Hi,

I am trying to compare two instances of a class.  I created a test program to try this, but every method I use to compare the instances always returns false.

this is my code to test comparison

class A
{
    this()
    {

    }
}

void main()
{
    A a = new A();
    A a2 = new A();

    writeln(equals(a, a2));
}

bool equals(Object obj1, Object obj2)
{
    return (obj1 is obj2);
}

I have tried 'a is a2', I have tried 'a1 == a2' and many other ways (including opEquals from object.d) among other things and every single time the comparison returns false.  The comparison always fails and never returns true.

I am trying to do something like Object.equals(Object o) in java, but so far, no success.

Am I missing something here?

On Friday, 10 March 2017 at 16:08:05 UTC, DRex wrote:
> Am I missing something here?

Yeah, you need to implement a custom equality operator.

class A {
   int member;
   override bool opEquals(const A rhs) {
        return this.member == rhs.member; // and other members that need to be equal
   }
}


The default opEquals sees if they are the same *instance* (same as `a is b`), and does not look at contents. You need to define that yourself.

On Friday, 10 March 2017 at 16:13:21 UTC, Adam D. Ruppe wrote:
> On Friday, 10 March 2017 at 16:08:05 UTC, DRex wrote:
>> Am I missing something here?
>
> Yeah, you need to implement a custom equality operator.
>
> class A {
>    int member;
>    override bool opEquals(const A rhs) {
>         return this.member == rhs.member; // and other members that need to be equal
>    }
> }
>
>
> The default opEquals sees if they are the same *instance* (same as `a is b`), and does not look at contents. You need to define that yourself.

I tried the above class A, and now the compiler fails with the following error:

Error: function app.A.opEquals does not override any function, did you mean to override 'object.Object.opEquals'?

My A class appears exactly as mentioned in your comment...

On Friday, 10 March 2017 at 16:22:18 UTC, DRex wrote:
> Error: function app.A.opEquals does not override any function, did you mean to override 'object.Object.opEquals'?

Oh sorry, maybe I messed up the const. Try:

override bool opEquals(A rhs) { ... }


and if the compiler still complains change the A to Object and cast it inside (but I'm pretty sure that will work, I think it is just const it is picky about)

On Friday, 10 March 2017 at 16:30:00 UTC, Adam D. Ruppe wrote:
> On Friday, 10 March 2017 at 16:22:18 UTC, DRex wrote:
>> Error: function app.A.opEquals does not override any function, did you mean to override 'object.Object.opEquals'?
>
> Oh sorry, maybe I messed up the const. Try:
>
> override bool opEquals(A rhs) { ... }
>
>
> and if the compiler still complains change the A to Object and cast it inside (but I'm pretty sure that will work, I think it is just const it is picky about)

Thanks.

I'm fairly new to D, but this seems to be quite a pain in the rear for a simple comparison of instances of classes...really odd that comparing instances of classes in D requires that messing around when D seems all about simplifying things...

On Friday, 10 March 2017 at 16:36:17 UTC, DRex wrote:
> I'm fairly new to D, but this seems to be quite a pain in the rear for a simple comparison of instances of classes...really odd that comparing instances of classes in D requires that messing around when D seems all about simplifying things...

There often is no sensible default comparison for class contents (now structs on the other hand do have a default comparison that usually works, but structs don't have to worry about polymorphism), so you just need to specify what fields actually matter to your code...

On Friday, 10 March 2017 at 16:47:47 UTC, Adam D. Ruppe wrote:
> On Friday, 10 March 2017 at 16:36:17 UTC, DRex wrote:
>> I'm fairly new to D, but this seems to be quite a pain in the rear for a simple comparison of instances of classes...really odd that comparing instances of classes in D requires that messing around when D seems all about simplifying things...
>
> There often is no sensible default comparison for class contents (now structs on the other hand do have a default comparison that usually works, but structs don't have to worry about polymorphism), so you just need to specify what fields actually matter to your code...

I guess i am just too used to the java way of x.equals(object) which at the source  is exactly 'return this == object'

On 03/10/2017 08:22 AM, DRex wrote:

> Error: function app.A.opEquals does not override any function, did you
> mean to override 'object.Object.opEquals'?
>
> My A class appears exactly as mentioned in your comment...

FWIW, here's some other info:

  http://ddili.org/ders/d.en/object.html#ix_object.opEquals

Ali

On Friday, 10 March 2017 at 17:08:42 UTC, Whatsthisnow wrote:
> I guess i am just too used to the java way of x.equals(object) which at the source  is exactly 'return this == object'

Java would return false here too, though, if it actually did `this == object` in its default compare method. If I remember correctly, comparing two objects with == in Java compares their addresses, not their contents.

On Friday, 10 March 2017 at 20:27:09 UTC, Meta wrote:
> On Friday, 10 March 2017 at 17:08:42 UTC, Whatsthisnow wrote:
>> I guess i am just too used to the java way of x.equals(object) which at the source  is exactly 'return this == object'
>
> Java would return false here too, though, if it actually did `this == object` in its default compare method. If I remember correctly, comparing two objects with == in Java compares their addresses, not their contents.

I must be losing my mind then