passing a class method pointer as a function pointer

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passing a class method pointer as a function pointer

(9 posts) (3 voices)

Started 2 years ago by bubulindo
Latest reply from bubulindo

bubulindo
Member

Hello,

I'm developing a class to use a timer as an encoder and am struggling with the timer interrupt.
Basically I want the timer interrupt to increment/decrement a variable that will be the number of turns on the encoder.
So inside the Encoder class I created a private method to do that (since the number of turns variable is private to the class), but when trying to pass a pointer of that method to the timer_attach_interrupt() it gives me an error that I can't seem to cast away (is this a pun?).

Any ideas on how to solve this?

Posted 2 years ago #
gbulmer
Moderator

bubulindo - Please post the text of the error messages and code for the things you have tried. (Preferably for the minimal case.)

Posted 2 years ago #
mlundinse
Member

The timer_attach_interupt expects a 'normal C' function pointer to a void function of a void argument.

Now a C++ class methods is a diferent creature, that sometimes can be called directly and sometimes looked up in Vtables from the class or parent classes.

Your best bet is to write a small void function of void, C function, and pass this to the timer_attach_interrupt and then call your Encoder class method from inside this function.

Posted 2 years ago #
bubulindo
Member

Hello again,

my timer interrupt is this:

void Encoder::timer_interrupt_handler(){
if (this->Direction() == POSITIVE) _revolutions++;
if (this->Direction() == NEGATIVE) _revolutions--;
}

And I try to attach it to the timer as this:

timer_attach_interrupt(_TIMER, TIMER_UPDATE_INTERRUPT, timer_interrupt_handler);

I get this error:

Also note that because of preprocessing, line numbers won't match up exactly. We're working on a fix.
/Users/bubulindo/Documents/MapleIDE/libraries/Encoder/encoder.cpp: In constructor 'Encoder::Encoder(timer_dev*, unsigned char, unsigned int, unsigned int)':
/Users/bubulindo/Documents/MapleIDE/libraries/Encoder/encoder.cpp:104: error: argument of type 'void (Encoder::)()' does not match 'void (*)()'

Casting gives me a similar error, if I try this:

timer_attach_interrupt(_TIMER, TIMER_UPDATE_INTERRUPT, (voidFuncPtr)timer_interrupt_handler);

I get this error:

Also note that because of preprocessing, line numbers won't match up exactly. We're working on a fix.
/Users/bubulindo/Documents/MapleIDE/libraries/Encoder/encoder.cpp: In constructor 'Encoder::Encoder(timer_dev*, unsigned char, unsigned int, unsigned int)':
/Users/bubulindo/Documents/MapleIDE/libraries/Encoder/encoder.cpp:104: error: invalid use of member (did you forget the '&' ?

Any thoughts?

Posted 2 years ago #
bubulindo
Member

@mlundinse

Not sure that is possible. The C function can't call a method that hasn't been instantiated... Since this is a general library, that may be a problem as I can't instantiate all timers as encoders.

I think the solution might be a two level approach where the C++ code is just a wrapper of the C functions. :\ I really hate doing it like this as it defeats the purpose. :(

Edit:

Thinking a bit more about it and a two level approach isn't efficient as I would increment/decrement one variable in a C file, but I can use 4 (on a Maple) timers as encoders. That would mess the whole thing up, or creating one of each for the possibility of the timers being used as an encoder is not great either. :\

Crap. :(

Posted 2 years ago #
mlundinse
Member

if the C function can call to a memeber, the that call is even less possible from inside the timer interrupt handler. There is no current Encoder object "this" in the interrupt handler context, so your approach doesnt work.

Your mtehod must be called like

anEncoderObject.timer_interrupt_handler()

and there is no way to pass the encoder object instance to the timer interrupt handler.

If you have several encoder objects, how should the handler know which to select ?
If you only got one then use a C adapter class.

The error message simply says that your class method is not possible to use as a C void func void.

Posted 2 years ago #
bubulindo
Member

Ok...

Thanks for the help. Guess the revolution counter will have to be done somewhere else then. :(

Posted 2 years ago #
mlundinse
Member

Use a global that is set somewhere, you can even let the timer handler switch the
current Encoder to the next object of an array of objects:

gCurrentEncoder * Encoder;

And then let your interrupthandler be:

void my_timer_handler()
{
gCurrentEncoder->timer_interrupt_handler();
/* Determine whitch Encoder object to call next ?? */
}

Posted 2 years ago #
bubulindo
Member

Yes, that seems to be the solution.

I'll leave the revolution counting bit out of the library for now.

I should have a working version in two weeks (after my vacation). :)

Posted 2 years ago #

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