struct stuff {
	private Exception delegate() exceptionBuilder = delegate Exception() {
		return new Exception("foobar");
	};
}

The following piece of code trigger a compiler error : delegate module.stuff.__dgliteral1 function literals cannot be class members

Why is that ? Is it a bug or a feature ?

On Tuesday, 21 February 2012 at 15:22:15 UTC, deadalnix wrote:
> struct stuff {
> 	private Exception delegate() exceptionBuilder = delegate Exception() {
> 		return new Exception("foobar");
> 	};
> }
>
> The following piece of code trigger a compiler error : delegate module.stuff.__dgliteral1 function literals cannot be class members
>
> Why is that ? Is it a bug or a feature ?

Delegates contain a context pointer. Your delegate literal has no context.

You can't initialize it with the address of a method, either. For struct methods, the context pointer is a pointer to the structure. You can't have a .init that contains a pointer to an instance. You probably want to use a function literal.

Le 21/02/2012 16:32, Vladimir Panteleev a écrit :
> On Tuesday, 21 February 2012 at 15:22:15 UTC, deadalnix wrote:
>> struct stuff {
>> private Exception delegate() exceptionBuilder = delegate Exception() {
>> return new Exception("foobar");
>> };
>> }
>>
>> The following piece of code trigger a compiler error : delegate
>> module.stuff.__dgliteral1 function literals cannot be class members
>>
>> Why is that ? Is it a bug or a feature ?
>
> Delegates contain a context pointer. Your delegate literal has no context.
>
> You can't initialize it with the address of a method, either. For struct
> methods, the context pointer is a pointer to the structure. You can't
> have a .init that contains a pointer to an instance. You probably want
> to use a function literal.

It doesn't work with function either.

But I need delegate here. The default one doesn't require a context, but isn't it possible to pass null as a context, as none is needed ? This value can be changer later, and definitively require to be a delegate.

A possible workaround is to initialize the delegate
in the object's constructor.

Le 21/02/2012 16:48, Adam D. Ruppe a écrit :
> A possible workaround is to initialize the delegate
> in the object's constructor.

It is a struct. And struct don't have default constructor. It lead to very segfault prone code (I did try that).

21.02.2012 17:24, deadalnix пишет:
> struct stuff {
> private Exception delegate() exceptionBuilder = delegate Exception() {
> return new Exception("foobar");
> };
> }
>
> The following piece of code trigger a compiler error : delegate module.stuff.__dgliteral1 function literals cannot be class members
>
> Why is that ? Is it a bug or a feature ?
>

The compiler expects member initializers to be known at compile-time. Since delegate carries closure, and closure is a run-time phenomena, you cannot put it there. That's how I understand it, and I might be wrong. Anyway, something like this is possible as a workaround:

struct Foo {
private Exception dg() {
if( m_Dg ) return m_Dg();
return new Exception( "foobar" );
}

private Exception delegate() m_Dg = null;
}

On 2012-02-21 16:55, deadalnix wrote:
> Le 21/02/2012 16:48, Adam D. Ruppe a écrit :
>> A possible workaround is to initialize the delegate
>> in the object's constructor.
>
> It is a struct. And struct don't have default constructor. It lead to
> very segfault prone code (I did try that).

You can implement a static opCall and use that instead of the constructor.

-- 
/Jacob Carlborg

Le 21/02/2012 17:30, Mantis a écrit :
> 21.02.2012 17:24, deadalnix пишет:
>> struct stuff {
>> private Exception delegate() exceptionBuilder = delegate Exception() {
>> return new Exception("foobar");
>> };
>> }
>>
>> The following piece of code trigger a compiler error : delegate
>> module.stuff.__dgliteral1 function literals cannot be class members
>>
>> Why is that ? Is it a bug or a feature ?
>>
>
> The compiler expects member initializers to be known at compile-time.
> Since delegate carries closure, and closure is a run-time phenomena, you
> cannot put it there. That's how I understand it, and I might be wrong.
> Anyway, something like this is possible as a workaround:
>
> struct Foo {
> private Exception dg() {
> if( m_Dg ) return m_Dg();
> return new Exception( "foobar" );
> }
>
> private Exception delegate() m_Dg = null;
> }

I think this the best solution after all.

But still I think the original code should be an error only if it use data out of the delegate scope. If it doesn't, frame pointer doesn't matter and null can be passed.

On Tue, Feb 21, 2012 at 08:01:18PM +0100, deadalnix wrote: [...]
> But still I think the original code should be an error only if it use data out of the delegate scope. If it doesn't, frame pointer doesn't matter and null can be passed.

You could file an enhancement request, if one hasn't already been filed.


T

-- 
I'm still trying to find a pun for "punishment"...

On Tuesday, 21 February 2012 at 15:41:58 UTC, deadalnix wrote:
> Le 21/02/2012 16:32, Vladimir Panteleev a écrit :
>> On Tuesday, 21 February 2012 at 15:22:15 UTC, deadalnix wrote:
>>> struct stuff {
>>> private Exception delegate() exceptionBuilder = delegate Exception() {
>>> return new Exception("foobar");
>>> };
>>> }
>>>
>>> The following piece of code trigger a compiler error : delegate
>>> module.stuff.__dgliteral1 function literals cannot be class members
>>>
>>> Why is that ? Is it a bug or a feature ?
>>
>> Delegates contain a context pointer. Your delegate literal has no context.
>>
>> You can't initialize it with the address of a method, either. For struct
>> methods, the context pointer is a pointer to the structure. You can't
>> have a .init that contains a pointer to an instance. You probably want
>> to use a function literal.
>
> It doesn't work with function either.
>
> But I need delegate here. The default one doesn't require a context, but isn't it possible to pass null as a context, as none is needed ? This value can be changer later, and definitively require to be a delegate.

struct stuff {
	private Exception function() exceptionBuilder =
		&defaultExceptionBuilder;
	
	private static Exception defaultExceptionBuilder() {
		return new Exception("foobar");
	};
}