May 06, 2012
I like to present my version of an NotNull Reference struct.
Two Bugs with "alias this" caused me a lot of trouble and one of them (the recursive infinite loop) i cannot fix by myself. Because of that you have to access the object in the struct with "obj.access" instead of "obj".
Little Example:
[code]
class A {
    // these mixin template contains the functionality to convert "A" to "Ref!(A)"
    mixin TRef!(A);

    void say_something() const {
        writeln("Hello");
    }
}

void foo(Ref!(A) ra) {
    // don't work until yet
    // ra.say();
    // so you have to write
    ra.access.say();
}

In my opinion a little annoying but i hope this bug will be fixed soon.
I think the reason for this behaviour is simple: if you have two structs/classes wich convert each to another and mark these convert methods with "alias this" the compiler found no solution and turns into an ininite loop. That could explain the "Stackoverflow" after compiling.
Or isn't that a real bug?

Well, here is the code:

[code]
template CreateMsg(string msg, string file = __FILE__, size_t line = __LINE__) {
	enum CreateMsg = msg ~  " @ " ~ file ~ " in Line " ~ to!(string)(line) ~ ".";
}

string CreateErrorMsg(string msg, string file = __FILE__, size_t line = __LINE__) {
	return msg ~  " @ " ~ file ~ " in Line " ~ to!(string)(line) ~ ".";
}

struct Ref(T : Object) {
private:
	 T _obj;

public:
	@disable
	this();

	@disable
	this(typeof(null));

	this(T obj) {
		assert(obj !is null, CreateMsg!("Object is null!"));

		this._obj = obj;
	}

	@disable
	typeof(this) opAssign()(typeof(null));

	typeof(this) opAssign(U : T)(Ref!(U) refer) {
		this._obj = refer.access;

		return this;
	}
	
	typeof(this) opAssign(U : T)(U refer) {
		assert(refer !is null, CreateMsg!("Object is null"));

		this._obj = refer;

		return this;
	}
	
	@property
	inout(T) access() inout {
		assert(this._obj !is null, CreateMsg!("Access: Object is null!"));

		return this._obj;
	}

	// DMD Bug: The compiler creates and infinite loop.
	// if uncomment, the compiler print "Stackoverflow" or "recursive expansion"
	//alias access this;
}

mixin template TRef(T : Object) {
	final Ref!(T) getRef(string file = __FILE__, size_t line = __LINE__) in {
		assert(this !is null, CreateErrorMsg("Object is null!", file, line));
	} body {
		return Ref!(T)(this);
	}
	
	final Ref!(const T) getRef(string file = __FILE__, size_t line = __LINE__) const in {
		assert(this !is null, CreateErrorMsg("Object is null!", file, line));
	} body {
		return Ref!(const T)(this);
	}
	
	// Hack for Bug 8001 (http://forum.dlang.org/thread/bug-8001-3@http.d.puremagic.com%2Fissues%2F)
	U opCast(U : Object)() inout {
		return *(cast(U*) &this);
	}

	alias getRef this;
}
[/code]

I'm using this construct already in one of my university projects and until now i found no errors. But for completeness: here the code of my test module: http://codepad.org/XGp3APCt

I would be glad for criticism and suggestions and maybe a solution/workaround for the infinite loop bug, if it's the case, that it's really a bug.
May 06, 2012

> I would be glad for criticism and suggestions and maybe a solution/workaround for the infinite loop bug, if it's the case, that it's really a bug.

First thought (after just copying the code and compiling it, not reading the
post :p), was 'where are the unittest blocks?' Then I saw you had a separate
module.

As for the infinite loop bug, consider this:

class A {
    B b;
    alias b this;
}

class B {
    A a;
    alias a this;
}

void main( ) {
    A a = 3;
}

This fails with the exact same message.

The compiler goes 'Eh, no int in A. But ooh, there's an alias this here!'
'Eh, no int in B. But ooh, there's an alias this here!'
Repeat as necessary.

Filed as 8053:
http://d.puremagic.com/issues/show_bug.cgi?id=8053
May 06, 2012
On Sunday, 6 May 2012 at 14:04:30 UTC, Simen Kjaeraas wrote:
>
>
>> I would be glad for criticism and suggestions and maybe a solution/workaround for the infinite loop bug, if it's the case, that it's really a bug.
>
> First thought (after just copying the code and compiling it, not reading the
> post :p), was 'where are the unittest blocks?' Then I saw you had a separate
> module.
>
> As for the infinite loop bug, consider this:
>
> class A {
>     B b;
>     alias b this;
> }
>
> class B {
>     A a;
>     alias a this;
> }
>
> void main( ) {
>     A a = 3;
> }
>
> This fails with the exact same message.
>
> The compiler goes 'Eh, no int in A. But ooh, there's an alias this here!'
> 'Eh, no int in B. But ooh, there's an alias this here!'
> Repeat as necessary.
>
> Filed as 8053:
> http://d.puremagic.com/issues/show_bug.cgi?id=8053

