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digitalmars.D.bugs - 'cast(Base )(derived)' is not an lvalue

reply "Kris" <someidiot earthlink.dot.dot.dot.net> writes:
Here's an opApply() with a problem: I wish to iterate over a list of derived
object instances, but pass each of them to the opApply() delegate as the
public (base class) instead.

However, invoking the delegate with the derived class produces a
compile-time error as follows: "'cast(Base )(derived)' is not an lvalue"

Note that the derived class is deliberately private (to hide implementation)
so I don't want to expose it at all. I can get around the problem by
creating a temporary Base variable and manually assigning it to derived,
before passing that to the delegate. On the face of it, this appears to be
an expression evaluation issue ...

class Base
{
        abstract void foo();
}

private class Derived : Base
{
        private Derived next;

        void foo(){}
}

class Test
{
        private Derived root;

        int opApply (int delegate(inout Base) dg)
        {
                int result = 0;
                Derived derived = root;

                while (derived)
                      {
                      // compile-time error here ...
                      if ((result = dg (derived)) != 0)
                           break;
                      derived = derived.next;
                      }
                return result;
        }
}
Jun 01 2004
parent reply "Walter" <newshound digitalmars.com> writes:
The issue here is the 'inout' on the argument. Allowing such an implicit
cast for an inout opens a huge type safety hole. The caller would know the
type as derived, however, it would be Base.

"Kris" <someidiot earthlink.dot.dot.dot.net> wrote in message
news:c9ic4p$1knk$1 digitaldaemon.com...
 Here's an opApply() with a problem: I wish to iterate over a list of

 object instances, but pass each of them to the opApply() delegate as the
 public (base class) instead.

 However, invoking the delegate with the derived class produces a
 compile-time error as follows: "'cast(Base )(derived)' is not an lvalue"

 Note that the derived class is deliberately private (to hide

 so I don't want to expose it at all. I can get around the problem by
 creating a temporary Base variable and manually assigning it to derived,
 before passing that to the delegate. On the face of it, this appears to be
 an expression evaluation issue ...

 class Base
 {
         abstract void foo();
 }

 private class Derived : Base
 {
         private Derived next;

         void foo(){}
 }

 class Test
 {
         private Derived root;

         int opApply (int delegate(inout Base) dg)
         {
                 int result = 0;
                 Derived derived = root;

                 while (derived)
                       {
                       // compile-time error here ...
                       if ((result = dg (derived)) != 0)
                            break;
                       derived = derived.next;
                       }
                 return result;
         }
 }

Jun 08 2004
next sibling parent "Kris" <someidiot earthlink.dot.dot.dot.net> writes:
That may be so, but an explicit cast is not allowed either. Currently, the
only way to do this is to assign a temporary variable as described below.
That's kinda' klutzy though; certainly not very elegant <g>

Any other method to work around this?

- Kris


"Walter" <newshound digitalmars.com> wrote in message
news:ca4pg1$1tfs$3 digitaldaemon.com...
 The issue here is the 'inout' on the argument. Allowing such an implicit
 cast for an inout opens a huge type safety hole. The caller would know the
 type as derived, however, it would be Base.

 "Kris" <someidiot earthlink.dot.dot.dot.net> wrote in message
 news:c9ic4p$1knk$1 digitaldaemon.com...
 Here's an opApply() with a problem: I wish to iterate over a list of

 object instances, but pass each of them to the opApply() delegate as the
 public (base class) instead.

 However, invoking the delegate with the derived class produces a
 compile-time error as follows: "'cast(Base )(derived)' is not an lvalue"

 Note that the derived class is deliberately private (to hide

 so I don't want to expose it at all. I can get around the problem by
 creating a temporary Base variable and manually assigning it to derived,
 before passing that to the delegate. On the face of it, this appears to


 an expression evaluation issue ...

 class Base
 {
         abstract void foo();
 }

 private class Derived : Base
 {
         private Derived next;

         void foo(){}
 }

 class Test
 {
         private Derived root;

         int opApply (int delegate(inout Base) dg)
         {
                 int result = 0;
                 Derived derived = root;

                 while (derived)
                       {
                       // compile-time error here ...
                       if ((result = dg (derived)) != 0)
                            break;
                       derived = derived.next;
                       }
                 return result;
         }
 }


Jun 08 2004
prev sibling parent reply "Kris" <someidiot earthlink.dot.dot.dot.net> writes:
"Walter"  wrote:
 The issue here is the 'inout' on the argument. Allowing such an implicit
 cast for an inout opens a huge type safety hole. The caller would know the
 type as derived, however, it would be Base.

