Using workspace variables in C++ Mex functions

When you have C or C++ files that you would like to use in your Simulink model, you have to interface them with a mex file. Variables in Matlab workspace can be used inside C++ mex functions. Example m file and mex function demonstrating scalar, vector, matrix, string and structure usage:

	%Tested with Matlab R2019b and Visual Studio 2015
	clc; clear all
	inScalar = 153;
	inVector = [1 2 3];
	inMatrix = [1 2 3; 11 22 33; 111 222 333; 1111 2222 3333];
	inCharArray = 'c:/temp/a/';
	inString = "c:/temp/a/"; %Note double quotes
	inStruct.x = [1 2 3];
	inStruct.y = [11 22; 111 222];
	mex mexWorkspaceDemo.cpp
	mexWorkspaceDemo();

view raw mexDriver.m hosted with ❤ by GitHub

	/* Demonstrate how to use Matlab workspace variables from C++ mex function.
	* Note that you can only set to or get from BASE or GLOBAL workspace, you cannot use caller workspace (workspace of the caller function).
	* For more, see https://www.mathworks.com/help/matlab/matlab_external/set-and-get-variables-in-matlab-workspace.html
	* See mexDriver.m on how to use this mex function.
	* Şamil Korkmaz, Jan 2020
	*/
	#include "mex.hpp"
	#include "mexAdapter.hpp"
	#include <iostream>
	using matlab::mex::ArgumentList;
	using namespace matlab::engine;
	using namespace matlab::data;
	using namespace std;
	class MexFunction : public matlab::mex::Function {
	private:
	ArrayFactory factory;
	shared_ptr<matlab::engine::MATLABEngine> matlabPtr = getEngine();
	void getCharArray(char* name, char* output) {
	CharArray inCharArray = matlabPtr->getVariable(name);
	for(size_t i=0; i < inCharArray.getNumberOfElements(); i++) {
	output[i] = inCharArray[i];
	}
	output[inCharArray.getNumberOfElements()] = 0; //ASCII Null or termination character denoting end of string
	}
	public:
	void operator()(ArgumentList outputs, ArgumentList ins) {
	//inputs from workspace:
	TypedArray<double> inScalarArray = matlabPtr->getVariable(u"inScalar");
	TypedArray<double> inVectorArray = matlabPtr->getVariable(u"inVector");
	TypedArray<double> inMatrixArray = matlabPtr->getVariable(u"inMatrix");
	TypedArray<MATLABString> inStringArray = matlabPtr->getVariable(u"inString");
	char inCharArrayOutput[255];
	getCharArray("inCharArray", inCharArrayOutput);
	StructArray inStructArray = matlabPtr->getVariable(u"inStruct");
	//printf does not print to Command Window, you have to use cout:
	cout <<"\nINPUTS FROM MATLAB WORKSPACE:"<<endl;
	cout << "inScalarArray[0] = " << inScalarArray[0] << endl; //You can only print if inScalarArray is of type TypedArray<double>, you can't if type ia Array, you will get error C2593: 'operator <<' is ambiguous
	cout << "inVectorArray";
	for(size_t i=0; i<inVectorArray.getNumberOfElements(); i++) {
	cout << "[" << i << "] = " << inVectorArray[i] << " ";
	}
	cout << endl;
	cout << "inMatrixArray.getNumberOfElements() = " << inMatrixArray.getNumberOfElements() << endl;
	ArrayDimensions inMatrixDims = inMatrixArray.getDimensions();
	size_t nRows = inMatrixDims[0];
	size_t nCols = inMatrixDims[1];
	for(size_t iRow=0; iRow < nRows; iRow++) {
	for(size_t iCol=0; iCol < nCols; iCol++) {
	cout << "inMatrixArray[" << iRow << "][" << iCol << "] = " << inMatrixArray[iRow][iCol] << "\t ";
	}
	cout << endl;
	}
	cout<<"inCharArray = " << inCharArrayOutput << endl;
	string stringVal = inStringArray[0]; //convert from Matlab string to standard C++ string
	cout << "stringVal = " << stringVal << endl; //You cannot use inStringArray[0] here.
	auto fields = inStructArray.getFieldNames();
	std::vector<std::string> fieldNames(fields.begin(), fields.end());
	//size_t nElements = inStructArray.getNumberOfElements(); cout << "nElements = " << nElements << endl;
	for(size_t i = 0; i<fieldNames.size(); i++) {
	//cout << "fieldNames[" << i << "] = " << fieldNames[i] << endl;
	TypedArray<double> const structField = inStructArray[0][fieldNames[i]];
	ArrayDimensions dims = structField.getDimensions();
	size_t nRows = dims[0];
	size_t nCols = dims[1];
	for(size_t iRow=0; iRow < nRows; iRow++) {
	for(size_t iCol=0; iCol < nCols; iCol++) {
	double value = structField[iRow][iCol];
	cout << "structField[" << iRow << "][" << iCol << "] = " << value << "\t ";
	}
	cout << endl;
	}
	cout << endl;
	}
	//outputs to workspace:
	double b = inScalarArray[0]; //convert to C++ double
	Array outScalarArray = factory.createScalar(b+10);
	double outScalar = outScalarArray[0]; //convert to C++ double
	matlabPtr->setVariable(u"outScalar", outScalarArray, WorkspaceType::GLOBAL);
	Array outVectorArray = factory.createArray<double>({1, 3}, {11, 22, 33});
	matlabPtr->setVariable(u"outVector", outVectorArray, WorkspaceType::GLOBAL);
	Array outMatrixArray = factory.createArray<double>({2, 2}, {1, 2, 11, 22});
	matlabPtr->setVariable(u"outMatrix", outMatrixArray, WorkspaceType::GLOBAL);
	Array outStringArray = factory.createArray<string>({1, 1}, {"samil was here"});
	matlabPtr->setVariable(u"outString", outStringArray, WorkspaceType::GLOBAL);
	cout <<"\nOUTPUTS FROM MATLAB WORKSPACE:"<<endl;
	cout << "outScalar + b = " << outScalar + b << endl; //You cannot use outScalarArray[0] + b, you will get error binary '+': 'matlab::data::ArrayElementRef<false>' does not define this operator
	}
	};

view raw mexWorkspaceDemo.cpp hosted with ❤ by GitHub

When you run mexDriver.m, you will get an output similar to this: