runtime type checking

[AutomationGuy]

I have a variable argument function:

int SetAttribute(object obj, int attrib, ...);

I want to check the type a user puts to the variable list at runtime. CVI itself can check the type put into SetCtrlAttribute(int panel, int ctrl, ..). Is it possible to check the type within a va_list at runtime?

Thanks in advance.

[farora]

Hi,

yould you please post more details about what you want to do? I do not really understand your question. Do you want to specify a data type at run-time, because at compliation time this is not known? 

Furthermore, where does the SetAttribute function comes from? It is not a standard CVI function I believe, 

Best Regards, Fabian

[AutomationGuy]

This is standard CVI:

int CVIFUNC_C SetCtrlAttribute(int panel, int control, int attribute, ...);

any attributes entered into SetCtrlAttribute() will be type checked by CVI. If you put a wrong type to the attribute value (the "..." parameter)  you will get a runtime error message.

This is NOT CVI:

typedef struct anonym* Object;

int Object_SetAttribute(Object mvalue, int attrib, ...)

{

   va_list args;

   va_start(args,attrib);

   // This is actualy a cast to int but you can not be sure if the parameter entered in "..." is realy int.

   // CVI can type check here. With RTTI (Runtime type checking) one can check if the attribute is realy of type int.

   // If CVI does it internaly, can a CVI user does it too?

   int x = va_arg(args,int);

  va_end(args);

  return x;

}

The compiler will not complain if you do:

Object_SetAttribute(object, 123,"ABC");

I would like to check the type of a parameter from a variable parameter list before I extract the parameter from the list. This is very helpful to avoid bugs and saves time when debugging.

I hope this explains my problem.

[farora]

Hi, 

the CVI Function works in the following way. see help documentation:

http://zone.ni.com/reference/en-XX/help/370051T-01/cvi/uiref/cvigetctrlattribute/ 

it is a void pointer.

you can also created a variable argument list:

http://www.ni.com/example/29450/en/

the only way how to do it would be via such a pointer and realize it on the same way

Best Regards, Fabian

[RobertoBozzolo]

I remember to have found an example which I can't locate at the moment that passes both the variable value and type to the variable-argument function. The structure is as follows:

// Typo of parameters passed to the function ("stdarg.h")
#define	INTPRM		1	// Integer
#define	DBLPRM		2	// Double
#define CHRPRM		3	// Char
#define STRPRM		4	// Structure

typedef struct {
	double	Dprm;
	int	Iprm;
	char	Cprm[16];
} dummy;

void VarParmFunc (int *parms, ...);


// Button callback which uses the variable-argument function
int CVICALLBACK varFunc (int panel, int control, int event,
		void *callbackData, int eventData1, int eventData2)
{
	int	cnt = 0;
	int	Iprm;
	double	Dprm;
	char	Cprm[16];
	dummy	*dd = NULL;

	if (event != EVENT_COMMIT) return 0;

        // Read values and fill-in type array
	GetCtrlVal (panel, PANEL_parm1, &Dprm);
	parms[cnt++] = DBLPRM;

	GetCtrlVal (panel, PANEL_parm2, &Iprm);
	parms[cnt++] = INTPRM;

	GetCtrlVal (panel, PANEL_parm3, Cprm);
	parms[cnt++] = CHRPRM;

	dd = malloc (sizeof (dummy));
	GetCtrlVal (panel, PANEL_Field1, &dd->Dprm);
	GetCtrlVal (panel, PANEL_Field2, &dd->Iprm);
	GetCtrlVal (panel, PANEL_Field3, dd->Cprm);
	parms[cnt++] = STRPRM;

	parms[cnt++] = 0;

	VarParmFunc (parms, Dprm, Iprm, Cprm, dd);

	if (dd) free (dd);

	return 0;
}

// Variable-arguments function
void VarParmFunc (int *parms, ...)

// This simply echos received parameters
{
	int	cnt = 0;
	char	type, msg[256];
	dummy	*dd;
	va_list	ap;

	if (!parms) goto Exit;

	// Initialize variable-argument list handling
	va_start (ap, parms);

	strcpy (msg, "Received parms:\n");

	// List parameters
	while ((type = *parms++) != 0) {
		switch (type) {
			case INTPRM:
				sprintf (msg, "%s\n  Integer: %d", msg, va_arg (ap, int));
				cnt++;
				break;
			case DBLPRM:
				sprintf (msg, "%s\n  Double %.2f", msg, va_arg (ap, double));
				cnt++;
				break;
			case CHRPRM:
				sprintf (msg, "%s\n  String: %s", msg, va_arg (ap, char *));
				cnt++;
				break;
			case STRPRM:
				dd = va_arg (ap, dummy *);
				sprintf (msg, "%s\n  Struct:\n      Double: %.2f\n      Integer: %d\n      String: %s", msg,
					dd->Dprm, dd->Iprm, dd->Cprm);
				cnt++;
				break;
		}
	}

	ResetTextBox (mainH, PANEL_res, msg);
Exit:
	sprintf (msg, "%d parameters received", cnt);
	MessagePopup ("Variable params", msg);

Error:
	// End variable arguments handling
	va_end (ap);
	return;
}

[RobertoBozzolo]

An alternative can be to use variants instead of simple variables: as you may know, variants can store different types of data and can be queried for the type store in them by means of CA_VariantGetType () function. This can simplify type checking inside your function, at the cost of a considerably more complex data handling.

[AutomationGuy]

The link above contains an ' ':

http://zone.ni.com/reference/en-XX/help/370051T-01/cvi/uiref/cvigetctrlattribute/

The help page does not tell anything about RTTI.

I copied the SetCtrlAttribute() function from the header of CVI2013. There is no void*. See userint.h.

And you can try to use a wrong type to the attribute setter of a control and you will get a runtime error message.

Yes and I did write an example already for using va_list. Another example dosen't help.

[AutomationGuy]

Thanks Roberto for your Tips. I was thinking on that already but I can not ask my users to striktly insert a correct type into the parameter list.

The probability of using a wrong type will not change with that aproach. I have to implement it without RTTI.

May be NI will publish a solution to that once. The best solution would be a RTTI implementation to the compiler. I don't know the effort to spend in devloping RTTI for CVI but that would improve stability to CVI generated products.

Regards

Andrej