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Subject: Re: [boost] New, powerful way to use enable_if in C++0x
From: Vicente BOTET (vicente.botet_at_[hidden])
Date: 2011-04-11 17:40:30

> Message du 11/04/11 20:16
> De : "Matt Calabrese"
> A : boost_at_[hidden]
> Copie à :
> Objet : [boost] New, powerful way to use enable_if in C++0x
> I'm not sure if there is much interest in this or if someone else discovered
> this as well, but I realized today that enable_if can be used in a new way
> in C++0x. Traditionally when working with enable_if with function templates
> you invoke it either in the return type or in an additional function
> parameter of the function. Each of these has advantages and disadvantages.
> The parameter form has the downside of augmenting the parameter list. The
> most notable problems with this approach are that it can't be used with
> overloaded operators that require a fixed number of arguments, and it can't
> be used after a variadic argument list.
> The return type use of enable_if is often preferred since it does not
> involve augmenting the parameter list, meaning that it can be used with most
> overloaded operators, but you still can't use it for conversion operations
> and you can't use it all with constructors as they have no explicit return
> type.
> However, in C++0x, there is a new way to use enable_if by altering neither
> the function parameter list nor the specification of the return type. It can
> be used with operators, constructors, variadic function templates, and even
> overloaded conversion operations (the latter of which was previously
> considered impossible). The way to do it is with C++0x default function
> template parameters.
> Rather than try to explain any further in English, the following code should
> demonstrate the approach fairly clearly (tested in GCC 4.5.2):

I like what you are trying to achieve, the fact that we could constraint the conversion operators and also the fact that the template parameter constraints are closer to the template parameters. There is yet another advantage in my opinion: the use of enable_is can be done in a uniform way for all the templates, classes or functions.

> There is only one drawback that I can think of off of the top of my head --
> you can't refer to the function parameters by name for the enable_if
> condition, whereas in C++0x if you use the return type form of enable_if
> along with the new trailing return type form of function declarations you
> can. I have never personally wanted to do this and I highly doubt that it's
> common so this is likely entirely a non-issue, plus you can always use
> std::declval anyway so you aren't actually losing any high-level
> functionality. Are there any thoughts on this or drawbacks other than what I
> mentioned? Should such use be added to the docs as supported behavior in 0x
> compilers?

I expected that this technique could not be used with variadic templates, but I see that you have used it in one of the examples.

Thanks Mat for sharing this with us,

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