Is there a way to encapsulate return value into scoped?

Say I have a function that returns a new object:

X new_x(T t...) {
    //Super complex input processing
    return new X(something);
}

And I want to encapsulate the result using scoped, is that possible? Can I just do:
    return scoped!X(something).
?

If I understand correctly, the data will be blitted, but the destructor will be called, so the returned object will be in invalid state.

On Thursday, 11 June 2015 at 08:48:22 UTC, Yuxuan Shui wrote:
> Is there a way to encapsulate return value into scoped?
>
> Say I have a function that returns a new object:
>
> X new_x(T t...) {
>     //Super complex input processing
>     return new X(something);
> }
>
> And I want to encapsulate the result using scoped, is that possible? Can I just do:
>     return scoped!X(something).
> ?
>
> If I understand correctly, the data will be blitted, but the destructor will be called, so the returned object will be in invalid state.
It's even weirder than I thought, this:

import std.stdio,
       std.typecons;
class A {
	int b;
	~this() {
		writeln("Des");
	}
	this(int x) {
		b=x;
		writeln("Cons");
	}
}
auto x() {
	A x = scoped!A(10);
	writeln(x.b);
	writeln("Return x");
	return x;
}
void main() {
	auto tx = x();
	writeln("Return main");
}

Produce output:
Cons
Des
0
Return x
Return main

Which I totally don't understand.

On Thu, 11 Jun 2015 09:01:04 +0000
Yuxuan Shui via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
wrote:
> 	A x = scoped!A(10);

use auto x = scoped!A(10);

On Thursday, 11 June 2015 at 09:11:47 UTC, Daniel Kozák wrote:
>
> On Thu, 11 Jun 2015 09:01:04 +0000
> Yuxuan Shui via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
> wrote:
>> 	A x = scoped!A(10);
>
> use auto x = scoped!A(10);

Thanks!

Curious question, why doesn't compiler reject this code?

On 6/11/15 1:28 PM, Yuxuan Shui wrote:
> On Thursday, 11 June 2015 at 09:11:47 UTC, Daniel Kozák wrote:
>>
>> On Thu, 11 Jun 2015 09:01:04 +0000
>> Yuxuan Shui via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
>> wrote:
>>>     A x = scoped!A(10);
>>
>> use auto x = scoped!A(10);
>
> Thanks!
>
> Curious question, why doesn't compiler reject this code?

Because scoped!A implicitly casts to A.

-Steve

On Thursday, 11 June 2015 at 17:34:56 UTC, Steven Schveighoffer wrote:
> On 6/11/15 1:28 PM, Yuxuan Shui wrote:
>> On Thursday, 11 June 2015 at 09:11:47 UTC, Daniel Kozák wrote:
>>>
>>> On Thu, 11 Jun 2015 09:01:04 +0000
>>> Yuxuan Shui via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
>>> wrote:
>>>>    A x = scoped!A(10);
>>>
>>> use auto x = scoped!A(10);
>>
>> Thanks!
>>
>> Curious question, why doesn't compiler reject this code?
>
> Because scoped!A implicitly casts to A.
>
> -Steve

Thanks!

I just found that out myself. Learned 'alias this' in the process.

On 06/11/2015 11:43 AM, Yuxuan Shui wrote:
> On Thursday, 11 June 2015 at 17:34:56 UTC, Steven Schveighoffer wrote:
>> On 6/11/15 1:28 PM, Yuxuan Shui wrote:
>>> On Thursday, 11 June 2015 at 09:11:47 UTC, Daniel Kozák wrote:
>>>>
>>>> On Thu, 11 Jun 2015 09:01:04 +0000
>>>> Yuxuan Shui via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
>>>> wrote:
>>>>>    A x = scoped!A(10);
>>>>
>>>> use auto x = scoped!A(10);
>>>
>>> Thanks!
>>>
>>> Curious question, why doesn't compiler reject this code?
>>
>> Because scoped!A implicitly casts to A.
>>
>> -Steve
>
> Thanks!
>
> I just found that out myself. Learned 'alias this' in the process.

