How to declare an opaque type?

[1997tn]

Reading through the language manual, it's still unclear to me. C's declaration is like this, what is the equivalent in M3?

typedef struct _TREEITEM *HTREEITEM;

@mikanystrom @mikanystrom-intel

[mikanystrom-intel]

It's not exactly equivalent to what you can do in C. In particular, you can only have opaque reference types in M3. (I seem to remember there is a way to do opaque structs in C, but I haven't actually seen it done much.)

What you do is declare a type to exist such as...

TYPE T <: REFANY;

This tells any importer of the declaration (since it would normally be in an interface file) that type T exists and is a reference type. The importer can do whatever it can do with REFANY to objects of type T, plus it can NEW a T.

More usefully,

TYPE T <: OBJECT METHODS init() : T END;

means to an importer that it can for example do

VAR t : T := NEW(T).init();

and not much else (well, whatever it can do with REFANY : store it, check it for equality, etc.)

The conventional way to use this (but obviously not the only way!) is as follows:

in your interface:

Declare T as an object with whatever methods you want to be public:

INTERFACE MyType;

TYPE T <: OBJECT METHODS init() : T; publicOp(); END;

Then in the implementation:

MODULE MyType;

REVEAL T = (OBJECT METHODS init() : T; publicOp(); END) BRANDED OBJECT privateField : PfType; OVERRIDES init := Init END;

The re-typing of "OBJECT METHODS init() : T; publicOp(); END" is painful so even more conventionally:

INTERFACE MyType;

TYPE T <: Public;

Public = OBJECT METHODS init() : T; publicOp(); END;

(giving a name to "OBJECT METHODS init() : T; publicOp(); END")

and then

MODULE MyType;

REVEAL T = Public BRANDED OBJECT privateField : PfType; OVERRIDES init := Init END;

BRANDED is required here so that a client of the interface can't construct its own matching type and sneak a peek at privateField.

[1997tn]

I should put it more clear. It's about mapping C's opaque type to M3. I think I will use the type ADDRESS for HTREEITEM. HTREEITEM is an opaque type defined on winuser.h, it definition is like what I posted. Our WinUser.i3 doesn't have it.

[mikanystrom-intel]

I should put it more clear. It's about mapping C's opaque type to M3. I think I will use the type ADDRESS for HTREEITEM. HTREEITEM is an opaque type defined on winuser.h, it definition is like what I posted. Our WinUser.i3 doesn't have it.

Oh, that's the question.

Yes, ADDRESS will work like "void *".