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digitalmars.D.bugs - [Issue 5328] New: The addressof-expression that should be rejected is accepted

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5328

           Summary: The addressof-expression that should be rejected is
                    accepted
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Keywords: accepts-invalid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: rayerd.wiz gmail.com



PST ---
// a.d
void f(void function() m) { m(); }

class A
{
    this()
    {
        f(&A.m); // rejects-invalid => OK
    }

    /+static+/ void m() {}
}

void main()
{
    f(&A.m); // accepts-invalid => BAD!!
}

$ dmd a.d
a.d(7): Error: function a.f (void function() m) is not callable using argument
types (void delegate())
a.d(7): Error: cannot implicitly convert expression (&this.A.m) of type void
delegate() to void function()

You will get the following result if you remove A.this constructor.

$ dmd a.d
object.Error: Access Violation

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Dec 06 2010
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5328


nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nfxjfg gmail.com
            Version|D2                          |D1 & D2
         OS/Version|Windows                     |All



It's a feature, not a bug. You can take the address of methods without
providing an object reference (i.e. no this pointer, or what would be the
context member of method delegates).

The returned function pointer is simply what the class' vtable contains. The
function signature is the same as the signature of a proper delegate to the
method. It doesn't make sense to call the function pointer by itself (wrong
calling convention, this pointer is missing), but it's useful for other things.

E.g. you can use the type of the function pointer value in meta programming to
get the method's signature. That's very important for meta programming. You
could just instantiate a new object, and then analyze the delegate, but why
should you need to construct a dummy object? For the actual value of the
function pointer there are also valid uses. Think about serialization of
delegates, for example.

Note that the delegate property .funcptr return a similarly non-sensical
function pointer. The context is missing and the calling convention is
incompatible. If you call it, you get an access violation as well.

What is happening here is that the language "designers" just didn't feel like
introducing a dedicated separate function type for this purpose. Instead, they
reused the "normal" function pointer type, even if calling it makes no sense.
Basically it's a quick language hack that endangers type-safety and confuses
users.

Btw. the code inside A.this is not accepted, because the "A." from "&A.m" just
qualifies the member "m". Inside the ctor it's exactly the same as writing
"&m". And &m is a delegate, not a function. So everything is alright. Though
this slight syntactic ambiguity is another hint that D as a language is
actually quite unorthogonal and full of special cases. Maybe "&typeof(this).m"
in the ctor would do the same as the &A.m in main()?

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au





 It's a feature, not a bug. You can take the address of methods without
 providing an object reference (i.e. no this pointer, or what would be the
 context member of method delegates).




 What is happening here is that the language "designers" just didn't feel like
 introducing a dedicated separate function type for this purpose. Instead, they
 reused the "normal" function pointer type, even if calling it makes no sense.
 Basically it's a quick language hack that endangers type-safety and confuses
 users.


Are you sure this is an intentional design decision, and not just an oversight?
Was it discussed by Walter and Andrei in the newsgroup?

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prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5328


yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |yebblies gmail.com
         Resolution|                            |DUPLICATE



*** This issue has been marked as a duplicate of issue 3720 ***

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Jun 18 2011