matlab-mexutils

C++ Utility Headers for Matlab MEX Development + CMake Template

Also listed on Matlab File Exchange at http://www.mathworks.com/matlabcentral/fileexchange/66217

This respository contains various utilities for Matlab MEX development in C++:

• Unification of Matlab & C++ Classs
• Other Utility Header Files
• Building MEX Functions with CMake

Demo Installation

To build and install the examples included in this repo, use CMake + your favorite C++ compiler. By default, the CMake installation process creates mexcpp_demo folder in your MATLAB user directory (e.g., My Documents/MATLAB/mexcpp_demo in Windows). To change the installation destination, modify MEXCPP_DEMO_INSTALL_DIR cache variable (not CMAKE_INSTALL_PREFIX). Note that the installation process is required to bring m-files and mex-files together.

Unification of Matlab & C++ Classs

include/mexObjectHandler.h

This portion is based on Oliver Woodford's implementation of a MATLAB class to wrap a C++ class. Three templates are found in mexObjectHandler.h:

Type	Name	Description
function	mexObjectHandler	All-in-one template function to be called in mexFunction
class	mexObjectHandle	Implements C++ object wrapping mechanism. This class is transparent in a mexObjectHandler-based MEX function.
class	mexSetGetClass	Base class with set/get/save/load actions

mexObjectHandler is designed to be paired with the MATLAB base/template handle class mexcpp.BaseClass. See the section below for the specifications of the m-file.

To implement a MEX function with this framework, mexFunction would require a single line:

#include <mex.h>
#include "mexObjectHandler.h"

class myClass;

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
  mexObjectHandler<myClass>(nlhs, plhs, nrhs, prhs);
}

Here, myClass is the target C++ class to be wrapped by the MATLAB class.

There are several requirements to specialize mexObjectHandler for myClass. First, it must have a constructor with signature:

myClass(const mxArray *mxObj, int nrhs, const mxArray *prhs[]);

The first argument mxObj points to the MATLAB class object, and nrhs and prhs get the varargin constructor arguments, which are as passed in from MATLAB. myClass must also define the following public functions:

C++ Signature	Description
static std::string get_classname()	Return the name of paired MATLAB class
bool action_handler(const mxArray *mxObj, const std::string &action, int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])	Perform the specified action
static bool static_handler(std::string action, int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])	Perform the specified static action

In MATLAB, the static MEX member function (assuming its name mexfcn in mexcpp.BaseClass) is then called with the following 4 signatures from the MATLAB class:

MATLAB Signature	Description
mexfcn(obj, varargin)	Create a new C++ class instance and store it as the backend MATLAB class property. Errors out if backend is not empty.
mexfcn(obj, 'delete')	Destruct the C++ class instance pointed by the backend property
varargout = mexfcn(obj, action, varargin)	Perform specified action of the wrapped C++ object
varargout = mexfcn('action', varargin)	Perform specified static action of the wrapped C++ object

The arguments int nlhs, mxArray *plhs[], int nrhs, and const mxArray *prhs[] in action_handler() and static_handler() carry varargout and varargin of their corresponding MATLAB calls.

+mexcpp/BaseClass.m

This abstract class is a bare-bone handle class to house the MEX function running mexObjectHandler() template function to wrap a C++ backend class instance.

It features:

• Protected backend property to hold a pointer to mexObjectHandle
• Abstract protected static method varargout = mexfcn(varargin) to reserve the MEX function as its protected method
• Constructor calls obj.mexfcn(obj, varargin{:}) to create a pairing C++ object (the mexFunction implicitly store it in the backend property)
• Deleter calls obj.mexfcn(obj, 'delete') to destroy the backend C++ object

Subclass inheriting mexcpp.BaseClass must:

• Pass necessary input arguments to mexcpp.BaseClass constructor to instantiate the C++ object
• Although accessible, leave backend property alone (unless the backend needs to be recreated). It is managed by mexObjectHandler().
• Implement various methods which call obj.mexfcn() to perform various C++ actions using the signature: [...] = obj.mexfcn(obj, 'action', ...) or statically [...] = obj.mexfcn('action', ...)
• Use ASCII action names as mexObjectHandler() does not accept multi-byte characters for the action name

