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To: Alexandre Oliva <aoliva@redhat.com>
Cc: martin@xemacs.org, gcc@gcc.gnu.org,
   Nathan Sidwell <nathan@codesourcery.com>
Subject: Re: C++ template parameter constraints
References: <15928.36654.134741.187229@gargle.gargle.HOWL>
	<orlm13mhn0.fsf@free.redhat.lsd.ic.unicamp.br>
From: Gabriel Dos Reis <gdr@integrable-solutions.net>
In-Reply-To: <orlm13mhn0.fsf@free.redhat.lsd.ic.unicamp.br>
Organization: Integrable Solutions
Date: Thu, 30 Jan 2003 17:59:00 -0000
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Alexandre Oliva <aoliva@redhat.com> writes:

| On Jan 30, 2003, Martin Buchholz <martin@xemacs.org> wrote:
| 
| > http://m17n.org/martin/writings/template-parameter-constraints.html
| 
| > (feedback desired)
| 
| /me thinks the syntax for satisfying might be something like Modula3's
| `T1 < T2' constraint, used to indicate that type T1 must be the same
| or derived from T2.  We could use something like:
| 
| template <class T : predicate<T>::satisfied> declaration;

This has been hashed several times (on the C++ committee reflectors,
at ACCU Spring conferences, with Bjarne Stroustrup, Jeremy Siek,
Fergus Henderson and many others)...  
The above doesn't scale to constraints on seperately declared
template-parameters.  The last syntaxes Bjarne Stroustrup discussed
were about

  template<class T, class U> [predicate<T, V>::holds]
    declaration;

Note where the constraints goes (in the square brackets just after the
template-parameters declaration).  The above does not conflict with
current grammar, allows for separately declared template-parameters
and works smoothly with other template meta-programming features.

[...]

| Comments?

I'll make more once I'm finished with 3.2.2 issues :-)

-- Gaby