayPersistent

Also see this answer on the Mathworks site: How can I make memory persistent between calls to a MEX-file in MATLAB

I have not done this myself so can't give more specific help but this might point you in the right direction.