How to decode mxGetData void pointer nicely

Question

Peter Li 2011-9-19

2
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https://ww2.mathworks.cn/matlabcentral/answers/16113-how-to-decode-mxgetdata-void-pointer-nicely

I'm trying to write a C++ mex function that can handle multiple datatypes. So I do something like:

void *data = mxGetData(mxarray);
switch (mxGetClassID(mxarray)) {
  case mxDOUBLE_CLASS:
    my_function((double *) data);
  ...

my_function is templated, so this handles different datatypes nicely. But it's still very annoying to need to have this switch for every possible my_function1, my_function2, etc.

So far, the solution I've come up with is to use a functional approach and have a method that accepts a functor:

template <typename ReturnType, typename FunctorType>
ReturnType mxarr_apply(const mxArray *inarr, FunctorType functor) {
  void *data = mxGetData(inarr);
  switch (mxGetClassID(inarr)) {
    case mxDOUBLE_CLASS:
      return (ReturnType) functor((double *) data);
    ...

This way I can put my logic in the functor (with the operator() templated) and not have to recreate the switch over and over.

But I wonder if there is some other way? In Java I think I could just have a function that translates the mxClassID directly into a class reference that could then be used to instantiate a type flexibly at runtime, but this doesn't seem to be an option in C++.

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Jan 2011-9-19

I agree, that this is a common problem. Both solutions are not really nice. You could use the first method and write it to a separate function, which is included at the compilation.

Peter Li 2011-9-20

Thanks. With the first method it seems I will always have the problem that I need to write a different switch for every possible my_function1, my_function2.

In my opinion the second method is much nicer, as it is now possible to put the mxarr_apply function in a header file and use it all the time and never have to write the switch again. It just requires writing the logic as a functor, which is not so onerous. The main problem with this approach is that it is still clumsy to get return values, so I find it is best to use this when you have only side-effects. Of course, you can assign returns by side-effect and it works okay this way, although this is a little ugly.

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How to decode mxGetData void pointer nicely

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