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digitalmars.D.bugs - [Issue 8520] New: Simple "in"-constrained opBinaryRight in interface doesn't work

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8520

           Summary: Simple "in"-constrained opBinaryRight in interface
                    doesn't work
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: alex lycus.org


--- Comment #0 from Alex Rønne Petersen <alex lycus.org> 2012-08-08 01:54:18
CEST ---
┌─[alexrp zor] - [~/Projects/tests] - [2012-08-08 01:52:24]
└─[$] <> cat test.d
import std.stdio;

interface I
{
    int* opBinaryRight(string op : "in")(int i);
}

class C : I
{
    int* opBinaryRight(string op : "in")(int i)
    {
        return null;
    }
}

void main()
{
    I i = new C;
    int* p = 42 in i;
    writeln(p);
}
┌─[alexrp zor] - [~/Projects/tests] - [2012-08-08 01:52:26]
└─[$] <> dmd test.d
test.o:test.d:function _Dmain: error: undefined reference to
'_D4test1I30__T13opBinaryRightVAyaa2_696eZ13opBinaryRightMFiZPi'
collect2: ld returned 1 exit status
--- errorlevel 1

It's my understanding that this is supposed to work since the opBinaryRight
template is constrained specifically to "in". This bug makes operators in
interfaces pretty unusable.

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8520


Simen Kjaeraas <simen.kjaras gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |simen.kjaras gmail.com


--- Comment #1 from Simen Kjaeraas <simen.kjaras gmail.com> 2012-08-07 18:26:30
PDT ---
Your understanding is wrong - templates never go in the vtable.

The solution is to use NVI and forwarding:

interface I
{
    int* opBinaryRight_in(int i);

    int* opBinaryRight(string op : "in")(int i)
    {
        return opBinaryRight_in(i);
    }
}

class C : I
{
    int* opBinaryRight_in(int i)
    {
        return null;
    }
}

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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--- Comment #2 from Alex Rønne Petersen <alex lycus.org> 2012-08-08 12:15:19
CEST ---
(In reply to comment #1)

 Your understanding is wrong - templates never go in the vtable.
 
 The solution is to use NVI and forwarding:
 
 interface I
 {
     int* opBinaryRight_in(int i);
 
     int* opBinaryRight(string op : "in")(int i)
     {
         return opBinaryRight_in(i);
     }
 }
 
 class C : I
 {
     int* opBinaryRight_in(int i)
     {
         return null;
     }
 }


I could understand if the opBinaryRight template wasn't constrained to "in",
but it is, so I see no reason why it cannot be in the vtable since it can only
ever have one instance in a class.

The NVI solution works, but it's very ugly.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Alex Rønne Petersen <alex lycus.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |enhancement


--- Comment #3 from Alex Rønne Petersen <alex lycus.org> 2012-08-08 12:15:50
CEST ---
Changing to enhancement.

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prev sibling parent d-bugmail puremagic.com writes:
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Jonathan M Davis <jmdavisProg gmx.com> changed:

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


--- Comment #4 from Jonathan M Davis <jmdavisProg gmx.com> 2012-08-08 10:11:34
PDT ---

 I could understand if the opBinaryRight template wasn't constrained to "in",


but it is, so I see no reason why it cannot be in the vtable since it can only
ever have one instance in a class.

Because templated functions _never_ end up in the vtable. Mixed in templates
may if they're fully instantiated when mixed in (I don't remember), but a
templated function written directly in the class is _never_ virtual.

That doesn't necessarily mean that there are no situations where the language
could be enhanced to make a templated function virtual if it's actually
restricted enough (and this may be such a case), but as it stands, being a
template is enough to make it non-virtual regardless of what it does or how
constrained it is or isn't. I don't believe that even

int func()() { return 2; }

would be virtual, and that's pretty much as constrained as it gets.

Of course, if such a function _did_ get changed to virtual, then it would
always defined, whereas right now it's only defined if used, which could be a
definite negative, depending on the programmer's intent. So, there's defintely
a tradeoff here even if it's possible to make some templated functions virtual.

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