Move Constructors - Converting Lvalues to Rvalues

[Manu]

On Thu, 3 Oct 2024 at 10:06, Timon Gehr via Digitalmars-d <
digitalmars-d at puremagic.com> wrote:

> On 10/3/24 01:07, Manu wrote:
> > On Thu, 3 Oct 2024 at 03:21, Walter Bright via Digitalmars-d
> > <digitalmars-d at puremagic.com <mailto:digitalmars-d at puremagic.com>>
> wrote:
> >
> >     On 10/1/2024 1:15 PM, Timon Gehr wrote:
> >      > I guess the new implementation you have in mind is something like
> >     the following?
> >      >
> >      > ```d
> >      > auto move(T)(ref T arg)=>__rvalue(arg);
> >      > ```
> >
> >     Not exactly. __rvalue would also convert an rvalue to an rvalue.
> >
> >
> > -preview=rvaluerefparams addresses this; it allows an rvalue to be
> > supplied to the lvalue there. It's actually an essential mechanic to
> > this whole thing, because it will allow the appropriate selection of
> > copy/move overloads where a type may define either one, or both. If a
> > copy and/or move constructor exists, it needs to select the proper one,
> > and the -preview handles that properly as is.
>
> `-preview=rvaluerefparam` allows you to treat an rvalue as an lvalue
> implicitly in some contexts.
>
> `__rvalue` allows you to treat an lvalue explicitly as an rvalue, so it
> will be moved.
>
> I fail to see how those are related.
>

__rvalue() needs to be callable with an lvalue or an rvalue.
That's the only specific interaction on this matter, but beyond that with
respect to move semantics in practice; in your application, you will
encounter types with copy and/or move constructors/assign operators,
general overloads, etc... you need to be able to pass the values that you
have and your code needs to work without friction.
If there is a copy constructor and no move constructor for instance (very
common situation) and you pass an rvalue, the code needs to compile. Later
if you add a move constructor, it will automatically select that as the
appropriate choice.
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