macOS Method pointers as arguments

Duke Leto · Jul 30, 2009

As discussed before, I started making my own view class for GLUT, and I made an event handling method that checks for hit rectangles and calls the appropriate method that was passed.

The method looks like this:

Code:

void View::mouseEvent(int x, int y, int button, void (View::*mfunc)(int, int, int))
{
	if(pointHitsView(x, y)) (this->*mfunc)(x, y, button);
	
	Enumerator* myEnum = subviews->getEnumerator();
	if(myEnum != NULL)
	{
		View* nextView = (View *)myEnum->getCurrentObject();
		while(nextView != NULL)
		{
			// check hit rect for subview
			if(nextView->pointHitsView(x,y)) (nextView->*mfunc)(x, y, button);
			nextView = (View *)myEnum->nextObject();
		}
	}
}

The function compiles with no errors, but when I actually use it, there is one error.

Code:

void mouse(int button, int state, int x, int y)
{
	void (View::*eventFunc)(int, int, int) = mouseDownAtPosition;
// this gives me "mouseDownAtPosition was not declared in scope"
	outerBox->mouseEvent(x, y, button, eventFunc);
	testCell->mouseEvent(x, y, button, eventFunc);
}

The mouse() function is in the main.cpp, which handles drawing, and GLUT events.

The mouseEvent() method is in View.cpp.

Why am I getting this error?

autorelease · Jul 30, 2009

Looks like

Code:

void (View::*eventFunc)(int, int, int) = mouseDownAtPosition;

should be

Code:

void (View::*eventFunc)(int, int, int) = &View::mouseDownAtPosition;

Though... do you really need method pointers for this? Couldn't you just require each View subclass to override a method called mouseDown?

Code:

void View::mouseEvent(int x, int y, int button)
{
	if(pointHitsView(x, y)) mouseDown(x, y, button);
	
	Enumerator* myEnum = subviews->getEnumerator();
	if(myEnum != NULL)
	{
		View* nextView = (View *)myEnum->getCurrentObject();
		while(nextView != NULL)
		{
			// check hit rect for subview
			if(nextView->pointHitsView(x,y)) nextView->mouseDown(x, y, button);
			nextView = (View *)myEnum->nextObject();
		}
	}
}

Also note that there's a bug in that method; it only works with a single level of nested views. (i.e. it will fail if you try to click on a subview of a subview of a subview.) You'll need to recursively descend the view hierarchy.

mslide · Jul 31, 2009

Are you sure you want to be using pointers to (non-static) member functions? This is a hairy subject and is not as simple as using pointers to C functions. I recommend not doing it.

Duke Leto · Jul 31, 2009

Thanks for the bug notice!

It works now.

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macOS Method pointers as arguments

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