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Subject: Re: [boost] review request: addition to type_traits library ofhas_operator_xxx
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-11-18 08:01:21

Nevin Liber wrote:
> On 17 November 2010 16:23, Edward Diener
> <eldiener_at_[hidden]> wrote:
> > Alternatively I could even see putting all the operator
> > traits in their own namespace of 'boost::operators',
> > dropping the 'operator_' in their name, and allowing the
> > end-user to surface them to just the 'boost' namespace via a
> > using declaration as they like.
> The problem, of course, is when you need them in a header, where using
> declarations cannot be scoped. This makes Boost.Multi-index a bit
> unwieldly, for example.
> I really don't mind if it gets spelled with namespaces instead of
> underscores, but a using declaration is not a motivation for it.


The "penalty" in a header, assuming no typedefs or using directives in class definitions or in template or inline function definitions, is two extra characters. In those contexts where a using directive is not a problem, the advantage is nine characters. I think the namespace wins pretty handily in that equation.

Obviously, one isn't too likely to use many such traits in any given inline or template function definition, so a using directive to use just one means an even worse penalty: "using namespace boost::operators; has_less_than" has 18 more characters than does "boost::has_operator_less_than." Thus, the namespace loses significantly in that comparison. Add the use of a second trait and the balance tips in favor of the namespace again.

There is, however, one additional benefit to the namespace approach: dealing with boost::has_new_operator. We can keep has_new_operator as a vestigial and even deprecated oddity and put the rest, including has_new, in the operators namespace. Thus, folks will be encouraged to use boost::operators::has_new due to its consistency with all of the rest. (One might even make has_new_operator hard to find in the documentation to further the adoption of the other.)

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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