Properties: a.b.c = 3

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
> On Wed, 29 Jul 2009 10:16:33 -0400, grauzone <none at example.net> wrote:
> 
>> Daniel Keep wrote:
>>> Maybe the compiler could rewrite the above as:
>>>  auto t = a.b;
>>> t.c = 3;
>>> a.b = t;
>>>  Unless it can prove it doesn't need to.  Same solution as to the op=
>>> conundrum.
>>
>> Yes! At least that's what the user wants.
>>
>> The compiler has to detect, that the object was modified at all. (To 
>> know whether it should generate code to write back the property.) 
>> Would this make the compiler much complexer?
>>
>> You also have to deal with nested properties:
>>
>> a.b.c.d = 3;
>>
>> turns to
>>
>> auto t = a.b;
>> auto t2 = t.c;
>> c.d = 3;
>> t.c = t2;
>> a.b = t;
>>
>> ???
> 
> Yeah, I think this idea is no good.  a.b.c.d.e.f = 3, results in one 
> gigantic mess, which the user might not want.
> 
> Properties don't have to be exactly like fields.  I think we need to get 
> away from that idea.

Agreed. But it would be nice if they didn't allowed code with identical 
syntax and useless semantics.

> It would be nice if the compiler could help by simply rejecting what it 
> can reject (assignment to rvalues), but other than that, there's not 
> much that can be done.
> 
> This can be detected in simple cases, but in the case where the end 
> point is a function, it will be difficult or impossible.

I think it is eminently possible, but we must figure a solution that 
doesn't complicate the language all too much.

> I don't believe the problem needs to be solved.

To me it looks like an essential problem.


Andrei