Re: [boost] New, powerful way to use enable_if in C++0x

Subject: Re: [boost] New, powerful way to use enable_if in C++0x
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-04-12 12:56:46

Matt Calabrese wrote:
> On Tue, Apr 12, 2011 at 9:00 AM, Stewart, Robert
> <Robert.Stewart_at_[hidden]>wrote:
>
> > I don't care for boost::enabler because "boost::" and
> > "enable" are repeated (the first "enable" is in
> > "enable_when") and a long line is made even longer.  Note
> > that the use of boost::enabler is optional: the type is void
> > *&, so, for example, "= 0" works just as well and might be
> > thought to negate enable_when.
>
> You can't initialize a reference template parameter to 0 (I
> don't believe you can for pointer parameters either for that
> matter).

You're right regarding references, of course, but a pointer to const should allow initialization from 0, right?

> If we go the route of making _when templates (which I'm not
> sure is a great idea as it doubles the number of templates in
> Boost.Enable_If),

That's a minor issue, compared to *& on the end which, as you noted previously, is unusual, and is easy to forget.  Consider the error messages if one forgets to add *&.

> the syntax that I think would be best would be:
>
> typename enable_when< is_arithmetic< T > >::type = {}

Interesting, but not nearly as clear as "= true."  OTOH, it eliminates the *& line noise.

Since I don't have access to a C++11 compiler that supports default function template arguments, I can't offer any other concrete suggestions.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer                     using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP  http://www.sig.com

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