sending a reference to a function in an argument

advanced
#1

hi all

i’ve got a class called Histogram which stores the data for and renders a histogram
I use this histogram class for a bunch of histograms on my screen

What i’m wondering is
Is it possible to / how can I send a function reference to the histogram
So that whenever i click on the histogram, it calls the correct function from the host class.

e.g.
class A hosts class B (histogram)
when i initialise class B i give it a reference to a function Gamma in class A
then, whenever i click on histogram in class B, i call function Gamma

thanks!
elliot

#2

you can use ofEvents for this, take a look at the advancedEventsExample

or pass a function pointer but the syntax is kind of horrible:

void passMeAFunction( void (*functionName)(int, int) ){
int a=0,b=0;
functionName(a,b);
}

if functionName is a plain function or:

void passMeAFunction( void (testApp::*functionName)(int, int) ){
int a=0,b=0;
functionName(a,b);
}

if functionName is a method of testApp

of course the original name of the function doesn’t need to be function name.

#3

is it possible to store that function reference for later use?

so could i use the following in my class:

  
  
h:  
private:  
    void * _onSelect();  
  
  
  
cpp:  
Histogram::Histogram(void (testApp::*onSelect)())  
{  
    _onSelect = onSelect;  
}  
  
Histogram::Selected()  
{  
    _onSelect();  
}

and would i call the constructor like

  
  
_histogram = new Histogram(histSelected());
#4

yes, it’s the same horrible syntax : )

  
h:  
private:  
void (testApp::*_onSelect)()  
  
cpp:  
Histogram::Histogram(void (testApp::*onSelect)())  
{  
    _onSelect = onSelect;  
}

to pass the argument to the constructor:

  
new Histogram(&testApp::onSelect);
#5

ok
now i’ve got around the maths bit i’m back to this

with:

  
  
class histThreshold : public PCHistogram  
{  
public:  
	histThreshold(string name, int nClasses, ofTrueTypeFont &Typer, void ( PCDecode::*onSelect) (int) ) :  
		PCHistogram(name, nClasses, Typer)  
	{  
		_onSelect = onSelect;  
	}  
	  
protected:  
	void	updateSelection()  
	{  
		_onSelect(_selectionIClass);     //error comes up on this line  
	}  
  
	void	(PCDecode::*_onSelect) (int);  
};

i get

error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘((histThreshold*)this)->histThreshold::_onSelect (…)’

i dont seem to get the problem when _onSelect has no arguments, and using things like
&_onSelect

changing the line for

  
  
		(*_onSelect) (_selectionIClass);

brings error

error: invalid use of ‘unary *’ on pointer to member

instead

any ideas :)?

#6

ok
i’ve got this working now with ofEvents

i realised what was confusing me about the example
it wasn’t really obvious to myself what eventsObject.h is
from the look of it (the name, the #ifndef OFXEVENTSADDON_H_, etc)
it seemed like it was a special file that you had to use to work with events
rather than an example of a class

so in an attempt to help others who might get lost, from my understanding:
if class A hosts class B, and you want class B to trigger functions in class A
then rather than have a function in class B which links to class A, you use a special object called an ofEvent

ofEvent can deal with different types of data, but you can only pass through one object at a time (unlike a function where you can pass many)

you define ofEvent and the argument in class B

e.g.

  
  
ofEvent<int> mousehit;          // (1)

this event has to be public, because you need to reference to it from class A

then in class A you add a listener for that event

e.g.

  
  
ofAddListener(classBinstance.mousehit,this,&classAdefinition::mouseWasHitInClassB); // (2)

then

mouseWasHitInClassB must be a function in class A which takes one argument (a reference to an int)

  
  
mouseWasHitInClassB(int &argumentName) // (3)  
{  
....  
}

the argument name isn’t important

finally,
you need to trigger the event somehow in class B
this is done with

  
  
ofNotifyEvent(mousehit,2,this); // result is evaluating mouseWasHitInClassB(2) in class A

this was probably obvious to a lot of people
but i just couldn’t see the wood from the trees
hope this helps anyone if you get stuck!

and special thanks to arturo for his help here

#7

Thanks for walking through the process Elliot - helped me a lot! :smiley: