DIP53 and DIP49 (ver2) - New definitions for qualified constructors and postblits

[Timon Gehr]

On 12/19/2013 02:27 AM, Kenji Hara wrote:
> 2013/12/19 Joseph Rushton Wakeling <joseph.wakeling at webdrake.net
> <mailto:joseph.wakeling at webdrake.net>>
>
>     And you have this(...) const (aka "unique"), which can be used to
>     construct both immutable and mutable instances.
>
>     It seems to me that this is a very natural relation to the existing
>     interpretation of mutable, immutable and const variables by the
>     language, and e.g. the way that immutable, mutable and const
>     function parameters are addressed.
>
>
> That's right. When 'const' constructor/postblit is called, the
> constructing object qualifier is either mutable or immutable, even if it
> is not visible inside const constructor/postblit.
> ...


That is how to treat a 'wildcard', and not a supertype. The semantics of 
the new const constructor are the same as for the existing inout 
constructor.


> ----
>
> I tweaked the descriptions in DIPs, about Const Constructor/Postblit
> concept.
>
> - I mostly removed words "unique constructor" and "unique postblit",
> because they would just increase reader's confusion.
>    DIPs can describe intrinsic concepts without these words.
>
> - I added following sections in DIPs:
>
>    Why 'const' postblit will called to copy arbitrary qualified object?
> http://wiki.dlang.org/DIP49#Why_.27const.27_postblit_will_called_to_copy_arbitrary_qualified_object.3F
>
>    Why 'const' constructor will be called to create arbitrary qualified
> object?
> http://wiki.dlang.org/DIP53#Why_.27const.27_constructor_will_be_called_to_create_arbitrary_qualified_object.3F
>
>    There's no violation against the widely known "const method" concept.
>
> Kenji Hara

Well, a constructor can be thought of as yielding a result. A const 
method returning 'this' cannot return an 'immutable' object. I do 
understand your rationale. What I am saying is that the DIP adds a 
wildcard meaning to 'const' that was not previously there, and I 
consider this to be more of a weakness than a strength.