• d-bugmail puremagic.com Jun 04 2009
• d-bugmail puremagic.com Jun 05 2009
• d-bugmail puremagic.com Jun 05 2009
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• d-bugmail puremagic.com Jun 10 2009

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3051

           Summary: Passing alias to member function does not work (1/2)
           Product: D
           Version: unspecified
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: andrei metalanguage.com


This is the first of two related bug reports.

class A
{
    A next;

    void fun(alias fun)()
    {
        assert(0);
    }

    void gun()
    {
        void hun(A a)
        {
        }
        next.fun!(hun)();
    }
}

Error: template instance cannot use local 'hun' as parameter to non-global
template fun(alias fun)

This might not work due to an implementation limitation in the current dmd (the
"this" pointer and the stack frame pointer compete for the same register), but
as the next bug will show, the code doesn't work even if said limitation is
worked around.

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--- Comment #1 from Sobirari Muhomori <maxmo pochta.ru>  2009-06-05 05:20:57
PDT ---
how on earth templates can be parameterized with values unevaluatable at
compile time?

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d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3051


Steven Schveighoffer <schveiguy yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |schveiguy yahoo.com




--- Comment #2 from Steven Schveighoffer <schveiguy yahoo.com>  2009-06-05
06:17:44 PDT ---
I thought the same as you at first, but I tried this code, and it works:

    void fun(alias fx)()
    {
        fx();
    }

    void main()
    {
        int x = 0;
        void hun()
        {
             x++;
        }
        fun!(hun)();
        writefln("%s", x); // outputs 1
    }

So I think Andrei is right, it is probably a conflict of 2 this pointers
required.  The second bug is legit, as it is equivalent to what I wrote here,
but I think this one is invalid (need 2 this pointers).

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--- Comment #3 from Sobirari Muhomori <maxmo pochta.ru>  2009-06-05 07:36:54
PDT ---
huh... &fun is a delegate! Does it take stack pointer with all needed
information? Quite hackish, I would say... foreign function messing my stack...
ugh...
So it behaves as nested function, but they are said to access this pointer
through stack, not register.

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--- Comment #4 from Steven Schveighoffer <schveiguy yahoo.com>  2009-06-05
08:00:23 PDT ---
Since the template is instantiated differently for each function that it is
called with, it's entirely possible to "know" the stack frame pointer since it
will be a constant offset from the current stack pointer.  I think that is why
it can work.

In fact, I'm now unsure why that can't work in the other bug too...

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--- Comment #5 from Steven Schveighoffer <schveiguy yahoo.com>  2009-06-05
08:10:38 PDT ---
(In reply to comment #4)
 In fact, I'm now unsure why that can't work for both these bugs too...


One other thing about having two "this" pointers, how come this can work
(tested with dmd 1.043)?

void foo(delegate void() dg)
{
  dg();
}

class C
{
   int y;
   void fun()
   {
       int x = 0;
       void gun()
       { x++; y++; } // 2 this pointers needed here!
       foo(&gun);
   }
}

So probably there should be no conflict.  And the "this" pointer of the alias'd
function in the bug's example should be statically calculatable, so it doesn't
need to occupy a register.

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--- Comment #6 from Sobirari Muhomori <maxmo pochta.ru>  2009-06-10 03:30:25
PDT ---
void foo(delegate void() dg)
{
  dg();
}

class C
{
   int y;
   void fun()
   {
       int x = 0;
       auto me=this;
       void gun()
       { stack.x++; stack.me.y++; }
       foo(&gun); //closure
   }
}

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