Hmmm ... I think I misunderstood your reply the first time around Walter. The caller is not expecting to see the Derived type at all (because it's completely private); only the Base class is visible. Hence the int opApply (int delegate(inout Base) dg) To restate the original problem: I'm trying to pass a Derived class as a Base implementation to the opApply() delegate. Your reply appears to indicate that I was attempting the reverse? - Kris
 "Kris" <someidiot earthlink.dot.dot.dot.net> wrote in message
 news:c9ic4p$1knk$1 digitaldaemon.com...
 Here's an opApply() with a problem: I wish to iterate over a list of

 object instances, but pass each of them to the opApply() delegate as the
 public (base class) instead.

 However, invoking the delegate with the derived class produces a
 compile-time error as follows: "'cast(Base )(derived)' is not an lvalue"

 Note that the derived class is deliberately private (to hide

 so I don't want to expose it at all. I can get around the problem by
 creating a temporary Base variable and manually assigning it to derived,
 before passing that to the delegate. On the face of it, this appears to


 an expression evaluation issue ...

 class Base
 {
         abstract void foo();
 }

 private class Derived : Base
 {
         private Derived next;

         void foo(){}
 }

 class Test
 {
         private Derived root;

         int opApply (int delegate(inout Base) dg)
         {
                 int result = 0;
                 Derived derived = root;

                 while (derived)
                       {
                       // compile-time error here ...
                       if ((result = dg (derived)) != 0)
                            break;
                       derived = derived.next;
                       }
                 return result;
         }
 }


Jun 08 2004
parent "Walter" <newshound digitalmars.com> writes:
The 'inout' means you can change the caller's reference from a Derived to
*any other class* derived from Base. This is a huge type error.

"Kris" <someidiot earthlink.dot.dot.dot.net> wrote in message
news:ca50ds$29qa$1 digitaldaemon.com...
 "Walter"  wrote:
 The issue here is the 'inout' on the argument. Allowing such an implicit
 cast for an inout opens a huge type safety hole. The caller would know


 type as derived, however, it would be Base.

Hmmm ... I think I misunderstood your reply the first time around Walter. The caller is not expecting to see the Derived type at all (because it's completely private); only the Base class is visible. Hence the int opApply (int delegate(inout Base) dg) To restate the original problem: I'm trying to pass a Derived class as a Base implementation to the opApply() delegate. Your reply appears to indicate that I was attempting the reverse? - Kris
 "Kris" <someidiot earthlink.dot.dot.dot.net> wrote in message
 news:c9ic4p$1knk$1 digitaldaemon.com...
 Here's an opApply() with a problem: I wish to iterate over a list of

 object instances, but pass each of them to the opApply() delegate as



 public (base class) instead.

 However, invoking the delegate with the derived class produces a
 compile-time error as follows: "'cast(Base )(derived)' is not an



 Note that the derived class is deliberately private (to hide

 so I don't want to expose it at all. I can get around the problem by
 creating a temporary Base variable and manually assigning it to



 before passing that to the delegate. On the face of it, this appears



 be
 an expression evaluation issue ...

 class Base
 {
         abstract void foo();
 }

 private class Derived : Base
 {
         private Derived next;

         void foo(){}
 }

 class Test
 {
         private Derived root;

         int opApply (int delegate(inout Base) dg)
         {
                 int result = 0;
                 Derived derived = root;

                 while (derived)
                       {
                       // compile-time error here ...
                       if ((result = dg (derived)) != 0)
                            break;
                       derived = derived.next;
                       }
                 return result;
         }
 }



Jun 08 2004