Operator overloading -- lets collect some use cases

[Denis Koroskin]

On Tue, 30 Dec 2008 22:26:19 +0300, Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org> wrote:

> Don wrote:
>> Frits van Bommel wrote:
>>> Don wrote:
>>>> Frits van Bommel wrote:
>>>>> Don wrote:
>>>>>> A straightforward first step would be to state in the spec that  
>>>>>> "the compiler is entitled to assume that X+=Y yields the same  
>>>>>> result as X=X+Y"
>>>>>
>>>>> That doesn't hold for reference types, does it?
>>>>
>>>> I thought it does? Got any counter examples?
>>>
>>> For any class type, with += modifying the object and + returning a new  
>>> one:
>>  Sure, you can do it (behaviour inherited from C++), but is that _EVER_  
>> a good idea? I can't think of any cases where that's anything other  
>> than a bug-breeder.
>>
>>> You can't just arbitrarily substitute between these two.
>> I'm still looking for a use case where that substitution doesn't make  
>> sense. No-one has yet come up with such a use case. I postulate that it  
>> doesn't exist.
>
> Well I forgot whether BigInt is a class, is it? Anyhow, suppose it *is*  
> a class and as such has reference semantics. Then a += b modifies an  
> object in-situ, whereas a = a + b creates a whole new object and happens  
> to bind a to that new object.
>
> Andrei

It was suggested 2 posts up the thread. I believe Don is looking for a use case where given

a1 = a + b;

a2 = a;
a2 += b;

the following check intentionally fails:

assert(a1 == a2); // not "a1 is a2"

He postulates that none exists.