Copy Constructor DIP and implementation

[Nicholas Wilson]

On Wednesday, 12 September 2018 at 22:11:20 UTC, Manu wrote:
> On Wed, 12 Sep 2018 at 04:40, Dejan Lekic via 
> Digitalmars-d-announce <digitalmars-d-announce at puremagic.com> 
> wrote:
>>
>> On Tuesday, 11 September 2018 at 15:22:55 UTC, rikki 
>> cattermole wrote:
>> >
>> > Here is a question (that I don't think has been asked) why 
>> > not
>> > @copy?
>> >
>> > @copy this(ref Foo other) { }
>> >
>> > It can be read as copy constructor, which would be excellent 
>> > for helping people learn what it is doing (spec lookup).
>> >
>> > Also can we really not come up with an alternative bit of 
>> > code than the tupleof to copying wholesale? E.g. 
>> > super(other);
>>
>> I could not agree more. @implicit can mean many things, while 
>> @copy is much more specific... For what is worth I vote for 
>> @copy ! :)
>
> @implicit may be attributed to any constructor allowing it to 
> be invoked implicitly. It's the inverse of C++'s `explicit` 
> keyword. As such, @implicit is overwhelmingly useful in its own 
> right.
>
> This will address my single biggest usability complaint of D as 
> compared to C++. @implicit is super awesome, and we must 
> embrace it.

https://stackoverflow.com/a/11480555/1112970

>I have no idea why you would want to declare a copy-constructor 
>as explicit

it seems that even if we were to want to have @implicit as an 
opposite of C++'s explicit it would _always_ be present on 
copy-constructors which means that @implicit for copy 
constructors should itself be implicit.

 From what I understand of explicit in C++, if we were to have 
@implicit construction it would be used for things like

struct Foo
{
     @implicit this(int) {}
}

void useFoo(Foo f) { ... }

void main()
{
     useFoo(0); // Fine, implicitly construct auto tmp = Foo(0); 
useFoo(tmp);
}

If at some point in the future we decide that we do want to add 
@implicit construction, then we can make the copy constructor 
always @implicit. Until that point I see no need for this, 
because it is replacing postblit which is always called 
implicitly.

@\all please remember to leave feedback on the actual draft 
review on the DIP PR.