Chris Nicholson-Sauls wrote:
> class Number {
>   int i;
> 
>   int opCmp (Object obj) {
>     if (auto other = cast(Number) obj) {
>       return this.i - other.i;
>     }
>     else {
>       throw new Exception("Can only compare Number with another Number.");
>     }
>   }
> }

Broken. (int.max - int.min) doesn't fit in an int...
This is a valid implementation only when the range is less than about half that of an int. In practice, that means this is a valid implementation if member 'i' is a bool, byte, ubyte, short, ushort, char or wchar, but not if it's an int, uint, long or ulong. Nor will it be correct for cent and ucent when they get implemented.
Any built-in type not mentioned don't work. (unless I missed any ;) )
Well, theoretically, on a machine with an address space of 31 bits or less it would also work for pointers on that machine...

A generic way to do it would be something like
  return typeid(i).compare(&i, &other.i);
That should always return a valid value for built-in types and arrays, and call opCmp for user-defined types[1].


[1]: With some wrapping. nulls are always considered 'less than', and structs without opCmp members default to calling memcmp instead.

Frits van Bommel wrote:
> Chris Nicholson-Sauls wrote:
>> class Number {
>>   int i;
>>
>>   int opCmp (Object obj) {
>>     if (auto other = cast(Number) obj) {
>>       return this.i - other.i;
>>     }
>>     else {
>>       throw new Exception("Can only compare Number with another Number.");
>>     }
>>   }
>> }
> 
> Broken. (int.max - int.min) doesn't fit in an int...

Granted.  :)  I was really just trying to be illustrative, not neccessarily thorough. (Take a look at the /glaring/ error in my other exmaple... which, in hindsight, probably should've been more complete|correct than this one.  Oh well, that's what I get for posting while half asleep.)

But you make a good point of the sort that belongs in a FAQ somewhere, actually.  I've seen plenty of opCmp's defined along these lines in other code.

-- Chris Nicholson-Sauls

Heinz wrote:
> Chris Nicholson-Sauls Wrote:
> 
>> Heinz wrote:
>>> Chris Nicholson-Sauls Wrote:
>>>
>>>> Heinz wrote:
>>>>> Carlos Santander Wrote:
>>>>>
>>>>>> Heinz escribió:
>>>>>>> torhu Wrote:
>>>>>>>
>>>>>>>> Heinz wrote:
>>>>>>>>> What does this sort property do? how can i use it? how do i implement opCmp. thanks in advance
>>>>>>>> .sort sorts an array, using some default sort order.  If you want to change the order, you implement opCmp.
>>>>>>>>
>>>>>>>>
>>>>>>>> opCmp has got these signatures, think.  It doesn't seem to be documented much:
>>>>>>>>
>>>>>>>> class C {
>>>>>>>>      // the argument is of type Object, not C
>>>>>>>>      int opCmp(Object other);
>>>>>>>> }
>>>>>>>>
>>>>>>>> or:
>>>>>>>>
>>>>>>>> struct S {
>>>>>>>>     int opCmp(S other);
>>>>>>>> }
>>>>>>>>
>>>>>>>>
>>>>>>>> opCmp() has to return less than zero if it's own object is smaller, more than zero if 'other' is smaller, and zero if they are equal.
>>>>>>>>
>>>>>>>> You can't define opCmp for any other types, if you want to change the sort order of ints, you have to write a separate sort function.
>>>>>>> So, lets assume we have the following class:
>>>>>>>
>>>>>>> class myclass
>>>>>>> {
>>>>>>>         char[] cs;
>>>>>>> }
>>>>>>>
>>>>>>> and then we have a dinamic array of myclass, can we sort this array by the cs property?
>>>>>> Yes, you would have to write an opCmp that does that.
>>>>>>
>>>>>> -- 
>>>>>> Carlos Santander Bernal
>>>>> Hi,
>>>>>
>>>>> Do i have to declare opCmp(Object) and it'll do it automatically? or
>>>>> Do i have to write the algorithm to sort the classes by the cs property? if so, what's the sense of having a sort property if i have to implement a propietary function, it acts as a link.
>>>> opCmp needs only return the sort-order of an object in relation to another given object. Using a simpler example:
>>>>
>>>> class Number {
>>>>    int i;
>>>>
>>>>    int opCmp (Object obj) {
>>>>      if (auto other = cast(Number) obj) {
>>>>        return this.i - other.i;
>>>>      }
>>>>      else {
>>>>        throw new Exception("Can only compare Number with another Number.");
>>>>      }
>>>>    }
>>>> }
>>>>
>>>> Otherwise the .sort property would have no idea how to order the objects (there really is no generic means to do this).  IIRC, there is/was a default Object.opCmp which compared the address of objects, but this is really useless in terms of proper sorting.  (Although it does have the usefulness of making all classes available as associative array keys.)
>>>>
>>>> -- Chris Nicholson-Sauls
>>>
>>> Hi Chris, i still don't get it hahaha. Replace the int in your example by a char[] str; How the hell opCmp can compare a string, what value of type int should return? thx
>> Return any value less than 0 for "less than", 0 for "equal to", and any value greater than zero for "greater than".  (Most people default to -1,0,1.)  For a char[] you'd have to decide how you want to order characters... but one generic means is just to compare their character codes.  So something like:
>>
>> class myclass {
>>    char[] cs;
>>
>>    int opCmp (Object obj) {
>>      char oc ;
>>
>>      if (auto other = cast(myclass) obj) {
>>        foreach (i, c; cs) {
>>          oc = other.cs[i];
>>          if (c < oc) {
>>            return -1;
>>          }
>>          else if (c > oc) {
>>            return 1;
>>          }
>>        }
>>        return 0;
>>      }
>>      else {
>>        throw new Exception("Can only compare myclass with myclass.");
>>      }
>>    }
>> }
>>
>> -- Chris Nicholson-Sauls
> 
> 
> Thanks chris, i finally understood. This example is much clearer than the other one.
> We could also use in this case std.string.icmp(), as it returns the same values.
> 
> Thanks man, good luck.

You know, I always forget about that function because I use it and its sort only rarely in my own code.  That's most likely the way you want to go.

-- Chris Nicholson-Sauls

Chris Nicholson-Sauls wrote:
> Frits van Bommel wrote:
>> Chris Nicholson-Sauls wrote:
>>> class Number {
>>>   int i;
>>>
>>>   int opCmp (Object obj) {
>>>     if (auto other = cast(Number) obj) {
>>>       return this.i - other.i;
>>>     }
>>>     else {
>>>       throw new Exception("Can only compare Number with another Number.");
>>>     }
>>>   }
>>> }
>>
>> Broken. (int.max - int.min) doesn't fit in an int...
> 
> Granted.  :)  I was really just trying to be illustrative, not neccessarily thorough. (Take a look at the /glaring/ error in my other exmaple... which, in hindsight, probably should've been more complete|correct than this one.  Oh well, that's what I get for posting while half asleep.)
> 
> But you make a good point of the sort that belongs in a FAQ somewhere, actually.  I've seen plenty of opCmp's defined along these lines in other code.

I usually prefer to write my opCmp in this fashion:
    return typeid(T).compare(&this.member, &other.member);

(replace T by whatever type you're comparing, or even "typeof(this.member)" if you're afraid you might forget to change this code if you change the type of 'member')

That way I don't make any stupid or annoying mistakes...