MATLAB: How to send vector of string to MATLAB compiled DLLs, using mwArray

cmatlab deploymentmatlab dllmwarray

Page at mwArray class describes about constructor to create a mwArray cell of array of string as: mwArray(mwSize num_strings, const char** str).

I have a vector of string which has been dynamically populated. I have converted vector<string> to char** and trying to make mwArray with this data. However, the constructor signature has const char**, so it is failing to convert char** to const char**

How else a vector of string can be set to a mwArray cell and sent to MATLAB DLLs?

Best Answer

Yes, C++ will not let you convert T** to const T**. While you probably could use an explicit cast, the best solution is to change your conversion from vector<string> so that it outputs directly const char**.

You must actually have purposefully cast away the constness of const char* returned by string.c_str, so don't do that!

std::vector<std::string> strings { "one", "two", "three"};
std::vector<const char*> cstrings;
for (size_t i = 0; i < strings.size(); ++i)
    cstrings.push_back(strings[i].c_str());  \\convert string to const char*
mwArray mstrings(cstrings.size(), cstrings.data());  \\cstrings.data() is const char**

Related Solutions

MATLAB: How to create a variable of type “voidPtr” which points to a string using the LIBPOINTER function in MATLAB

This enhancement has been incorporated into the documentation as of MATLAB 7.2 (R2006a). For previous product releases, read below for any possible workarounds:

In C, characters are represented as unsigned 8-bit integers. In order to create a variable of type "voidPtr" containing a string, the string must first be cast to one of this numeric type as in the following code:

p=libpointer('voidPtr',[ uint8('this is a string') 0]);

The data in the pointer can then be accessed:

p.value
char(p.value)

To call a function that takes a voidPtr as input, use the following syntax:

foo(uint8(str));

where "foo" is the name of your function and "str" is the string.

MATLAB: How to convert a UTF-8 C-string to an mxArray of mxChar in C code

This is a two step process. The first phase is performed in C and the second phase in MATLAB (called from C).

(1) In the C code, you will need to create an mxArray of UINT8 from the UTF-8 C-string. If you intend to have multiple rows of characters in your array, keep in mind that MATLAB stores arrays in column-major order.

(2) In MATLAB you will need to use the 'native2unicode' function to convert the array of 'uint8' to an array of 'char'.

For example, if this conversion is taking place in a MEX file, the code would look something like this: (NOTE: This code does not compile as-is)

      //UTF-8 string to convert
      char utf8str[] = {65 66 67 68 207 163};
      // Step 1 - create a mwArray of uint8 from UTF-8 encoded C-string
      // **YOUR FUNCTION** Convert C-string to mwArray of uint8
      mwArray* uint8Array = UTF8CStringToMwArrayUint8(utf8str, length);
    //Step 2 - convert mwArray of uint8 to mwArray of char using 'native2unicode'
    mwArray* encodingString = mxCreateString("UTF-8");
    mwArray* lhs_native2unicode[1]; 
    mwArray* rhs_native2unicode[] = {uint8Array,  encodingString};
      // Calling 'native2unicode'
      mexCallMATLAB(1, &lhs_native2unicode, 2, &rhs_native2unicode, 'native2unicode'); 
      // this is the converted char array
      mwArray* charArray = lhs_native2unicode[1];

Best Answer

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MATLAB: How to create a variable of type “voidPtr” which points to a string using the LIBPOINTER function in MATLAB

MATLAB: How to convert a UTF-8 C-string to an mxArray of mxChar in C code

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