Math Forum Discussions

mathias

Posts: 6
Registered: 1/20/10

Re: Making C++ objects persistent between mex calls, and robust.
Posted: Jul 6, 2011 2:31 AM

Plain Text

Just for the record, in order to complete what has been proposed :

I faced the same issue (which is why I ended up on this thread).

I just want to point out a side problem :

the reinterpret_cast loose the type_id and therefore handling object of a class hierarchy through the mother_class and dynamic_cast is made difficult.

For instance, imagine the class hierachy :
class A {};
class B : public A{};
class C : public B {};

imagine in "mex_function1 " you create
B * pB = new B();
C * pC = new C();

then you send the pointers to matlab, and provide the pointer back to another mex function "mex_function2", and want to use a A * interface :
A * pA = createObjectPointerFromMxArray(prhs[0)

and later on :

pB * = dynamic_cast<B*>(pA) ;
if(NULL!=pB) etc .....

This will not work, since the type_id of the object is lost

As a consequence, I ended up creating a class for storing object.

The class mainly contains a pointer to a map :

class storage
{
public :
storage();

object * retrieveObject(string key);
string registerObject(object * inObject);

private :
static map<string,object * > * _myMap;
}

the storage is used for registering and retrieving objects through a key (string). What is transmitted to matlab and then back to mexes is just the string key. This permits keeping the type_id of created objects.

Then the storage is made global, basically using the following constructor

// static init
map<string,object * > storage::_myMap =NULL;

// constructor
storage:storage()
{
if(NULL == _myMap)
{
// look for the global adress
mxArray * mxContainer = mexGetVariable("global","globalObjectStorageAdress");

if( NULL == mxContainer)
{
mexPrintf("Storage container does not exist, creating one\n");

_myMap = new map<string,object * >;
mxContainer = createMxArrayFromObjectPointer(_matlabStorage);

int status = mexPutVariable("global","globalObjectStorageAdressr",mxContainer);

std::cout << "status " << status;
}
else
{
mexPrintf("Using existing storage\n");

_myMap = createObjectPointerFromMxArray<map<string,object * >>(mxContainer);
}

}
}

I hope this can help.

Here are the two helpers written by a collegue of mine

template <typename TOBJECT>
TOBJECT * createObjectPointerFromMxArray(const mxArray * inArray) throw (LOCException)
{
if (mxGetClassID(inArray) != mxINDEX_CLASS)
throw LOCException("The input mxArray must be of type mxUINT32_CLASS or mxUINT64_CLASS to store an object pointer.");
if ((mxGetM(inArray) != 1) || (mxGetM(inArray) != 1))
throw LOCException("The input mxArray must be scalar to store an object pointer.");

return *(reinterpret_cast<TOBJECT**>(mxGetData(inArray)));
}

template <typename TOBJECT>
mxArray * createMxArrayFromObjectPointer(TOBJECT * ptrObject)
{
mxArray * outArray = mxCreateNumericMatrix(1, 1, mxINDEX_CLASS, mxREAL);
TOBJECT * * ptrData = reinterpret_cast<TOBJECT**>(mxGetData(outArray));
*ptrData = ptrObject;
return outArray;
}

As