Re: [Boost-users] [Proto] Recreating an expression

Subject: Re: [Boost-users] [Proto] Recreating an expression
From: Eric Niebler (eric_at_[hidden])
Date: 2009-07-03 13:09:07

christophe henry wrote:
> Thanks Eric for your fast answer!
> Unfortunately, this doesn't solve my problem. To make a very very long 
> discussion long, here a small
> example showing the problem from another side:
> I have an expression wrapper called mp_terminal and I define terminals, 
> but not in the documented
> way (which I suspect is the problem) 

Indeed.

> but instead deriving from them (for 
> convenience purposes), for example:
> struct True : mp_terminal<typename proto::terminal<guard_tag>::type>
> {...}

<snip>

> Then, why do most use cases work like a charm? Only unary operators seem 
> to fail.

It's because (a) True is a Proto type, but hasn't been made so by 
directly using proto::extends, and (b) Proto's unary operators are being 
a little too nit-picky about (a).

I have fixed (b) on trunk (changelist 54610). In the mean time, you can 
change your definition of True and False to use this formulation:

   struct True
     : proto::extends<
           mp_terminal<typename proto::terminal<guard_tag>::type>
         , True
         , sm_domain
       >

or the simpler and more direct

   struct True
     : proto::extends<
           typename proto::terminal<guard_tag>::type
         , True
         , sm_domain
       >

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com