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digitalmars.D.bugs - [Issue 5217] New: Permit static+abstract

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5217

           Summary: Permit static+abstract
           Product: D
           Version: D2
          Platform: All
        OS/Version: Windows
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: ah08010-d yahoo.com


--- Comment #0 from Austin Hastings <ah08010-d yahoo.com> 2010-11-14 12:41:13
PST ---
The abstract keyword currently cannot be used with "non virtual" functions.
IMO, this is an error. 

Non-virtual simply means that only one layer of implementation will be possible
- since a vtable would be required to disambiguate multiple layers. It does not
mean that the implementation is automatically obvious, nor does it mean that
multiple layers of interfaces cannot be composed into the single layer of
implementation.

Factory methods, for example, are usually static. And in the presence of
templates, the expectation should be for a high degree of fan-out, but not much
depth. (That is, a template can be realized with many type parameters, but is
less likely to be stacked up in a deep inheritance tree, although it does
happen.)

Thus, I would very much like to be able to require a static method, via the
abstract keyword.

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Jonathan M Davis <jmdavisProg gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jmdavisProg gmx.com


--- Comment #1 from Jonathan M Davis <jmdavisProg gmx.com> 2010-11-14 16:21:20
PST ---
I'm afraid that you're going to have to be more clear in what you mean.

abstract indicates that a function does not have a body and that derived types
must give an implementation if they are to be instantiatable.

static functions have nothing to do with the virtual table, and so they don't
care about base or derived types. They're essentially just namespaced within a
class or struct. abstract has no meaning for them.

Templates _always_ result in code. What does it matter if that code is a static
function, within a class, a struct, or whatever? An abstract template makes no
sense. Even if you're instantiating a template within a base class, it must be
fully instantiated within that class regardless of whether any classes will
ever be derived from that class.

What are you trying to do here? You need to be clearer, because as it stands,
what you're asking for makes no sense.

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--- Comment #2 from Austin Hastings <ah08010-d yahoo.com> 2010-11-16 16:00:37
PST ---
What I'm asking for is "abstract static". That is, a syntax for imposing a
requirement that a class (or struct) provide a static member function before it
can be instantiated.

In my particular use case, I've got a template and would like to impose a
requirement for a factory method on any realizations of the template.

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--- Comment #3 from Jonathan M Davis <jmdavisProg gmx.com> 2010-11-16 17:01:57
PST ---
Do template constraints not work for you in this case?

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--- Comment #4 from Austin Hastings <ah08010-d yahoo.com> 2010-11-16 18:53:00
PST ---
Template constraints do not work. The problem is not "The input type must
behave in such-and-such manner," but rather "Here is a template for most, but
not all, of the class: you must provide two static methods that I have marked
as abstract."

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--- Comment #5 from Jonathan M Davis <jmdavisProg gmx.com> 2010-11-16 19:42:08
PST ---
Hmm. I think that I get what you're getting at, though it is a bit odd.
Certainly, requiring static functions is something different. I've never seen a
language do that. I'd still think that you could do it with a static if or
template constraint, but without seeing the code, I'm not sure that it's clear
enough exactly what you're trying to for me to give any kind of real suggestion
about how to g about it with D as it is.

In general, I don't think that I support the idea of enforcing that a type have
a particular set of static functions, but it does seem like a perfectly
legitimate enhancement request. I'm inclined to think that you should probably
be doing things a different way. But I'm not familiar with your code, so I
obviously can't say for sure.

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