CODEWITHSUNDEEP
c++cmatlabmexextern
Tyler Shellberg
Tyler Shellberg

Reputation: 1356

Trouble calling C++ code from MATLAB 2016 using a helper C file w/MEX Wrapper

I need to call a C++ non-member function from MATLAB 2016. MATLAB did not support C++ MEX until 2018, so this poses an issue.

I am doing this in Windows under mingw64.

To get around this, I attempted to make a C file with a MEX wrapper, with the C implementation purely just calling the C++ function using a shared header file with extern "C" based on some advice online.

However, I am very new to MATLAB, calling C++ from C and some other concepts here. So nothing compiles properly whatsoever.

Any advice on properly approaching this is greatly appreciated.

The header file myFunc.h:

#ifndef CPP_H
#define CPP_H

#ifdef __cplusplus
void myFunc(const std::vector<myStruct>& a,
            const std::vector<myStruct>& b,
            const double c,
            std::vector<std::pair>& d,
            mxArray **e
            );
extern "C" {
#endif
void myFunc(const std::vector<myStruct>& a,
            const std::vector<myStruct>& b,
            const double c,
            std::vector<std::pair>& d,
            mxArray **e
            );
#ifdef __cplusplus
}
#endif

#endif

The C file to call it, myFuncCaller.c:

#include "myFunc.h"
#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    myFunc(prhs[0], prhs[1], prhs[2], prhs[3], prhs[4]);
}

The actual implementation:myFunc.cpp:

#include "myFunc.h"

void myFunc(const std::vector<myStruct>& a,
            const std::vector<myStruct>& b,
            const double c,
            std::vector<std::pair>& d,
            mxArray **e
            )
{
  //things done here, pushes results to d
}

The end result of trying to compile both of these is that:

One large hurdle is the approach of passing input arguments from MATLAB -> C -> C++. I don't want things to be "lost" and ideally don't want to do conversion unless necessary, and I'm not sure where that would be.

Upvotes: 0

Views: 171

Answers (1)

Cris Luengo
Cris Luengo

Reputation: 60770

  1. You cannot put std::vector<> in an extern "C" section, as it's not valid C syntax. You need to create a pure C function stub, compiled as C++, that calls the C++ function.

  2. But, you don't need to do any of that because you can compile a C++ MEX-file just fine. The API is C, but you can call C functions from C++ without problem.

  3. The actual problem in your mexFunction is that you pass pointers to mxArray objects where the function expects references to std::vector<> etc. The only way to do this right is to copy the data from the MATLAB array into a C++ vector:

#include "myFunc.h"
#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
   if (nrhs < 5) {
      mexErrMsgTxt("Not enough input arguments! 5 expected.");
   }

   // arg 1: std::vector<myStruct>
   if (!mxIsDouble(prhs[0])) {
      mexErrMsgTxt("First input argument must be a double array.");
   }
   std::size_t N = mxGetNumberOfElements(prhs[0]);
   double* pr = mxGetPr(prhs[0]);
   std::vector<myStruct> a(N);
   std::copy(pr, pr+N, a.begin());

   // arg 2: std::vector<myStruct>
   std::vector<myStruct> b; 
   // ...

   // arg 3: double
   double c = mxGetScalar(prhs[2]);

   // arg 4: std::vector<std::pair> // This makes no sense in C++, std::pair<> is a template, 2 template arguments missing!!
   std::vector<std::pair> d;
   // ...

   // arg 5: mxArray **  // is this the output? Why does your C++ function take an mxArray?

   myFunc(a, b, c, d, &prhs[4]);
}

Upvotes: 1

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