C++ Matlab MEX development kit.

The kit contains a couple of C++ classes and macros to make MEX development easy in Matlab. There are 3 major components in the development kit.

• mexplus/dispatch.h Macros to dispatch function calls within a MEX binary.
• mexplus/arguments.h MEX function argument wrappers.
• mexplus/mxarray.h MxArray data conversion and access class.

All classes are located in mexplus namespace, and you can use all of them by including the mexplus.h header file.

The library depends on a few C++11 features, and might not be compatible with older compilers. For older g++, make sure to add -std=c++11 flag at compile time, or in the CXXFLAGS variable in the MEX options located at $HOME/.matlab/$VERSION/mexopts.sh, or in Matlab R2014a or later, at $HOME/.matlab/$VERSION/mex_C++_$ARCH.xml.

Suppose we have the following Database class in C++, and we would like to create a Matlab wrapper.

// Database.h

// Hypothetical database class to be wrapped.
class Database {
public:
  Database(const std::string& filename);
  virtual ~Database();
  std::string query(const std::string& key) const;
};

We will need to create two files.

• Database_.cc: C++ interface file.
• Database.m: Matlab interface file.

Database_.cc

C++ implementation of the MEX interface. It provides MEX entry points by MEX_DEFINE macros and MEX_DISPATCH macro at the end. Notice how inputs and outputs are wrapped by mexplus InputArguments and OutputArguments class. They automatically convert majority of C++ types to/from mxArray, using C++ template. The Session class keeps Database instances between MEX calls, allowing the MEX binary to be stateful.

// Database_.cc: C++ interface file to the Database class.
#include <mexplus.h>
#include "Database.h"

using namespace mexplus;
using namespace std;

// This initializes a session storage for Database instances.
template class mexplus::Session<Database>;

// Create a new instance of Database and return its session id.
MEX_DEFINE(new) (int nlhs, mxArray* plhs[],
                 int nrhs, const mxArray* prhs[]) {
  InputArguments input(nrhs, prhs, 1);
  OutputArguments output(nlhs, plhs, 1);
  output.set(0, Session<Database>::create(
      new Database(input.get<string>(0))));
}

// Delete the Database instance specified by its id.
MEX_DEFINE(delete) (int nlhs, mxArray* plhs[],
                    int nrhs, const mxArray* prhs[]) {
  InputArguments input(nrhs, prhs, 1);
  OutputArguments output(nlhs, plhs, 0);
  Session<Database>::destroy(input.get(0));
}

// Query to the Database instance specified by its id with a string argument.
MEX_DEFINE(query) (int nlhs, mxArray* plhs[],
                   int nrhs, const mxArray* prhs[]) {
  InputArguments input(nrhs, prhs, 2);
  OutputArguments output(nlhs, plhs, 1);
  const Database& database = Session<Database>::getConst(input.get(0));
  output.set(0, database.query(input.get<string>(1)));
}

MEX_DISPATCH

Database.m

Matlab class interface file. The id_ property keeps the session ID (handle) in the MEX binary. Each method is a wrapper around corresponding MEX entry points defined in the C++ file. The first argument of Database_() MEX function is the name defined using MEX_DEFINE() macro in the above file.

classdef Database < handle
%DATABASE Matlab interface to Database.

properties (Access = private)
  id_ % ID of the session instance.
end

methods
  function this = Database(filename)
  %DATABASE Create a new database.
    assert(ischar(filename));
    this.id_ = Database_('new', filename);
  end

  function delete(this)
  %DELETE Destructor.
    Database_('delete', this.id_);
  end

  function result = query(this, key)
  %QUERY Query something to the database.
    assert(isscalar(this));
    result = Database_('query', this.id_, key);
  end
end

end

Build

The above C++ can be compiled by mex command. The output name Database_ is the MEX function name used in Database.m.

mex -Iinclude Database_.cc -output Database_

In Linux, you might need to add CXXFLAGS="$CXXFLAGS -std=c++11" to mex command. i.e.,

mex -Iinclude Database_.cc -output Database_ CXXFLAGS="\$CXXFLAGS -std=c++11"

Once compiled, the Database class is available in Matlab.

database = Database('mydatabase.db');
result = database.query('something');
clear database;

The development kit also contains make.m build function to make a build process easier. Customize this file to build your own MEX interface. The kit depends on some of the C++11 features.

