Re: Defeating (N)RVO

From:

"ThosRTanner" <ttanner2@bloomberg.net>

Newsgroups:

comp.lang.c++.moderated

Date:

22 Nov 2006 10:18:05 -0500

Message-ID:

<1164202286.423642.43890@e3g2000cwe.googlegroups.com>

peter koch larsen wrote:

Hi group

I've had the need to prevent the (named) return value optimisation from
taking place for a given class. One na?ve approach was to make my copy
constructor private and instead relying on an implicit conversion:

class cheater
{
public:
    struct local {...};
    operator local() const;
    cheater(local const &);
private:
    cheater(cheater&);
};

But this does not work - of course.
(In real life, cheater is an "error return code" and I want to verify
that its value has been examined. RVO prevents this for obvious
reasons).

I thought I'd seen articles on writing error codes that throw if you
don't access the value?

Anyway, wouldn't you get what you wanted if your copy constructor set
that the thing being copied had been accessed. Thus
class cheater
{
   private stuff stuff;
   private bool referenced;
   public:
   cheater(cheater &x) : stuff(x.stuff), referenced(false)
   {
     x.referenced = true;
  }
}

With usual warnings about throwing from destructors, etc, etc.

Forgive me if I'm missing something obvious - I've had many years to
practice and I'm getting quite good at it now.

--
      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]