This MATLAB class could be used as the superclass for a user's wrapper class as outline thus far, or it can be used as a template for a standalone class. Such class must be a handle class.

include/mexSetGetClass.m

Accessing member variables of the C++ backend object from MATLAB is often important, and mexSetGetClass implements set and get actions, which call the derived class' set_prop and get_prop, respectively. Note that this implementation is not the most efficient but may be useful to separate the set/get actions from other actions for a large-scale class object. In addition to set/get, load and save actions are suggested to be used with saveobj and loadobj MATLAB class functions.

Standalone Usage of mexObjectHandle Template Class

mexObjectHandle may be used on its own without mexObjectHandler(). See examples/mexCounter.cpp and examples/mexCounter_demo.m for such an example. Note that the wrapped C++ "object" in this example is a plain integer to store the counter state. This demo also demonstrates that you can have multiple handles of the same MEX function. Last, Use onCleanup class in MATLAB to guarantee that the MEX object gets deleted when MATLAB workspace is cleared. As illustrated in the demo, the handle stored in a MATLAB variable could easily be overwritten and without the onCleanup mechanism, the C++ object gets completely lost and the lock on the MEX function will never be removed.

Other Utility Header Files

include/mexRuntimeError.h

Derived class of C++ std::runtime_error to log the exception id in addition so that mexObjectHandler() can catch the exception and call mexErrMsgIdAndTxt().

include/mexGetString.h

Defines a function mexGetString(), which converts char/cellstr mxArray to std::string. This function is based on mxGetString() LIBMX function and only supports single-byte character strings.

include/mexAllocator.h

Defines mexAllocator class, which is a custom C++ allocator. It wraps mxCalloc(), mxRealloc(), and mxFree(). This allocator is useful to write a template class, which dynamically allocates memory and the allocated memory is later used in Matlab (i.e., set to an mxArray object) completely detached from the template class. Such template class could be written independent of Matlab with an Allocator template.

Building MEX Functions with CMake

CMake is one of the most widely used cross-platform build automation software, and it meshes well with MATLAB, which is also a cross-platform environment. While mex command in MATLAB does exactly that, configuring it becomes cumbersome quickly as the scale of the project grows. The couple features of CMake makes it very attractive platform to build MEX functions:

• Built-in support to configure header and library files via find_package(Matlab) command, which runs Modules/FindMatlab.cmake.
• matlab_add_mex macro in Modules/FindMatlab.cmake to create and configure the MEX target.
• Target installation. The built targets as well as any files in the project can be made to be "installed" to a separate directory.

This repository presents a way to set up CMake for a MEX project,utilizing these features. For the first two points, readers are referred to the CMake documentation for the details.

The installation process of CMake is what makes it a great MEX development environment. Here is how to set up a MEX project.

1. Create a MEX project folder, which only holds the related m-files (e.g., help text file for a mex function or other class functions for a mex class) and C/C++ source files.
   • The directory structure should reflect how files will be installed. For example, have +package folders to define a MATLAB package or @class folders to define a MATLAB class with multiple m-files.
   • Place mexFunction C/C++ source files at exactly where the compiled MEX files should be placed. For example, the source file for the mexfcn MEX function for the myClass example above should be placed inside of @myClass folder.
2. CMake build process takes the C/C++ source files and place the compiled binary files in the build directory.
3. CMake installation process can be set to copy all the m-files in the MEX project directory to the installation location, keeping the directory structure intact. Then, make it copy all the MEX binaries to the appropriate place in the install directory (same relative locations as the source files in the project directory).

This keeps all the non-Matlab files (i.e., source files and any intermediate build files) away from the final product while presenting the m-files and C/C++ files in the easy-to-relate locations.

Note on building MEX function with MS Visual C++ compiler

While the Mathwork's documentation suggests using a def file to export mexFunction symbol, it could be achieved simply with a linker flag /EXPORT:mexFunction. In CMake, this flag can be set by the command:

set_target_properties(${MEX_TARGET} PROPERTIES LINK_FLAGS /EXPORT:mexFunction)