Dynamic function pointer (delegate)

Forum 2005-2010 (read only) Hardware Bugs & Suggestions
#1

Hi, I don't know if this is a the matter of the gcc, or Arduino. I feel the need to pass function pointer (delegate) of an object instace. (Passing function pointers as a parameter.) E.g. interrupt handling from libraries.

Unfortunately I can only pass static function pointers.

My suggestion is to allow function pointers of object instances. (And the compiler error message in other case is awful.)

Thanks, Balázs

#2

Below is example code I use to pass a class definition as a delegate:

void PrintXmlTime(Print &pr,uint32_t t)
{
  char str[11];
  time_s ts;
  
      local_time(t,ts);
      //2008-10-31T15:07:38
      sprintf_P(str,PSTR("%04d-%02d-%02d"),ts.year+2000,ts.month+1,ts.day+1);
      pr.print(str);
      sprintf_P(str,PSTR("T%02d:%02d:%02d"),ts.hour,ts.minute,ts.second);
      pr.print(str);
}/*PrintXmlTime*/

  SdPrint Log;

  PrintXmlTime(Serial);
  PrintXmlTime(Log);

Serial is the reference to the standard hardware serial class and Log is a class reference to a data logger (SD card) class. Both of these classes define Print as a subclass and this allows me to use a single subroutine for writing to an SD card and printing to serial. This is all standard AVR C++.

Is this along the lines of what you're trying to do?

#3

No. What I try to do is this:

class Rc5
{
  public:
    Rc5(int interrupt);
  private:
    void onInfraEvent();
}

Rc5::Rc5(int interrupt) {
  attachInterrupt(interrupt, this->onInfraEvent, CHANGE);
}

In this example the onIfraEvent() is my delegate function, which should be a member function. But the C compiler complains, so I had to do it this way:

class Rc5Class
{
  public:
    void init(int interrupt, int irDataPin);
  private:
    static void onInfraEvent();
}

Rc5Class::init(int interrupt) {
  attachInterrupt(interrupt, Rc5Class::onInfraEvent, CHANGE);
}
Rc5Class Rc5;

Where the onInfraEvent is a static function. So there can be only one Rc5 class instances, that can handle interrupts. :(

#4

The attachInterrupt() function requires a simple pointer to a parameterless function returning void. A member function pointer on the other hand is a compound pointer that include a reference to "this" AND a reference to the function within "this". As such there is a parameter type conflict that triggers the compile error.

So much for understanding why, but I would agree with you that "this is what we want, so give us syntax to do so". For all I know this may already be possible (e.g. with some special typecast/dereference keyword), but I haven't seen any.

Member function pointers is a somewhat dark area when it comes to C++ in general. Part of the reason is that existing implementations were challenged (e.g. with backwards compatibility) when inheritance and later multiple inheritance was formally included in C++. If you're interested in the topic there is a decent article here:

http://www.codeproject.com/KB/cpp/FastDelegate.aspx

If you want to propose changes/extensions to gcc however I think you're better off addressing this to the gcc community directly.

#5

Okay. Now I understand that this is nothing to do with Arduino. So I go to other forums. I'll be back when I've got solution. Btw: I copy the error messages here for documentation reasons.

If I try to pass instance function I've got this:

argument of type 'void (Rc5::)()' does not match 'void (*)()'

If I try to cast "attachInterrupt(interrupt, (void (*)())(this->onInfraEvent), CHANGE);", I get this error message:

invalid use of member (did you forget the '&' ?)

If I put the "&" mark, I get this:

ISO C++ forbids taking the address of a bound member function to form a pointer to member function.  Say '&Rc5::onInfraEvent'

I'm still not sure about this member function pointer stuff. I don't want member function pointer, I simply want function ponter of an instance.

#6

I don't want member function pointer, I simply want function ponter of an instance.

A function pointer of an instance IS a member function pointer.

#7