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 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
{
   template
   <
      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  http://www.sig.com

IMPORTANT: The information contained in this email and/or its attachments is confidential. If you are not the intended recipient, please notify the sender immediately by reply and immediately delete this message and all its attachments. Any review, use, reproduction, disclosure or dissemination of this message or any attachment by an unintended recipient is strictly prohibited. Neither this message nor any attachment is intended as or should be construed as an offer, solicitation or recommendation to buy or sell any security or other financial instrument. Neither the sender, his or her employer nor any of their respective affiliates makes any warranties as to the completeness or accuracy of any of the information contained herein or that this message or any of its attachments is free of viruses.