How to use imread from OpenCV in Matlab?

I am trying to use `imread` from OpenCV within Matlab. I have the following files:

`imreadCV.cpp`:

#include "opencvmex.hpp"
    #include "mxarray.hpp"

    using namespace cv;


    void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
    {  
        std::string imname= prhs[0]->toString();
        cv::Mat img = imread(imname);
        cv::cvtColor(img, img, cv::COLOR_BGR2GRAY);
    	img.convertTo(img, CV_32F);
    	cv::Scalar s = 255;
	divide(img, s, img);
        
        plhs[0] = ocvMxArrayFromImage_double(img);
    }

and the Matlab code:

function runMatlab
    imname=['lenna.bmp'];
    BW=im2double(imread(imname));
    imname_CV=imreadCV(imname);

When I try `mexOpenCV imreadCV.cpp`, I get the error:

c:\me\mxarray.hpp(460):
	warning C4099: 'matlab::ArgumentParser::Variant': type name first seen using 'struct' now seen using 'class'
	c:\me\mxarray.hpp(460):
	note: see declaration of 'matlab::ArgumentParser::Variant'
	c:\me\mxarray.hpp(523):
	warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
	c:\me\mxarray.hpp(525):
	warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
	c:\me\mxarray.hpp(528):
	warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
	c:\me\mxarray.hpp(530):
	warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
	C:\me\imreadCV.cpp(9): error C2027: use of
	undefined type 'mxArray_tag'
	c:\program files\matlab\r2018b\extern\include\matrix.h(687): note: see declaration of 'mxArray_tag'
	C:\me\imreadCV.cpp(12): error C3861: 'imread':
	identifier not found

why is this?