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Subject: Re: [boost] New, powerful way to use enable_if in C++0x
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-04-12 07:50:02

Matt Calabrese wrote:
> I realized today that enable_if can be used in a new way
> in C++0x.

This is very slick, as so many have noted already. However, I wonder if you couldn't repackage your idea in, say, enable_when and disable_when (plus variants) so that the usage is slightly simpler:

class test
      class... T
      , typename boost::enable_when_c<sizeof...(T) == 10>::type = true
   test(T &&...);

IOW, enable_when_c would expose a nested type, type, of type bool that makes forming the default easy[*]. You could use a pointer or integer type and default to 0, for example, but expressing the default as "= true" just looks good and is easy to remember.

enable_when can derive from enable_if, reusing that machinery with your void *& trick, but hiding it from normal use as shown above. enable_when only needs one template argument, too, as there's no need for overriding the type of the nested type.

[*] I was wondering if I could fit "type" any more times in that sentence, especially in close proximity to the three it already contains!

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

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