Get pointer to underlying data in mex C++ API

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Nicholas Maxwell
Nicholas Maxwell 2020-1-27
回答： Andrew Janke 2020-1-31
I would like to get a pointer to the underlying data of an input array, using the C++ mex API, while avoiding copies of the data.
This was partially answered in
https://www.mathworks.com/matlabcentral/answers/436916-how-to-access-raw-data-via-the-mex-c-api
eg. I tried
matlab::data::TypedArray<double> ar(std::move(inputs[0]));
double* a = ar.release().get();
and
matlab::data::TypedArray<double> ar(inputs[0]);
double* a = ar.release().get();
However, I've compared that to the result with the C-API,
double *p = mxGetPr(prhs[i]);
and the results are different.
That is, the C++ API must be copying the data. I get that this probably supports safety, but I still want to get at the underlying data, without unnecessary memory copying.
My use case is that I want to call libraries written with xtensor [1], using xt::adapt. I need a pointer to do that.
[1] https://xtensor.readthedocs.io/en/latest/

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回答（1 个）

Andrew Janke
Andrew Janke 2020-1-31
That mxGetPr() call is copying the data. The MEX API changed in R2018a when Matlab switched to the "interleaved complex data model". See https://www.mathworks.com/help/matlab/matlab_external/matlab-support-for-interleaved-complex.html. Now, when you call parts of the MEX API that use the old "separate complex" data model, it does a memory copy to convert the data to the old format. mxGetPr is one of those legacy functions.
Use the new mxGetDoubles() or similar functions instead. https://www.mathworks.com/help/matlab/apiref/mxgetdoubles.html. Those will get you direct access to the new interleaved-complex representation without a copy step.

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