Ah, I see. Thanks for this. I'm glad when dmd 2.060 comes and fix
this.
That is really a little bit annoying. I hate the ".acces"
notation to access the object which is holding by Ref.
But I have a workaround for the problem:
use mixin Proxy!(this._obj); and it works. Now the only problem
is, that Ref!(Foo) cannot be downcast to Foo, but you have now
full object access wihtout the ".access" notation. Great step
forward i think. :)
And as far as i used it, i needn't the conversion from Ref!Foo to
Foo.
Here the changed Code with test calls below:
http://codepad.org/lfxe5plJ

I must excuse me at this point again for my bad english.

May 06, 2012
I found an error: if you have something like this

[code]
void const_ref_foo(const Ref!(Foo) rf) {
	write("Const: ");
	rf.say();
}
[/code]

and call it with

[code]
Foo f1 = new Foo();
const_ref_foo(f1);
[/code]

You get this error:

ref_test.d(101): Error: template ref_test.Ref!(Foo).Ref.Proxy!(_obj).opDispatch!
("say").opDispatch does not match any function template declaration
D:\D\dmd2\windows\bin\..\..\src\phobos\std\typecons.d(2842): Error: template ref
_test.Ref!(Foo).Ref.Proxy!(_obj).opDispatch!("say").opDispatch(this X,Args...) c
annot deduce template function from argument types !()()

To avoid this you must change line 2842 in std.typecons from

[code]
auto ref opDispatch(this X, Args...)(Args args) { return mixin("a."~name~"(args)"); }
[/code]

in

[code]
auto ref opDispatch(this X, Args...)(Args args) inout { return mixin("a."~name~"(args)"); }
[/code]

It seems to be a bug. But must i create a new bug report for something like this?
May 06, 2012
It seems that "inout" isn't enough to fix the problem.

So i extend the template to this:
http://codepad.org/SwBPoVM2

I wrote a copy of the function and now there is one with and one
without const. I thought inout would do the job alone but it
seems that it doesn't.

Now all of my code works as i expected. But because of my bad
english i would prefer that one of you report the bug and the fix.
May 07, 2012
On Sunday, 6 May 2012 at 17:58:22 UTC, Namespace wrote:
> It seems that "inout" isn't enough to fix the problem.
>
> So i extend the template to this:
> http://codepad.org/SwBPoVM2
>
> I wrote a copy of the function and now there is one with and one
> without const. I thought inout would do the job alone but it
> seems that it doesn't.
>
> Now all of my code works as i expected. But because of my bad
> english i would prefer that one of you report the bug and the fix.

I hoped for a little more interest/suggestions/criticism.
Particularly for the missed implementation of "Proxy".
May 07, 2012
On Monday, 7 May 2012 at 10:37:57 UTC, Namespace wrote:
> I hoped for a little more interest/suggestions/criticism.
> Particularly for the missed implementation of "Proxy".

I think it's great! I'm planning on trying it out on a few things and I'll let you know if I have any suggestions. I _almost_ suggested Proxy earlier, but I had never used it and I wasn't sure whether it was appropriate for the problem you were having.

Overall, I think a lot of people will get a lot of use out of it. It's something I know people have been looking for, but I think you'll have to save the link and when someone asks/talks about NonNull references/pointers you can refer them here.
May 07, 2012
> I think it's great! I'm planning on trying it out on a few things and I'll let you know if I have any suggestions. I _almost_ suggested Proxy earlier, but I had never used it and I wasn't sure whether it was appropriate for the problem you were having.

I would be glad if you will do this. :)

> Overall, I think a lot of people will get a lot of use out of it. It's something I know people have been looking for, but I think you'll have to save the link and when someone asks/talks about NonNull references/pointers you can refer them here.

That's a good idea.
May 07, 2012
OK, I tried a few things, but something that I think might need some attention: UFCS is very problematic with this.

https://gist.github.com/6198938b9c6d40a53ca1
May 07, 2012
On Monday, 7 May 2012 at 18:46:55 UTC, Chris Cain wrote:
> OK, I tried a few things, but something that I think might need some attention: UFCS is very problematic with this.
>
> https://gist.github.com/6198938b9c6d40a53ca1

I see. I think that is the same problem as "alias this". The compiler doesn't know what he has to do. I hope that functionality like this and the conversion from Foo to Ref!Foo and in reverse works in the next version. But I don't know how i can solve the problem. :/
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