C++ 对象的 C++ vector 到 mex 中的 mxArray

Question

我有以下 C++ vector :

std::vector<myStruct> myVec;

其中 myStruct 包含:

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

现在在我的 mex 文件中，我想将其转换为 mxArray，同时保持相同的结构。

当前(非功能)代码:

// 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;

如您所见，重要的是我可以有 1 个输出值，它可以包含多个“点”- 结构，其中每个点都有一个开始(xyz)结构、结束(xyz)结构和长度值。

到目前为止，此代码仅以正确的格式输出 1 个单一结构，但是对于第二次迭代，它仅在“点”所在的级别直接创建一个空的“结束”值。

我必须更改什么才能使其正常工作？

提前致谢!

Answer 1

这是我的实现:

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));
    }
}

我们从 MATLAB 编译并调用 MEX 函数:

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

如您所见，这会创建一个 structure array长度为 10。例如，第 5 个结构被访问为:

>> 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

---

上述 MEX 文件等效于以下 MATLAB 代码:

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