Simple inheritance Q

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3 total messages Started by Eric Simon Tue, 27 Feb 1996 10:34

#56

Author: Eric Simon
Date: Tue, 27 Feb 1996 10:34

37 lines
1542 bytes

I have a base class, called Animal, and a derived class, called Cat.

In my main program I have:

1.    Animal *p;
2.    Cat a(args);
3.    p = &a;

Now, if I define
    class Cat:Animal{...

then I get the error in line 3:
  Can not convert Cat * to Animal *

But if I define
   class Cat:public Animal{...

everything is fine.

Can anyone tell me why?

Thanks,

Eric

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Re: Simple inheritance Q

#57

Author: Wes Cowley
Date: Wed, 28 Feb 1996 13:11

31 lines
1417 bytes

Eric Simon (erics@access2.digex.net) wrote:
: 1.    Animal *p;
: 2.    Cat a(args);
: 3.    p = &a;
: 
: Now, if I define
:     class Cat:Animal{...
: 
: then I get the error in line 3:
:   Can not convert Cat * to Animal *
: 
: But if I define
:    class Cat:public Animal{...
: 
: everything is fine.
: 
: Can anyone tell me why?

Inheritance defaults to private, which essentially means that if you use:

   class Cat : Animal { ...

code outside your Cat class has no way of knowing that Cat inherits from 
Animal and cannot use the implicit conversion from Cat* to Animal*. 

Wes

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Re: Simple inheritance Q

#58

Author: Stephen C. Marney
Date: Fri, 01 Mar 1996 13:31

37 lines
1556 bytes

Eric Simon wrote:
> 
> I have a base class, called Animal, and a derived class, called Cat.
> 
> In my main program I have:
> 
> 1.    Animal *p;
> 2.    Cat a(args);
> 3.    p = &a;
> 
> Now, if I define
>     class Cat:Animal{...
> 
> then I get the error in line 3:
>   Can not convert Cat * to Animal *
> 
> But if I define
>    class Cat:public Animal{...
> 
> everything is fine.
> 
> Can anyone tell me why?
> 	[clip]

The default access for classes is private.
p = &a; 	// asks for an implicit conversion.

"Pointers and references to derived classes can be implicitly converted to
pointers and references 
to their base classes if there is an accessible, unambiguous base class."

Key word in this case is accessible (i.e., at least, not private.)

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