I didn't get any responses in D.learn for this question, so I decided maybe it was advanced enough for this group.

I was hoping I could do this:

typedef void* CFTypeRef;
typedef CFTypeRef CFStringRef;

class DCFObject
{
  CFTypeRef getRef() {}
}

class DString : DCFObject
{
  CFStringRef getRef() {}
}

But the compiler gives me:

src/d/darbon/corefoundation/DString.d:32: function darbon.corefoundation.DString.DString.getRef of type CFStringRef() overrides but is not covariant with darbon.corefoundation.DCFObject.DCFObject.getRef of type CFTypeRef()

From what I gather in the docs, CFStringRef is derived from CFTypeRef, so this should be acceptable. Am I missing something?

TIA,
Rick

Rick Mann wrote:
> I didn't get any responses in D.learn for this question, so I decided maybe it was advanced enough for this group.
> 
> I was hoping I could do this:
> 
> typedef void* CFTypeRef;
> typedef CFTypeRef CFStringRef;
> 
> class DCFObject
> {
>   CFTypeRef getRef() {}
> }
> 
> class DString : DCFObject
> {
>   CFStringRef getRef() {}
> }
> 
> But the compiler gives me:
> 
> src/d/darbon/corefoundation/DString.d:32: function darbon.corefoundation.DString.DString.getRef of type CFStringRef() overrides but is not covariant with darbon.corefoundation.DCFObject.DCFObject.getRef of type CFTypeRef()
> 
> From what I gather in the docs, CFStringRef is derived from CFTypeRef, so this should be acceptable. Am I missing something?

That's not the message I get from reading the docs.  What I see:
 "Strong types are semantically a distinct type to the type checking system, for function overloading, and for the debugger."
 (http://www.digitalmars.com/d/declaration.html)

Basically CFStringRef becomes something completely new and different from CTypeRef as far as the compiler is concerned.

What are you reading that makes you think CFStringRef is "derived" from CFTypeRef?

--bb

Bill Baxter Wrote:

> Rick Mann wrote:
>> I didn't get any responses in D.learn for this question, so I decided maybe it was advanced enough for this group.
>> 
>> I was hoping I could do this:
>> 
>> typedef void* CFTypeRef;
>> typedef CFTypeRef CFStringRef;
<snip>
>> From what I gather in the docs, CFStringRef is derived from CFTypeRef, so this should be acceptable.  Am I missing something?
> 
> That's not the message I get from reading the docs.  What I see: "Strong types are semantically a distinct type to the type checking system, for function overloading, and for the debugger." (http://www.digitalmars.com/d/declaration.html)

Are you taking this to mean distinct from each other or distinct from their underlying types?  If the latter, this is much the same a class being different from its base class.

> Basically CFStringRef becomes something completely new and different from CTypeRef as far as the compiler is concerned.

What are you reading that makes you think it's _completely_ different?  This compiles:

----------
typedef void* CFTypeRef;
typedef CFTypeRef CFStringRef;

void func(CFTypeRef p) {}

void main() {
    CFStringRef s;
    func(s);
}
----------

> What are you reading that makes you think CFStringRef is "derived" from CFTypeRef?

Probably that typedefs implicitly convert to their underlying types.  Moreover, typedefs behave in a number of respects very similarly to enums, for which the notation

    enum Enum : int

suggests type derivation as with classes.

Stewart.