I can see that one way would let RenderMessage act upon a Message and
cause problems, but what about the other way around?  An base object
pointer is safe to operate on a derived class.  As an example:

struct Message { };
struct RenderMsg : Message { };

class Visual {
 public:
    virtual void Render1( RenderMsg *msg ) { }
    virtual void Render2( Message *msg ) { }
};

void (Visual::*method1)( Message* ) = &Visual::Render1;
void (Visual::*method2)( RenderMsg* ) = &Visual::Render2;

Neither of the last two lines compile.


Mike

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Mike Austin wrote:

> I can see that one way would let RenderMessage act upon a Message and
> cause problems, but what about the other way around?  An base object
> pointer is safe to operate on a derived class.  As an example:
>
> struct Message { };
> struct RenderMsg : Message { };
>
> class Visual {
>  public:

>     virtual void Render2( Message *msg ) { }
> };

> void (Visual::*method2)( RenderMsg* ) = &Visual::Render2;

1) C++, unlike ML (or GNU C), does _not_ support closures;
   C++ only supports simple pointer to function

2) RenderMsg* doesn't necessarily have the same representation
   as Message* (real work is done when converting a derived
   pointer to a base pointer)

--

Valentin Bonnard

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