Re: [boost] Finalizing a class

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2002-02-14 04:29:20

Mark Rodgers wrote:
> 
> From: "Daniel Frey" <daniel.frey_at_[hidden]>
> 
> >   template< typename T > class finalize
> >   {
> >      friend T;
> >      finalize() {}
> >   };
> 
> This has been suggested many times in the past but unfortunately
> the standard gets in the way.  See 7.1.5.3p2:
> 
>    "If the identifier resolves to a typedef-name or a template
>    type-parameter, the elaborated-type-specifier is ill-formed.
>    [Note: this implies that, within a class template with a
>    template type-parameter T, the declaration
> 
>           friend class T;
> 
>    is ill-formed. ]"

I do not have the standard, but I always thought that 'friend T;' is
legal while 'friend class T;' isn't. Are you saying that they are both
illegal? Can someone please confirm this? If yes: Is there any (legal)
way to make a template parameter a friend?

> Otherwise, yes, your finalize template would be a useful addition
> to utility.hpp, although I'd prefer a different name maybe "key"
> since it had other uses before it was made illegal.  For example,
> you could use it to restrict a function so that only a certain class
> can call it:
> 
> [example snipped]
> 
> Since only Y can construct a key<Y>, no-one else can call X::f
> (unless Y gave them the key).

I'd like to see a reasonable example where this is needed. If I have a
function that should only be called by Y, it'd be a private function of
Y or X would be a friend of Y.

Anyway, you need to think of the resulting error and what it tells the
user. 'finalize' is a good name to give the user a hint of what's wrong.
Probably a second, mostly identical class with a better name is needed
for your case as I (personal opinion follows...) can not see why it
should be called 'key'. Probably 'restrict'? But I don't think there is
a general name for all cases.

Regards, Daniel

--
Daniel Frey
aixigo AG - financial training, research and technology
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eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de