MATLAB: How to access a single instance of the C++ object from several S-Functions in Simulink 7.1 (R2008a)

Question

I have a C++ class with several methods that I am creating S-Functions for. I want to be able to create an instance of the class and then have each of my S-Functions use that instance during simulation. I would like to know how I may accomplish this task.

Answer 1

You cannot access a single instance of your C++ object between different S-Functions but you can share access to a single object between multiple instances of a single S-function. Attached is a modified version of the C++ 'Counter' S-Function demo (sfncdemo_counter_cpp.mdl) that uses the STATIC keyword to create a single instance of the counter class that is accessed by both instances of the S-Function.

In the original file the Start and Output functions are written as follows:

 #define MDL_START  /* Change to #undef to remove function */
 #if defined(MDL_START) 
   /* Function: mdlStart =======================================================
    * Abstract:
    *    This function is called once at start of model execution. If you
    *    have states that should be initialized once, this is the place
    *    to do it.
    */
   static void mdlStart(SimStruct *S)
   {
       ssGetPWork(S)[0] = (void *) new counter; // store new C++ object in the
   }                                            // pointers vector
 #endif /*  MDL_START */
 /* Function: mdlOutputs =======================================================
* Abstract:
*    In this function, you compute the outputs of your S-function
*    block.
*/
 static void mdlOutputs(SimStruct *S, int_T tid)
 {
     counter *c = (counter *) ssGetPWork(S)[0];
     real_T  *y = ssGetOutputPortRealSignal(S,0); // the pointers vector and use
     y[0] = c->output();                          // member functions of the
     UNUSED_ARG(tid);                             // object
 }

The modified version replaces the line

counter *c = (counter *) ssGetPWork(S)[0];

with

static counter *c = (counter *) ssGetPWork(S)[0];

The "static" keyword prevents the creation of a second counter by the second instance of the S-Function block.