What can the implicit type of mxGetPr return argument be?

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Peter Cotton 2011-2-17

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https://ww2.mathworks.cn/matlabcentral/answers/1537-what-can-the-implicit-type-of-mxgetpr-return-argument-be

The following code returns a compiler warning because the pointer retrieved is of incompatible type.

 mwSize countDims[2] = {nP,1};
 plhs[1] = mxCreateNumericArray(ndim,countDims,mxINT64_CLASS,mxREAL);
 int* count;
 count = mxGetPr(plhs[1]);

This appears to be the case no matter what integer type I use in mxCreateNumericArray, or whether I use int, short etc.

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Peter Cotton 2011-3-17

Small suggestion for Mathworks. It would be nice if the mxGetData help was a little more explicit in regard to what is very much a standard procedure (extracting, casting etc) and alternatives. For example mxGetLogical might appear in the "See Also" for mxGetData. I wonder aloud if specialized mxGet's for integer types would be appreciated by users.

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Answer 1

James Tursa 2011-2-17

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https://ww2.mathworks.cn/matlabcentral/answers/1537-what-can-the-implicit-type-of-mxgetpr-return-argument-be#answer_2280

mxGetPr returns a (double *) type and is typically only used for double class variables, although most compilers will let you get away with just a warning if you convert it to another non-(void *) pointer type. Use the mxGetData function (and mxGetImagData for complex variables) for other data types, such as your int64 class variable. mxGetData and mxGetImagData return a (void *) type. You may want to cast the result explicitly. Also, using an int * will almost certainly not give you a 64-bit integer type on a typical compiler these days. You will need to use something else, like long long or int64_t.