using do_line in a class

no-reply@allegro.cc (HardTranceFan) — Fri, 20 Oct 2006 10:56:13 +0000

I've defined a class, and within that I'm trying to use the do_line function, passing one of the methods of that class as the supplied function. However, when I try, the compiler complains with:

argument of type `void (class::)(BITMAP*, int, int, int)' does not match `void (*)(BITMAP*, int, int, int)'

Is there a way of using a class's method in do_line?

Jeroen

no-reply@allegro.cc (orz) — Fri, 20 Oct 2006 14:03:06 +0000

class Bob {
public:
  virtual void draw_line_callback_in_class1(BITMAP *bmp, int x, int y) {
  }
};

void draw_line_callback_helper_1(BITMAP *bmp, int x, int y, int d) {
  Bob *bob = (Bob*)d;//only valid if sizeof(int) >= sizeof(void*)
  //if sizeof(int) is < sizeof(void*) you might try having a table of sime kind to look up which Bob you want in using d
  //if you need more parameters, instead of going for a Bob you could go for an intermediate object that contained a Bob* and something else
  ((Bob*)d)->draw_line_callback_in_class1(bmp, x, y);
}

...
do_line(screen, 0, 0, SCREEN_W-1, SCREEN_H-2, (int)bob_ptr, draw_line_callback_helper_1);

Not the best way of doing things, but it works on most 32 bit compilers. I think some people create templates to replace the helper function. Also, if you don't need a 'this' pointer, you can declare the in-class callback as static and then you can do away with the helper. See the comments for ideas on what to do on 64 bit compilers or other circumstances.

no-reply@allegro.cc (HardTranceFan) — Sat, 21 Oct 2006 00:09:02 +0000

Hey Orz,

Your solution worked a treat. Thanks

Jeroen

no-reply@allegro.cc (ImLeftFooted) — Sat, 21 Oct 2006 00:49:50 +0000

1// in header file
2 
3class Bob {
4public:
5  virtual void draw_line_callback_in_class1(BITMAP *bmp, int x, int y) {
6  }
7};
8 
9// ... in some source file
10static int bobPtrsIndex = 0;
11static Bob *bobPtrs[5]; // Allows max 5 threads accessing callback at once
12 
13void draw_line_callback_helper_1(BITMAP *bmp, int x, int y, int d) {
14  
15  bobPtrs[d]->draw_line_callback_in_class1(bmp, x, y);
16}
17 
18// ... in some method
19int i = bobPtrsIndex++;
20bobPtrs<i> = this;
21do_line(screen, 0, 0, SCREEN_W-1, SCREEN_H-2, i, draw_line_callback_helper_1);
22--bobPtrsIndex;

1	// in header file
2
3	class Bob {
4	public:
5	virtual void draw_line_callback_in_class1(BITMAP *bmp, int x, int y) {
6	}
7	};
8
9	// ... in some source file
10	static int bobPtrsIndex = 0;
11	static Bob *bobPtrs[5]; // Allows max 5 threads accessing callback at once
12
13	void draw_line_callback_helper_1(BITMAP *bmp, int x, int y, int d) {
14
15	bobPtrs[d]->draw_line_callback_in_class1(bmp, x, y);
16	}
17
18	// ... in some method
19	int i = bobPtrsIndex++;
20	bobPtrs<i> = this;
21	do_line(screen, 0, 0, SCREEN_W-1, SCREEN_H-2, i, draw_line_callback_helper_1);
22	--bobPtrsIndex;