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digitalmars.D.bugs - [Issue 5123] New: Cannot assign null to a class with 'alias this'

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5123

           Summary: Cannot assign null to a class with 'alias this'
           Product: D
           Version: D2
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: osa8aso gmail.com



If class has 'alias this' for a struct member, attempt to assign null to a
class instance fails:
------
struct Foo {}
class Bar {
    Foo foo_;
    alias foo_ this;
}
void main() {
    Bar a;
    a = null;
}
------
aliasthis1.d(8): Error: cannot implicitly convert expression (null) of type
void* to Foo

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Oct 26 2010
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5123


bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs eml.cc



Here alias this is doing its work of "let's pretend" a Bar instance is a Foo
instance. To solve this problem you may need something more refined than alias
this, something that allows a partial aliasing. I don't think it's an easy
thing to do.

A related program shows a different outcome:

struct Foo {}
class Bar {
    Foo foo_;
    alias foo_ this;
}
void main() {
    Bar a;
    //a = null;
    Foo f;
    a = f; // Access violation!
}

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Oct 26 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5123





 A related program shows a different outcome:
     Bar a;
     Foo f;
     a = f; // Access violation!


This is expected. 'a' is an uninitialized, and attempt to assign to a Foo part
of null instance causes access violation. This is no different from

  Bar a;
  a.foo_ = Foo();

"Partial" assignment of Foo to a valid Bar instance is fine and works as
expected:
-----
struct Foo { int x; }
class Bar {
    Foo foo_;
    alias foo_ this;
    int y;
}
void main() {
    Bar a = new Bar;
    a.x = 1;
    a.y = 42;
    assert( a.foo_.x == 1 && a.y == 42 );
    a = Foo( 2 );
    assert( a.foo_.x == 2 && a.y == 42 );
}
-----

My problem is only with special treatment of null: I want to nullify local
reference to a Bar instance, not change Bar or Foo inside it.

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Oct 26 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5123






 This is expected. 'a' is an uninitialized, and attempt to assign to a Foo part
 of null instance causes access violation. This is no different from
 
   Bar a;
   a.foo_ = Foo();


Right, sorry for the noise.

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Oct 26 2010
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5123


Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED



This issue was already fixed by bug 2943.

And this was related to bug 6630.

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Oct 23 2011