As discussed deep in the thread "Is there any reason why arithmetic operation
on shorts and bytes return int?"[1], D currently does not support this behavior:

struct bbyte {
    byte b;
    alias b this;
}

void foo(bbyte b) {}

void baz() {
    byte b;
    foo(b); // Cannot implicitly convert byte to bbyte.
}

bbyte baz( ) {
    byte b;
    return b; // Cannot implicitly convert byte to bbyte.
}

Kenji Hara points out, and I myself thought, that this was a deliberate design choice. Walter's post[2] in the aforementioned thread indicates (but does not make clear-cut) that he also thinks this implicit construction is desirable.

A previous discussion with Andrei[3] about implicit conversion of nameless tuples to named tuples resulted in a bug report[4], and it is clear that his view also supports some such form of implicit conversion.

A long time ago, when dinosaurs roamed the earth, walterandrei.pdf[5] suggested that opImplicitCastTo and opImplicitCastFrom take care of this conversion. Is anything like this still on the drawing board? Should alias this do it? How do we deal with cases were one field is alias this'd, and other fields are not?


[1]: http://forum.dlang.org/thread/mailman.2599.1355228650.5162.digitalmars-d@puremagic.com?page=3#post-mailman.2625.1355305365.5162.digitalmars-d:40puremagic.com

[2]: http://forum.dlang.org/thread/mailman.2599.1355228650.5162.digitalmars-d@puremagic.com?page=3#post-kaatc0:24hgn:242:40digitalmars.com

[3]: http://forum.dlang.org/thread/sedknwtlaefrxuflnbez@forum.dlang.org?page=8#postjul0qv:242l9d:241:40digitalmars.com

[4]: http://d.puremagic.com/issues/show_bug.cgi?id=8570

[5]: http://s3.amazonaws.com/dconf2007/WalterAndrei.pdf


-- 
Simen

On Thursday, 13 December 2012 at 14:25:27 UTC, Simen Kjaeraas wrote:
>     foo(b); // Cannot implicitly convert byte to bbyte.

I think the way it is now is correct for alias this.. it is kinda like implicitly casting to a base class. That's correct, but going to a superclass isn't necessarily right. You might only want it to be one way. Suppose you had something like this:

struct SafeText {
  string text;
  alias this text;
}

Safe text should implicitly convert to plain string text, but it shouldn't go the other way automatically.

> A long time ago, when dinosaurs roamed the earth, walterandrei.pdf[5] suggested that opImplicitCastTo and opImplicitCastFrom take care of this conversion.

A separate function or facility for implicit cast to might be ok though.

On 2012-38-13 15:12, Adam D. Ruppe <destructionator@gmail.com> wrote:

> On Thursday, 13 December 2012 at 14:25:27 UTC, Simen Kjaeraas wrote:
>>     foo(b); // Cannot implicitly convert byte to bbyte.
>
> I think the way it is now is correct for alias this.. it is kinda like implicitly casting to a base class. That's correct, but going to a superclass isn't necessarily right. You might only want it to be one way. Suppose you had something like this:
>
> struct SafeText {
>    string text;
>    alias this text;
> }
>
> Safe text should implicitly convert to plain string text, but it shouldn't go the other way automatically.

But this is easily solved:

struct SafeText {
    string text;
    string get( ) @property {
        return text;
    }
    alias get this;
}

This is just as explicit as what you propose, and possible with what we
currently have.


>> A long time ago, when dinosaurs roamed the earth, walterandrei.pdf[5] suggested that opImplicitCastTo and opImplicitCastFrom take care of this conversion.
>
> A separate function or facility for implicit cast to might be ok though.


-- 
Simen

On Thursday, 13 December 2012 at 16:15:16 UTC, Simen Kjaeraas wrote:
> But this is easily solved:

Hmmm, indeed. This might just work then...

12/13/2012 6:25 PM, Simen Kjaeraas пишет:
> As discussed deep in the thread "Is there any reason why arithmetic
> operation
> on shorts and bytes return int?"[1], D currently does not support this
> behavior:
>
> struct bbyte {
>      byte b;
>      alias b this;
> }
>
> void foo(bbyte b) {}
>
> void baz() {
>      byte b;
>      foo(b); // Cannot implicitly convert byte to bbyte.
> }
>
> bbyte baz( ) {
>      byte b;
>      return b; // Cannot implicitly convert byte to bbyte.
> }
>
> Kenji Hara points out, and I myself thought, that this was a deliberate
> design choice. Walter's post[2] in the aforementioned thread indicates
> (but does not make clear-cut) that he also thinks this implicit
> construction is desirable.
>
> A previous discussion with Andrei[3] about implicit conversion of
> nameless tuples to named tuples resulted in a bug report[4], and it is
> clear that his view also supports some such form of implicit conversion.
>

User defined implicit conversion should be possible IMO. The reason is a proverbial "user-defined types that behave like built-ins" promise that D (and many other languages do) but it doesn't hold it yet

Basically we have "explicitly constructed from", and "implicitly converts to" but there is yet not provided "implicitly converts from" to put it into the seamless interaction with built-ins.

Being a checkable opt-in and staying separate from ctor it won't be as readily susceptible to abuse unlike implicit construction in C++ _by default_.

> A long time ago, when dinosaurs roamed the earth, walterandrei.pdf[5]
> suggested that opImplicitCastTo and opImplicitCastFrom take care of this
> conversion. Is anything like this still on the drawing board? Should
> alias this do it? How do we deal with cases were one field is alias
> this'd, and other fields are not?
>



-- 
Dmitry Olshansky