See example directory for a complete demonstration.

MEXPLUS defines a few macros in mexplus/dispatch.h that help to create a single MEX binary with multiple function entries. Create a C++ file that looks like this:

//mylibrary.cc
#include <mexplus/dispatch.h>

MEX_DEFINE(myfunc) (int nlhs, mxArray* plhs[],
                    int nrhs, const mxArray* prhs[]) {
  // Do something.
}

MEX_DEFINE(myfunc2) (int nlhs, mxArray* plhs[],
                     int nrhs, const mxArray* prhs[]) {
  // Do another thing.
}

MEX_DISPATCH

Notice how MEX_DEFINE and MEX_DISPATCH macros are used. Then build this file in Matlab.

mex -Iinclude mylibrary.cc

The built MEX binary can now call two entries by the first argument.

Note that MEX_DISPATCH is only required per MEX binary. If you split the MEX_DEFINE entries across multiple files, you only need to instantiate MEX_DISPATCH in one file.

mylibrary('myfunc', varargin{:})  % myfunc is called.
mylibrary('myfunc2', varargin{:}) % myfunc2 is called.

To prevent from unexpected use, it is a good practice to wrap these MEX calls in a Matlab class or namescoped functions and place the MEX binary in a private directory:

@MyClass/MyClass.m
@MyClass/myfunc.m
@MyClass/myfunc2.m
@MyClass/private/mylibrary.mex*

or,

+mylibrary/myfunc.m
+mylibrary/myfunc2.m
+mylibrary/private/mylibrary.mex*

Inside of myfunc.m and myfunc2.m, call the mylibrary MEX binary. This design pattern is useful to wrap a C++ class in Matlab. See the example directory in the package.

MEXPLUS provides InputArguments and OutputArguments classes to ease parsing, validation, and data conversion of input and output arguments to MEX functions.

The class provides a wrapper around input arguments to validate and convert Matlab variables. You can define a single or multiple input formats to accept. The get() method automatically converts Matlab's mxArray to most of the basic C++ types using a template parameter. Also it can convert to a custom data type when a template specialization to MxArray::to() method is provided. (See the next section.)

Example: The MEX function takes a single numeric input argument.

// C++
InputArguments input(nrhs, prhs, 1);
myFunction(input.get<double>(0));

% Matlab
myFunction(1.0);

Example: The MEX function takes two numeric arguments, and two optional arguments specified by name-value pairs. When optional arguments are not given, the function uses a default value.

// C++
InputArguments input(nrhs, prhs, 2, 2, "option1", "option2");
myFunction(input.get<double>(0),
           input.get<int>(1),
           input.get<string>("option1", "foo"), // default: "foo".
           input.get<int>("option2", 10)); // default: 10.

% Matlab
myFunction(1.0, 2);
myFunction(1.0, 2, 'option2', 11);
myFunction(1.0, 2, 'option1', 'bar');
myFunction(1.0, 2, 'option1', 'baz', 'option2', 12);
myFunction(1.0, 2, 'option2', 12, 'option1', 'baz');

Example: The MEX function has two input formats: 1 + 2 arguments or 2 + 2 arguments.

// C++
InputArguments input;
input.define("format1", 1, 2, "option1", "option2");
input.define("format2", 2, 2, "option1", "option2");
input.parse(nrhs, prhs);
if (input.is("format1"))
    myFunction(input.get<int>(0),
               input.get<string>("option1", "foo"),
               input.get<int>("option2", 10));
else if (input.is("format2"))
    myFunction(input.get<int>(0),
               input.get<vector<double> >(1),
               input.get<string>("option1", "foo"),
               input.get<int>("option2", 10));

% Matlab
myFunction(1.0);
myFunction(1.0, 'option1', 'foo', 'option2', 10);
myFunction(1.0, [1,2,3,4]);
myFunction(1.0, [1,2,3,4], 'option1', 'foo', 'option2', 10);

The class provides a wrapper around output arguments to validate and convert Matlab variables. The set() method automatically converts most of the basic C++ types to Matlab's mxArray using a template parameter.

Example: The MEX function returns at most 3 arguments. The wrapper doesn't assign to the output when the number of outputs are less than 3.

