I'm debugging some code I wrote back in 2017 and a bounding box collision detection kept giving spurious answers till I resorted to assuming nothing and dumped every variable and alias.

I kept getting results like it was checking against itself, and of course, that would result in finding a collision. So I threw an assert in to check if it was identical objects (as in an error outside this function), and it didn't fire off. It appears (unless my eyes are deceiving me?) that variable aliases themselves are broken.

$ dmd --version
DMD64 D Compiler v2.098.0-beta.2

code:

class drawable_object_t obj;

bool is_colliding_with(drawable_object_t obj) //was a class member
	{
	assert(this != obj);  //does not fire off

	alias x1 = x;
	alias y1 = y;
	alias w1 = w;
	alias h1 = h;
		
	alias x2 = obj.x;
	alias y2 = obj.y;
	alias w2 = obj.w;
	alias h2 = obj.h;
		
	writeln("x1: ", x1, " y1: ", y1, " w1: ", w1, " h1: ", h1);
	writeln("x2: ", x2, " y2: ", y2, " w2: ", w2, " h2: ", h2);

	writeln("x1: ", x, " y1: ", y, " w1: ", w, " h1: ", h);
	writeln("x2: ", obj.x, " y2: ", obj.y, " w2: ", obj.w, " h2: ", obj.h);
        }
/*
output:

x1: 50 y1: 81 w1: 5 h1: 6
x2: 50 y2: 81 w2: 5 h2: 6		<------------
x1: 50 y1: 81 w1: 5 h1: 6
x2: 200 y2: 86.54 w2: 26 h2: 16		<------------
*/

The arrows point to the dependency. The top two sets of numbers should not be identical.

On Friday, 17 September 2021 at 09:44:53 UTC, Chris Katko wrote:

class drawable_object_t obj;

bool is_colliding_with(drawable_object_t obj) //was a class member
	{
	assert(this != obj);  //does not fire off

	alias x1 = x;
	alias y1 = y;
	alias w1 = w;
	alias h1 = h;
		
	alias x2 = obj.x;
	alias y2 = obj.y;
	alias w2 = obj.w;
	alias h2 = obj.h;
		
	writeln("x1: ", x1, " y1: ", y1, " w1: ", w1, " h1: ", h1);
	writeln("x2: ", x2, " y2: ", y2, " w2: ", w2, " h2: ", h2);

	writeln("x1: ", x, " y1: ", y, " w1: ", w, " h1: ", h);
	writeln("x2: ", obj.x, " y2: ", obj.y, " w2: ", obj.w, " h2: ", obj.h);
        }
/*
output:

x1: 50 y1: 81 w1: 5 h1: 6
x2: 50 y2: 81 w2: 5 h2: 6		<------------
x1: 50 y1: 81 w1: 5 h1: 6
x2: 200 y2: 86.54 w2: 26 h2: 16		<------------
*/

It's not a bug because "obj.x" referes to the same symbol that is "this.x"

Alias will create an alias for a symbol, not an expression or the like.

So

obj.x is the same as this.x and in that case the alias will refer to the same thing.

The problem here is that the function is local to the class, so the alias will always refer to the class members directly and not the passed instance.

You really shouldn't use alias like this anyway and should just use auto, if it's because you want to save typing.

It shouldn't have any impact at all tbh.

On Friday, 17 September 2021 at 10:29:12 UTC, bauss wrote:

Thanks everyone!

Okay, so yeah, back in 2017 I definitely was still learning D. And, after a year or two, I was re-learning it... using this bad learner codebase! There's been plenty of other things I had to fix that are "no-nos" in my modern codebases.