Shameless plug: :)

  http://ddili.org/ders/d.en/destroy.html#ix_destroy.scoped

This issue is explained at the end of that section.

Ali

On Thursday, 11 June 2015 at 19:23:49 UTC, Ali Çehreli wrote:
> On 06/11/2015 11:43 AM, Yuxuan Shui wrote:
>> On Thursday, 11 June 2015 at 17:34:56 UTC, Steven Schveighoffer wrote:
>>> On 6/11/15 1:28 PM, Yuxuan Shui wrote:
>>>> On Thursday, 11 June 2015 at 09:11:47 UTC, Daniel Kozák wrote:
>>>>>
>>>>> On Thu, 11 Jun 2015 09:01:04 +0000
>>>>> Yuxuan Shui via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com>
>>>>> wrote:
>>>>>>   A x = scoped!A(10);
>>>>>
>>>>> use auto x = scoped!A(10);
>>>>
>>>> Thanks!
>>>>
>>>> Curious question, why doesn't compiler reject this code?
>>>
>>> Because scoped!A implicitly casts to A.
>>>
>>> -Steve
>>
>> Thanks!
>>
>> I just found that out myself. Learned 'alias this' in the process.
>
> Shameless plug: :)
>
>   http://ddili.org/ders/d.en/destroy.html#ix_destroy.scoped
>
> This issue is explained at the end of that section.
>
> Ali

Can you explain more about why the destructor is not called when returning a struct?

Can't seem to find it in the document.

On 06/11/2015 12:51 PM, Yuxuan Shui wrote:
> On Thursday, 11 June 2015 at 19:23:49 UTC, Ali Çehreli wrote:

>>   http://ddili.org/ders/d.en/destroy.html#ix_destroy.scoped
>>
>> This issue is explained at the end of that section.
>>
>> Ali
>
> Can you explain more about why the destructor is not called when
> returning a struct?

Are you asking in general or specific to scoped!C?

In general, D has move semantics built into the language. It depends on whether the returned expression is an rvalue or an lvalue: rvalues are moved, lvalues are copied. And the destructor will not be called for a moved object.

About returning scoped!C, I think it works:

import std.stdio;
import std.typecons;

class C
{
    ~this()
    {
        writeln("dtor");
    }
}

auto foo()
{
    auto c = scoped!C();
    return c;
}

void main()
{
    writeln("entering scope");
    {
        writeln("calling");
        auto s = foo();
        writeln("returned");
    }
    writeln("leaving scope");
}

"dtor" is printed upon leaving the scope:

entering scope
calling
returned
dtor
leaving scope

Ali

On Thursday, 11 June 2015 at 21:38:59 UTC, Ali Çehreli wrote:
> On 06/11/2015 12:51 PM, Yuxuan Shui wrote:
> > On Thursday, 11 June 2015 at 19:23:49 UTC, Ali Çehreli wrote:
>
> >> [...]
> >
> > Can you explain more about why the destructor is not called
> when
> > returning a struct?
>
> Are you asking in general or specific to scoped!C?
>
> In general, D has move semantics built into the language. It depends on whether the returned expression is an rvalue or an lvalue: rvalues are moved, lvalues are copied. And the destructor will not be called for a moved object.
>
> About returning scoped!C, I think it works:
>
> import std.stdio;
> import std.typecons;
>
> class C
> {
>     ~this()
>     {
>         writeln("dtor");
>     }
> }
>
> auto foo()
> {
>     auto c = scoped!C();
>     return c;
> }
>
> void main()
> {
>     writeln("entering scope");
>     {
>         writeln("calling");
>         auto s = foo();
>         writeln("returned");
>     }
>     writeln("leaving scope");
> }
>
> "dtor" is printed upon leaving the scope:
>
> entering scope
> calling
> returned
> dtor
> leaving scope
>
> Ali

I just find out that the document of scoped says that "It's illegal to move a class instance even if you are sure there are no pointers to it. As such, it is illegal to move a scoped object."

So this is not a solution?