Boost logo

Boost :

From: Fernando Cacciola (fcacciola_at_[hidden])
Date: 2002-02-13 11:01:18

----- Original Message -----
From: "Geurt Vos" <G.Vos_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Wednesday, February 13, 2002 12:17 PM
Subject: RE: [boost] properties in C++

> > For instance, this straight-forward example looks great but
> > it doesn't work:
> >
> > > boost::property<
> > > boost::read<int,A,&A::get_it>,
> > > boost::write<int,A,&A::set_it>
> > > > value;
> >
> > &A::get_it can only be a constant expression if get_it is a
> > 'static' member;
> > but a static member cannot access a property value.
> >
> Why isn't &A::get_it (as non-static) a constant expression?
> I have to admit I didn't look it up in the standard, but when
> I find that every real C++ compiler(*) eats it, I am not tempted
> to look it up.
> Geurt
OK. I mess up with the wording...

I'll try it again...

What I meant to say was that a member function must be bounded to an object
in order to call it.
This 'bounding' is known as a 'closure'. A closure, which doesn't exist in
C++ as a language feature, is basically a pointer to an object plus a
pointer to a member function.
Although the address of a member function is certainly a constant
expression -so you can use it as a template parameter- the address of an
object isn't, so you can't use a closure (object*+memfun*) as a template

In your example, the object address is totally missing from the interface
and the sketch you've shown.

When you do
   A.value = 100 ;
how does the property 'value' receives &A?

My point is that there is NO WAY to store &A inside 'value' at compile time.
You can store &A::foo, but without &A this still requires runtime support.

The typical solutions that I'm aware of require the property constructor to
receive A*. Thus, 'value' cannot be default constructed and every
constructor for A requires a proper initialization of 'value'. This is the
sort of complications that usually renders the technique not too friendly
and ultimately discarded.

Fernando Cacciola
Sierra s.r.l.

Boost list run by bdawes at, gregod at, cpdaniel at, john at