Properties: a.b.c = 3

[Bill Baxter]

On Wed, Jul 29, 2009 at 10:11 AM, KennyTM~<kennytm at gmail.com> wrote:
> Bill Baxter wrote:
>>
>> On Wed, Jul 29, 2009 at 9:23 AM, Jarrett
>> Billingsley<jarrett.billingsley at gmail.com> wrote:
>>>
>>> On Wed, Jul 29, 2009 at 10:44 AM, Ary Borenszweig<ary at esperanto.org.ar>
>>> wrote:
>>>>
>>>> Andrei Alexandrescu wrote:
>>>>>
>>>>> Kagamin wrote:
>>>>>>
>>>>>> Andrei Alexandrescu Wrote:
>>>>>>
>>>>>>> Kagamin wrote:
>>>>>>>>
>>>>>>>> Andrei Alexandrescu Wrote:
>>>>>>>>>
>>>>>>>>> The question is very simple: given that we're used with a
>>>>>>>>> specific semantics for a.b.c = 3, how can we address the fact
>>>>>>>>> that the semantics of this familiar operation is so different
>>>>>>>>> (and likely so useless) when properties replace fields?
>>>>>>>>
>>>>>>>> You're solving problems that never came to life. Well... only as
>>>>>>>> syntetic examples.
>>>>>>>
>>>>>>> IMHO it's quite the contrary, a.b.c = 3 is a very simple and
>>>>>>> concrete problem that emphatically shows we haven't gotten
>>>>>>> properties up to snuff.
>>>>>>
>>>>>> Never saw this problem in C#.
>>>>>
>>>>> Of course you didn't. This is because C# doesn't have it - their
>>>>> structs
>>>>> can't define properties.
>>>>>
>>>>> Andrei
>>>>
>>>> Yes they can. And also C# shows us the solution to the problem (similar
>>>> to
>>>> what Walter proposed).
>>>>
>>>> ---
>>>> public class Bar
>>>> {
>>>>   public Foo Foo { get; set; }
>>>> }
>>>>
>>>> public struct Foo
>>>> {
>>>>   public int Property { get; set; }
>>>> }
>>>>
>>>> Bar bar = new Bar();
>>>> Foo foo = new Foo();
>>>> foo.Property = 10;
>>>> bar.Foo = foo;
>>>>
>>>> bar.Foo.Property = 20; // line 16
>>>> ---
>>>>
>>>> Error on line 16: Cannot modify the return value of 'Bar.Foo' because it
>>>> is
>>>> not a variable
>>>
>>> Booom, exactly what I said about rvalues.
>>>
>>
>> Yeh, I don't understand how any of this has anything to do with
>> properties.  It's the same question if you ask what should
>>
>>  a.b().c = 5
>>
>>  do.  It's the same issue whether you have properties or not, and
>> needs a solution whether you have properties or not.
>>
>> --bb
>
> Probably Walter means
>
> a.b.c = 3;      // b is a field
>
> works, while
>
> a.b.c = 3;      // b is a getter
>
> is a useless call. Nevertheless, if b() uses ref-return then it is
> meaningful.

Similarly, taking the address should lead to an error.

auto ptr = &a.b;  // ok if b is a field
auto ptr = &a.b; // bogus if b is a property returning a non-lvalue

--bb