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Subject: Re: [boost] [variant] match()
From: TONGARI J (tongari95_at_[hidden])
Date: 2015-01-08 04:44:27

2015-01-08 17:39 GMT+08:00 TONGARI J <tongari95_at_[hidden]>:

> 2015-01-08 3:14 GMT+08:00 Matt Calabrese <rivorus_at_[hidden]>:
>> On Wed, Jan 7, 2015 at 10:48 AM, Peter Dimov <lists_at_[hidden]> wrote:
>> > Matt Calabrese wrote:
>> >
>> >> The main limitation of this approach is that overloads must copy/move
>> the
>> >> passed-in function objects. I.E. there is no known tie_overloads that
>> would
>> >> be able to exhibit the same behavior.
>> >>
>> >
>> > Hmm. If you had a reference_wrapper<F> which SFINAEd its operator() on
>> > whether F::operator() compiles, could you not then pack those reference
>> > wrappers into an overloads object?
>> >
>> Unfortunately, no, because at that point you've "flattened" the operator()
>> to having all template parameters. Overload resolution would no longer be
>> able to produce better or worse matches when one or more of the passed-in
>> function objects are callable. For instance, if you pass in tie_overloads(
>> [](int a) {}, [](auto a) ) to apply_visitor, if the variant contained an
>> "int" then the function call would actually be ambiguous rather than
>> preferring the int overload since both overloads are callable and now just
>> have template parameters as arguments. I've put a lot of thought (on/off
>> for years) into trying to come up with a tie_overloads that actually works
>> precisely as an overload set and I'm reasonably certain that it cannot be
>> done, but I am unable to say that for certain.
>> On the plus side, I've never actually found the lack of a tie_overloads to
>> be a problem, since in times that I've personally wanted it it's been easy
>> to manually make a reference-semantic function object at the call-site via
>> lambdas. The main difficulty is that this just can't be done automatically
>> inside of generic code, so the value-semantics of the function object
>> passing sometimes bleeds out to the user a little bit in generic code (the
>> user just needs to be aware the function objects are copied/moved in). In
>> practice this isn't much of a problem since standard library algorithms
>> take function objects by value anyway and so people are familiar with
>> those
>> semantics.
> At least, for functors that don't overload themselves (e.g. lambdas), you
> can do something like this:
> That said, I don't have such a need myself though, just for fun :p


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