OutputArguments output(nlhs, plhs, 3);
output.set(0, 1);
output.set(1, "foo");
MxArray cell_array(MxArray::Cell(1, 2));
cell_array.set(0, 0);
cell_array.set(1, "value");
output.set(2, cell_array.release());

Data conversion in MEXPLUS is provided by MxArray class.

The MxArray class provides common data conversion methods between mxArray and C++ types, as well as serving itself as a unique_ptr to manage memory.

Two static methods: MxArray::to() and MxArray::from() are the core of the high-level conversions. Give a desired type in the template parameter. The MxArray::to() method has two function signatures. The one with a second pointer argument is to avoid extra copy assignment in the return value.

int value = MxArray::to<int>(prhs[0]);
string value = MxArray::to<string>(prhs[0]);
vector<double> value = MxArray::to<vector<double> >(prhs[0]);
vector<double> value2;
MxArray::to<vector<double> >(prhs[0], &value2); // No extra copy.

plhs[0] = MxArray::from(20);
plhs[0] = MxArray::from("text value.");
plhs[0] = MxArray::from(vector<double>(20, 0));

Additionally, the following object API's are to wrap around a complicated data construction with automatic memory management. Use MxArray::release() to get a mutable mxArray pointer after construction.

// Read access.
MxArray cell_array(prhs[0]);   // {x, y, ...}
int x = cell_array.at<int>(0);
vector<double> y = cell_array.at<vector<double> >(1);

MxArray struct_array(prhs[0]);   // struct('field1', x, ...)
int x = struct_array.at<int>("field1");
vector<double> y = struct_array.at<vector<double> >("field2");

MxArray numeric_array(prhs[0]);   // [x, y, ...]
double x = numeric_array.at<double>(0);
int y = numeric_array.at<int>(1);

// Write access.
MxArray cell_array(MxArray::Cell(1, 3));
cell_array.set(0, 12);
cell_array.set(1, "text value.");
cell_array.set(2, vector<double>(4, 0));
plhs[0] = cell_array.release(); // {12, 'text value.', [0, 0, 0, 0]}

MxArray struct_array(MxArray::Struct());
struct_array.set("field1", 12);
struct_array.set("field2", "text value.");
struct_array.set("field3", vector<double>(4, 0));
plhs[0] = struct_array.release(); // struct('field1', 12, ...)

MxArray numeric_array(MxArray::Numeric<double>(2, 2));
numeric_array.set(0, 0, 1);
numeric_array.set(0, 1, 2);
numeric_array.set(1, 0, 3);
numeric_array.set(1, 1, 4);
plhs[0] = numeric_array.release(); // [1, 2; 3, 4]

To add your own data conversion, define in namespace mexplus a template specialization of MxArray::from() and MxArray::to() with a pointer argument. This will also enable automatic conversion in InputArguments and OutputArguments class.

class MyObject; // This is your custom data class.

namespace mexplus {
// Define two template specializations.
template <>
mxArray* MxArray::from(const MyObject& value) {
  // Write your conversion code. For example,
  MxArray struct_array(MxArray::Struct());
  struct_array.set("x", value.x);
  struct_array.set("y", value.y);
  // And so on...
  return struct_array.release();
}

template <>
void MxArray::to(const mxArray* array, MyObject* value) {
  // Write your conversion code. For example,
  MxArray struct_array(array);
  value->x = struct_array.at<double>("x");
  value->y = struct_array.at<double>("y");
  // And so on...
}
} // namespace mexplus

// Then you can use any of the following.
MyObject object;
std::vector<MyObject> object_vector;
MxArray::to<MyObject>(prhs[0], &object);
MxArray::to<std::vector<MyObject> >(prhs[1], &object_vector);
plhs[0] = MxArray::from(object);
plhs[1] = MxArray::from(object_vector);

Run the following to test MEXPLUS.

make test

• Matlab keeps a string in uint16 while the std::string in C++ is actually std::basic_string<char>. Because of this, signed integers might break if saved inside std::string. To do unicode-safe conversion, use unicode2native and native2unicode before and after calling a MEX function.

General

• Add a script to generate wrapper templates.
• Maybe, use a compiler front-end to automatically generate a wrapper?
• Runtime dependency checker.

MxArray

• N-D array composition and decomposition. See this.
• Sparse arrays.