== and equals()

[nevdelap]

Hey guys,

I'm writing a Cairo app and when I couldn't compare PointD's (2D point with doubles) I thought I had to add an equals extension method because == couldn't be used, but then discovered that it already has an equals so that the equals here works, but the == doesn't.

Code: Select all

use Cairo from Mono.Cairo

class Program

   def main
      assert PointD(1, 2).equals(PointD(1, 2))
      assert PointD(1, 2) == PointD(1, 2) # error: Operator "==" cannot be applied to operands of type "Cairo.PointD" and "Cairo.PointD"

I was under the impression that the equals() was used to implement the == and <>.

Can you explain what's going on?

Thanks,

Nev

[hopscc]

From a quick code skim, For reference types it uses .Equals eventually but on its voyage to that it looks like its doing a check for existence of .CompareTo ( IComparable) which is
where the error message is coming from..
( are you sure it sez 'cannot be applied' and not 'cannot be compared' ?)

This seems a bit of a miss for syntax checking vs codegen but chuck can probably clarify.

[nevdelap]

Yep. That's def a copy and paste of the message.

[hopscc]

Hey Chuck, can ya take a quick look at this and give us an idea whether either the (compiler) code is wrong ( . -> CompareTo -> Equals) or the assumption around 'equals' is..

Is it just that '==' needs 'CompareTo' rather than an 'Equals' override ?

[nevdelap]

Once I understood that CompareTo was used for ==, <> from Mike's post, I wondered if for things where <, > etc aren't meaningful it would preclude using IComparable/CompareTo. ?

[Charles]

Sorry I missed this thread. I'm traveling today and visiting family for the next several days. Maybe I can look into this tomorrow/Friday.

[Charles]

One thing I notice is that this is a C# error. And PointD is a struct so we may have a bug here in Cobra's treatment of == with structs. It looks like Cobra accepts "a == b" for structs, but does not generate the C# for that expression that would make the C# compiler happy.

[Charles]

This is now fixed.