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Subject: Re: [boost] [gsoc] Interest in BGL v2?
From: Dave Abrahams (dave_at_[hidden])
Date: 2011-04-01 11:53:40

At Thu, 31 Mar 2011 19:20:47 -0700,
Mostafa wrote:
> On Thu, 31 Mar 2011 07:02:43 -0700, Dave Abrahams <dave_at_[hidden]>
> wrote:
> > At Thu, 31 Mar 2011 08:53:02 -0500,
> > Andrew Sutton wrote:
> >>
> >> I will say that our design does take a slightly more object-oriented
> >> approach (g.size(), for example), but that hardly precludes
> >> adaptation. Adaptors would have to be written as classes, but that
> >> shouldn't be a big deal.
> >
> > I think you underestimate the cost of adaptation-via-wrapper and urge
> > you to stick with free functions unless you have a really good reason
> > to do otherwise.
> Can you elaborate?

Briefly: if you have to wrap things to get them to conform to a concept,
then they are no longer the original thing anymore. For example:

     T x, y;
     Wrapper w(y);
     swap(x,w); // <=== compilation fails

A bigger issue is the question of whether the wrapper has value
semantics or not: do you pay to copy y into w, or do you merely refer
to y from w? The former can be costly, but the latter can be wrong.

I believe other cases of this general problem will crop up repeatedly
in real code.

Dave Abrahams
BoostPro Computing

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