So, here we have an opCmp function for a class named shape. It compares the .volume of each shape:

 int opCmp(shape rhs)
{
    int exp;
    if(this.volume == rhs.volume)
        exp =  0;
    else if(this.volume > rhs.volume)
        exp =  1;
    else if(this.volume < rhs.volume)
        exp = -1;
    return exp;

}

here is how I would use it:
shape test = new shape(pos);/*Ignore the constructor argument it's not relevant here*/
shape test1 = new shape(pos); //same variable
writeln(test == test1);
writeln(test.opCmp(test1) == 0);

Both .volume are the same, but the output says this:

false
true

note that the documentation for comparing classes with an opCmp function defined is said to be this: x.opCmp(y) op 0

can anybody explain this to me?

On Wednesday, 23 September 2020 at 10:37:09 UTC, Ruby The Roobster wrote:
> So, here we have an opCmp function for a class named shape. It compares the .volume of each shape:
>
>  int opCmp(shape rhs)
> {
>     int exp;
>     if(this.volume == rhs.volume)
>         exp =  0;
>     else if(this.volume > rhs.volume)
>         exp =  1;
>     else if(this.volume < rhs.volume)
>         exp = -1;
>     return exp;
>
> }
>
> here is how I would use it:
> shape test = new shape(pos);/*Ignore the constructor argument it's not relevant here*/
> shape test1 = new shape(pos); //same variable
> writeln(test == test1);
> writeln(test.opCmp(test1) == 0);
>
> Both .volume are the same, but the output says this:
>
> false
> true
>
> note that the documentation for comparing classes with an opCmp function defined is said to be this: x.opCmp(y) op 0
>
> can anybody explain this to me?

Equality is defined by opEquals.
Which for classes defaults to pointer identity comparison.

On Wednesday, 23 September 2020 at 10:38:42 UTC, Stefan Koch wrote:
> Equality is defined by opEquals.
> Which for classes defaults to pointer identity comparison.
Using opEquals, it still returns false. Does this have something to do with volume being a property?:

int volume() @property
{
//...
}

note that this returns true:
test.opEquals(test1)
but not this:
test == test1

here is the function:
bool opEquals(shape rhs)
{
return(rhs.volume == this.volume);
}
despite the fact that x == y should be the same as x.opEquals(y),
calling opEquals directly returns true, but otherwise it returns false
also, compiler complains if I don't have this alias: alias opEquals = Object.opEquals. that might have something to do with it.

On Wednesday, 23 September 2020 at 11:19:15 UTC, Ruby The Roobster wrote:
> bool opEquals(shape rhs)
> {
> return(rhs.volume == this.volume);
> }

This is a wrong implementation of opEquals for a class. The documentation mentions (https://dlang.org/spec/operatoroverloading.html#equals):

> 4. If overridding Object.opEquals() for classes, the class member function signature should look like:
> 
> class C
> {
>     override bool opEquals(Object o) { ... }
> }

A correct implementation would be:

override bool opEquals(Object o) {
    if (!cast(Shape)o) return false;
    return (cast(Shape)o).volume == this.volume;
}

--
  Simen

On 9/23/20 7:28 AM, Simen Kjærås wrote:
> On Wednesday, 23 September 2020 at 11:19:15 UTC, Ruby The Roobster wrote:
>> bool opEquals(shape rhs)
>> {
>> return(rhs.volume == this.volume);
>> }
> 
> This is a wrong implementation of opEquals for a class. The documentation mentions (https://dlang.org/spec/operatoroverloading.html#equals):
> 
>> 4. If overridding Object.opEquals() for classes, the class member function signature should look like:
>>
>> class C
>> {
>>     override bool opEquals(Object o) { ... }
>> }
> 
> A correct implementation would be:
> 
> override bool opEquals(Object o) {
>      if (!cast(Shape)o) return false;
>      return (cast(Shape)o).volume == this.volume;
> }

Or, more D way:

if(auto other = cast(Shape)o) return other.volume == this.volume;
return false;

-Steve

On Wednesday, 23 September 2020 at 11:28:11 UTC, Simen Kjærås wrote:
> A correct implementation would be:
>
> override bool opEquals(Object o) {
>     if (!cast(Shape)o) return false;
>     return (cast(Shape)o).volume == this.volume;
> }
>
> --
>   Simen

Thanks. This solved the problem.