How can I transfer a complex matrix from Matlab R2018a to Eigen

Question

I am currently importing the real and imaginary parts of a matrix separately from Matlab to C++. I then also map the real and imaginary parts to Eigen separately. I also perform the calculation and map the final result separately, as shown below:

//import real and imaginary parts from matlab 
mwSize     M = mxGetM (prhs[1]);
mwSize     N = mxGetN (prhs[1]);
double  * PR = mxGetPr (prhs[1]);
double  * PI = mxGetPi (prhs[1]);

//map real and imaginary parts to Eigen
Map<Matrix<double,Dynamic,Dynamic,ColMajor> > Br (PR, M, N );
Map<Matrix<double,Dynamic,Dynamic,ColMajor> > Bi (PI, M, N );

//map real and imaginary parts of result 
plhs[0] = mxCreateDoubleMatrix(M, N, mxCOMPLEX);
Map<Matrix<double,Dynamic,Dynamic,ColMajor> > resultr (mxGetPr(plhs[0]), M, N);
Map<Matrix<double,Dynamic,Dynamic,ColMajor> > resulti (mxGetPi(plhs[0]), M, N);

//calculate real and imaginary parts of A*B separately
resultr=A*Br;
resulti=A*Bi;

However, as of R2018a, Matlab allows importing the real and imaginary parts together.

How can I do this? I tried the following:

//import complex matrix from matlab 
mwSize     N = mxGetN (prhs[1]);
mxComplexDouble  * PR = mxGetComplexDoubles (prhs[1]);

//map complex matrix to eigen
Map<Matrix<mxComplexDouble,Dynamic,Dynamic,ColMajor> > B (PR, M, N );

//map complex result
plhs[0] = mxCreateDoubleMatrix(M, N, mxCOMPLEX);
Map<Matrix<mxComplexDouble,Dynamic,Dynamic,ColMajor> > result (mxGetDoubles(plhs[0]), M, N);

//calculate real and imaginary parts together
result=A*B;

But it does not compile because Eigen does not like the mxComplexDouble type.

Answer 1

If I understood correctly, mxComplexDouble has the same layout as std::complex<double>, so you should be able to simply cast:

auto* PR = reinterpret_cast<std::complex<double>*>(mxGetComplexDoubles(prhs[1]));

Reference:

In the -R2018a API, the mxGetElementSize function returns sizeof(std::complex<T>) for a complex mxArray with data type T. This value is twice the value returned by the function in the -R2017b API.

From MATLAB documentation page "Upgrade MEX Files to Use Interleaved Complex API"