C++ Vector of C++ objects to mxArray in mex

Question

I have the following c++ vector:

std::vector<myStruct> myVec;

Where myStruct contains:

struct myStruct {
  Point3D start;
  Point3D end;
  double lenght;
}
struct Point3D {
  double x, y, z;
}

Now in my mex file I would like to convert this into a mxArray while keeping the same structure.

current (non functioning) code:

// basic 'out' 
mxArray *out;
mwSize dims[2] = { 1, 1 };
const char *pointField[] = {"point"};
const char *fields[] = {"start", "end","lenght"};
const char *3dFields[] =  {"x", "y", "z"};
out = mxCreateStructArray(2,dims,(int)myVec.size(),pointField);
for( int i = 0 ; i < myVec.size() ; ++i)
{
    // create struct to fill
    mxArray *point = mxCreateStructArray(2, dims, 3, fields);

    // create struct for start
    mxArray *start= mxCreateStructArray(2, dims, 3, 3dFields);
    mxSetField( start, 0, "x", mxCreateDoubleScalar(myVec[i].start.x) );
    mxSetField( start, 0, "y", mxCreateDoubleScalar(myVec[i].start.y) );
    mxSetField( start, 0, "z", mxCreateDoubleScalar(myVec[i].start.z) );

    // create struct for end
    mxArray *end= mxCreateStructArray(2, dims, 3, 3dFields);
    mxSetField( end, 0, "x", mxCreateDoubleScalar(myVec[i].end.x) );
    mxSetField( end, 0, "y", mxCreateDoubleScalar(myVec[i].end.y) );
    mxSetField( end, 0, "z", mxCreateDoubleScalar(myVec[i].end.z) );

    // add start + end to 'point'
    mxSetField( point, 0, "start", start);
    mxSetField( point, 0, "end", end); 
    mxSetField( point, 0, "lenght", mxCreateDoubleScalar(myVec[i].lenght) );

    // add completed 'point' struct to out at given index i
    mxSetField(out, i, "point", point );
}
plhs[0] = out;

As you can see, it's important that I can have 1 output value, which can contain multiple "points"-structs of which each point has a start(xyz)-struct, end(xyz)-struct and lenght value.

So far this code only outputs 1 single structure in the correct format, but then for the 2nd iteration, it only creates an empty 'end' value directly at the level where 'point' is.

What do I have to change to have this work correctly?

Thanks in advance!

Answer 1

Here is my implementation:

vec_struct.cpp

#include "mex.h"
#include <vector>

// C++ struct
struct Point3D {
    double x, y, z;
};
struct myStruct {
    Point3D start;
    Point3D end;
    double length;
};

const char *fieldsPoint[] = {"x", "y", "z"};
const char *fieldsStruct[] = {"start", "end", "length"};

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    if (nrhs != 0 || nlhs > 1) {
        mexErrMsgIdAndTxt("MATLAB:MEX", "Wrong number of arguments.");
    }

    // create C++ vector of structs
    std::vector<myStruct> v;
    for (int i=0; i<10; i++) {
        myStruct s = {
            {1.0+i, 2.0+i, 3.0+i},
            {4.0+i, 5.0+i, 6.0+i},
            i
        };
        v.push_back(s);
    }

    // convert it to MATLAB struct array and return it as output
    mxArray *p;
    plhs[0] = mxCreateStructMatrix(1, v.size(), 3, fieldsStruct);
    for (int i=0; i<v.size(); i++) {
        // start point
        p = mxCreateStructMatrix(1, 1, 3, fieldsPoint);
        mxSetField(p, 0, "x", mxCreateDoubleScalar(v[i].start.x));
        mxSetField(p, 0, "y", mxCreateDoubleScalar(v[i].start.y));
        mxSetField(p, 0, "z", mxCreateDoubleScalar(v[i].start.z));
        mxSetField(plhs[0], i, "start", p);

        // end point
        p = mxCreateStructMatrix(1, 1, 3, fieldsPoint);
        mxSetField(p, 0, "x", mxCreateDoubleScalar(v[i].end.x));
        mxSetField(p, 0, "y", mxCreateDoubleScalar(v[i].end.y));
        mxSetField(p, 0, "z", mxCreateDoubleScalar(v[i].end.z));
        mxSetField(plhs[0], i, "end", p);

        // length
        mxSetField(plhs[0], i, "length", mxCreateDoubleScalar(v[i].length));
    }
}

We compile and call the MEX-function from MATLAB:

>> mex -largeArrayDims vec_struct.cpp
>> s = vec_struct()
s = 
1x10 struct array with fields:
    start
    end
    length

As you can see, this creates a structure array of length 10. For instance the 5th structure is accessed as:

>> s(5)
ans = 
     start: [1x1 struct]
       end: [1x1 struct]
    length: 4
>> s(5).start
ans = 
    x: 5
    y: 6
    z: 7
>> s(5).start.x
ans =
     5

---

The above MEX-file is equivalent to the following MATLAB code:

ss = struct();
for i=1:10
    ss(i).start  = struct('x',1+i-1, 'y',2+i-1, 'z',3+i-1);
    ss(i).end    = struct('x',4+i-1, 'y',5+i-1, 'z',6+i-1);
    ss(i).length = i-1;
end

Answer 2

I see two issues (not necessarily complete):

1. out is created as a scalar structure in your code, instead it should probably be

   out = mxCreateStructMatrix(1, (int)myVec.size(), 3, pointField);

2. You specify const char *fields[] = {"start", "end","depth"}; but then you try to set the lenght field.

The first issue might be the reason for your observation - in mex you'll have to pre-allocate the structure matrix appropriately.