Re: type constraint specification

From: dexsch (DavidS_at_[hidden])
Date: 2002-02-19 22:19:07

--- In boost_at_y..., "Noah Stein" <noah_at_v...> wrote:
> > -----Original Message-----
> > From: dexsch [mailto:DavidS_at_a...]
> > Sent: Tuesday, February 19, 2002 5:48 PM
> 
> >
> > Here is an example of a meta type definition.
> >
> > // This says ListNode types must
> > // declare a typedef value_type,
> > // have a copy constructor,
> > // and a next() method which returns a ListNode*
> > // and an equality comparison operator
> > // Remember the name ListNode is a place holder
> > // for the actual type(s) being constrained
> > typespec ListNode
> > {
> > public:
> > 	typedef value_type;
> > 	ListNode(const ListNode&)
> > 	ListNode* next();
> > 	bool operator==(const ListNode&)
> > };
> 
> 
> I like the idea here, but I would prefer a notation that does not 
introduce
> a keyword.  I would suggest using a notation similar to that used 
by pure
> virtual functions:
> 
> typename ListNode = 0
> {
> public:
> 	typedef value_type;
> 	ListNode(const ListNode&);
> 	ListNode* next();
> 	bool operator==(const ListNode&);
> };
> 
> The issues I still see with the above is that the "typedef 
value_type;" is
> different than the normal syntax for typedef.

You're right that should read
typename value_type;
// which indicates that ListNode introduces a new type name

The pure syntax (= 0) following typename would be unnecessary
typename could be a direct replacement for typespec.
This elevates the typename keyword from a place holder
to a syntactic element in it's own right and create
a metatypes name space outside the class/struct/typedef namespace(s).

dex