Properties: a.b.c = 3

[Chad J]

Steven Schveighoffer wrote:
> On Wed, 29 Jul 2009 14:22:26 -0400, grauzone <none at example.net> wrote:
> 
>> 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.
>>
>> I don't want to type out that mess as a user either...
> 
> What I meant was, I wouldn't want something like a.b.c.d.e.f = 3 to
> generate the equivalent of 25 lines of code.
> 
>> Design changes to avoid that mentioned mess would interfere with the
>> goal of abstraction (e.g. assume you have widget.position, now how do
>> you set only the x coordinate? yeah, split the property into
>> position_x and position_y. Result is you have more noise, and you
>> can't use a Point struct.)
> 
> option 1, return a ref Point struct
> option 2, return a special struct which uses properties to set the
> values in the original widget.
> 
> I don't think it's an impossible problem to solve, I just don't think
> the compiler should be involved, because it makes it too easy to
> gerenate horrible code.
> 

So we could have semantics that actually work, but you don't want them
because, oh man, my code might have to do a few more assignments.  A few
assignments.  Really?!

!!!!

Assignments aren't even that expensive!  They are one of the easiest
operations your CPU can perform!  It's like you'll have a few more MOV
operations laying around in the worst case.

If there are dereferences involved it has to break up the expression
ANYWAYS.

ARGH!

You'll have to forgive me.  What I'm reading here is quite frustrating.

> Now, having the compiler reject invalid assignments is definitely
> something I can live with.
> 
> -Steve