Hey,

(I think I'm still a beginner)

I can't define a class member operator which allows pointer conversion between
a BASE class and one of its derived class... Here is what I've tried
(simplified) :

        class Mother
        {
          ...
          operator Child*();
          ...
        }

        Mother::Child*()
        {
          return (Child*) this;
        }

And when I use an instruction such :

        pM* Mother;
        pC* Child;

        pC = pMother;

the compiler ignores the operator I defined and still says that there's any
conversion possible between "class Mother *" and "class Child *" ...


Can you help me?

Thanks a lot in advance!





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In article <DwL25G.Jwt@news.cern.ch> Thomas.Gilbert@cern.ch (Thomas
GILBERT) writes:

|> I can't define a class member operator which allows pointer conversion between
|> a BASE class and one of its derived class...

You don't need an operator for this; it can be done by a simple cast.

|> Here is what I've tried
|> (simplified) :

|>         class Mother
|>         {
|>           ...
|>           operator Child*();
|>           ...
|>         }

|>         Mother::Child*()
|>         {
|>           return (Child*) this;
|>         }

First, the operator you have defined does not convert a pointer to
Mother (Mother*) to a pointer to Child, but rather converts a Mother
object (what is pointed to) to a Child*.  This is *NOT* the same as
converting between two pointers.

Second, I don't see any inheritance.  So it is a question of converting
between two unrelated classes.  Unless Mother really does derive from
child, your operator is a sure recipe for core dumps, and other
disagreeable behavior.  Within Mother, this points to an object of type
Mother.  Saying it is a Child will not make it one.

If Mother actually does derive from Child, then you don't need a
conversion operator to convert the pointers; the compiler will do so
automatically.  If Mother is a base class of child, you must explicitly
convert; in this case, I would recommend using dynamic_cast, if you
compiler supports it.  In general, I would suggest using the new cast
syntax whenever it is available.  If Mother is in fact unrelated to
Child (as a class), then the only cast that the compiler will allow is
reinterpret_cast; reinterpret_cast is only for very special systems
level programming, and the fact that no other cast works is a sign that
you are doing something wrong.

|> And when I use an instruction such :

|>         pM* Mother;
|>         pC* Child;

|>         pC = pMother;

I presume you mean "pChild = pMother" here, where "pChild" and "pMother"
are defined as pointers to "Child" and to "Mother", respectively.

|> the compiler ignores the operator I defined and still says that there's any
|> conversion possible between "class Mother *" and "class Child *" ...

This is because your cast operator converts a "Mother", and not a
"Mother*", and in the expression in question, you are only using the
pointers.  If Mother derives from Child, the assignment above is legal
as it stands; no further operators or whatever are necessary.  If Child
derives from Mother, then you need to write:

 pChild = dynamic_cast< Mother* >( pMother ) ;

You cannot define a new conversion operator between two pointers.  You
can define new conversions (to or from class types), but you cannot
redefine existing conversions.
--
James Kanze         Tel.: (+33) 88 14 49 00        email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs-Bourgeois, F-67000 Strasbourg, France
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