How do I pass a string from a C++ Mex file to the MATLAB workspace?

I am trying to pass a string from a C++ Mex file I am working on to the MATLAB workspace for plotting purposes. It is a fairly complicated program, but all I want to do is pass a std::string in my MexFunction (which I defined as "var") to the MATLAB workspace. This is the important part of the code:

#include "mex.hpp"
#include "mexAdapter.hpp"

using namespace matlab::data;
using namespace matlab::engine;
using namespace std;
using matlab::mex::ArgumentList;

// "some code here" //

class MexFunction : public matlab::mex::Function{
public:
    void operator()(ArgumentList outputs, ArgumentList inputs){
        std::shared_ptr<matlab::engine::MATLABEngine> matlabPtr = getEngine();
        ArrayFactory factory;
        
        // "more code here" //
        
        for(int i = 0; i < 100; i++){
            std::vector<ToolData> toolData =  apiSupportsBX2 ? capi.getTrackingDataBX2() : capi.getTrackingDataBX();

            for (int i = 0; i < toolData.size(); i++)
            {
                std::string var = toolDataToCSV(toolData[i]);
                
                std::cout << var << std::endl;
                
                // Pass "var" to MATLAB here //
            }
            usleep(5000);
        }
        
        matlabPtr->eval(u"printData = true");        
        
        // "more code here" //

        std::cout << "Press Enter to continue...";
        std::cin.ignore();
    }
};

I'm fairly new to C++/MATLAB-implementations and don't have that much experience with Mex file programming, so thanks in advance!

Answer 1

[Guillaume]

matlab:engine::setvariable seems to be the function to do that. With your original C++ code (not the C interface)

matlabPtr->setVariable(u"variablename", var);  //%don't use var as variable name in matlab. It's the variance function.

Untested. I've never used the C++ interface.

[André C. D.]

Thanks for the suggestion. I tried using that function before and got the following error(s):

Error using mex

/home/PATH/track_cpp_new.cpp: In member function ‘virtual void

MexFunction::operator()(matlab::mex::ArgumentList, matlab::mex::ArgumentList)’:

/home/PATH/track_cpp_new.cpp:514:60: error: no matching function for call to

‘matlab::engine::MATLABEngine::setVariable(const char16_t [9], std::__cxx11::string&)’

matlabPtr->setVariable(u"var_name",variable);

^

In file included from /home/PATH/mexAdapter.hpp:18:0,

from /home/PATH/track_cpp_new.cpp:35:

/home/PATH/cppmex/detail/mexApiAdapterImpl.hpp:313:6: note: candidate: void

matlab::engine::MATLABEngine::setVariable(const u16string&, const matlab::data::Array&, matlab::engine::WorkspaceType)

void matlab::engine::MATLABEngine::setVariable(const std::u16string &varName,

^~~~~~

/home/PATH/cppmex/detail/mexApiAdapterImpl.hpp:313:6: note: no known conversion

for argument 2 from ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘const matlab::data::Array&’

/home/PATH/cppmex/detail/mexApiAdapterImpl.hpp:327:6: note: candidate: void

matlab::engine::MATLABEngine::setVariable(const string&, const matlab::data::Array&, matlab::engine::WorkspaceType)

void matlab::engine::MATLABEngine::setVariable(const std::string &varName,

^~~~~~

/home/PATH/cppmex/detail/mexApiAdapterImpl.hpp:327:6: note: no known conversion

for argument 1 from ‘const char16_t [9]’ to ‘const string& {aka const std::__cxx11::basic_string<char>&}’

Without the setVariable function, the code compiles without any problems. I'm assuming this is a data type conversion problem, but I am not familiar with the way MATLAB handles its data types.

[Guillaume]

I'm assuming this is a data type conversion problem, but I am not familiar with the way MATLAB handles its data types.

I would say it's a purely C++ problem. You try to call a C++ function with arguments that don't match the function signature. In particular, as documented in the link in my answer, the second argument to setVariable has to be of type matlab::data::Array.

You can easily construct that object as demonstrated here:

matlab::data::ArrayFactory factory;
matlabPtr->setVariable(u"variablename", factory.createCharArray(var));  //assuming var is std::string

[André C. D.]

EDIT: I managed to make it work now, I had forgotten to use factory.createCharArray() before setVariable and now it works.

So at the end, I used the original code and added this:

Array str = factory.createCharArray(var); //var is of type std::string
matlabPrt->setVariable(u"StringName",str);

Thank you for the tip!

Answer 2

[Sreelakshmi S.B]

You can convert var to an mxArray and use 'engPutVariable' to write the mxArray into the Matlab Engine Worspace.You can find details about using this function in C and Fortran at this link: https://in.mathworks.com/help/matlab/apiref/engputvariable.html

This will work with C++ too.

[André C. D.]

Hi, thanks for the response. The problem with using engPutVariable, is that whenever I include "engine.h", I get the following error when compiling the Mex file:

#error Using MATLAB Data API with C Matrix API is not supported

I am assuming this is because I have included "mex.hpp" and/or "mexAdapter.hpp". How should I go about solving this problem?

EDIT:

I have replaced the header files "mex.hpp" and "mexAdapter.hpp" with only "engine.h" and used the same method as explained in engdemo.c ('extern/examples/eng_mat/engdemo.c') and now have the following code:

#include "engine.h"

#pragma comment(lib,"libmat.so")
#pragma comment(lib,"libmx.so")
#pragma comment(lib,"libmex.so")
#pragma comment(lib,"libeng.so")

// "Code" //

int main(){

    Engine *ep;
    mxArray *pointer;
    
    for(int i = 0; i < 100; i++){
        std::vector<ToolData> toolData = apiSupportsBX2 ? capi.getTrackingDataBX2() : capi.getTrackingDataBX();

        for (int i = 0; i < toolData.size(); i++)
        {
            std::string var = toolDataToCSV(toolData[i]); //toolDataToCSV returns string
            std::cout << var << std::endl;
            
            char ch[var.size() + 1];
            strcpy(ch, var.c_str()); //conversion from std::string to char
            
            pointer = mxCreateString(ch);
            
            memcpy((void *)mxGetPr(ch), (void *)ch, sizeof(ch));
            
            engPutVariable(ep, "ch", ch);
        }
        usleep(10000);
    }
    
    mxDestroyArray(pointer);

    engEvalString(ep, "close;");
    return 0;
}

And now the issues I get are:

cannot convert 'char*' to 'const mxArray* {aka const mxArray_tag*}' for argument '1' to 'double* mxGetPr(const mxArray*)';

cannot convert 'char*' to 'const mxArray* {aka const mxArray_tag*}' for argument '3' to 'int engPutVariable(ep, "ch", ch);

Using mxGetString instead of mxGetPr also leads to a smilar data conversion issue.

[Sreelakshmi S.B]

"The MATLAB Data API supports modern C++ features and is not compatible with the C Matrix API" ------- as taken from the official documentation page.This is why the initial error is happening.

You can try using 'setVariable' instead.This link has more details on how to go about doing that:

https://in.mathworks.com/help/matlab/matlab_external/set-and-get-variables-in-matlab-workspace.html