Re: [boost] [GSoC] [Boost.Hana] Formal review request

Subject: Re: [boost] [GSoC] [Boost.Hana] Formal review request
From: pfultz2 (pfultz2_at_[hidden])
Date: 2014-08-03 09:56:55

> Could you post an example? Is the concept check done in the return type
> as with enable_if? 

In C++11, we can use a default template parameter, rather than the return
type, which is what the `CONCEPT_REQUIRES_` uses. So for example, we can
write:

    template<typename... T,
CONCEPT_REQUIRES_(BidirectionalIterator<T...>())>
    auto foo(T&&... xs)
    {
        ...
    }

This works as long as all the varidiac template types are to be deduced by
the
compiler. For classes this won't work:

    // ERROR
    template<typename... T,
CONCEPT_REQUIRES_(BidirectionalIterator<T...>())>
    struct foo
    {
        ...
    };

Because you can't have a default template parameter after the varidiac
template parameters in this context(you can when using speczializations).

Of course, you can't use `CONCEPT_REQUIRES_` for class specialization. In my
library, I do provide a `TICK_CLASS_REQUIRES` which can be used for class
specializations. Note that since it relies on `enable_if` it won't resolve
ambiguities like Concepts Lite will. For example:

    template<typename T>
    struct foo;

    template<typename T>
    struct foo<T, 
        TICK_CLASS_REQUIRES(RandomAccessIterator&lt;T>())>
    {
        ...
    };
    // Here we have to add a check for `RandomAccessIterator` to avoid
ambiguities
    template<typename T>
    struct foo<T, 
        TICK_CLASS_REQUIRES(BidirectionalIterator&lt;T>() &&
!RandomAccessIterator<T>())>
    {
        ...
    };

Perhaps, there is a way to this using tag dispatching, I'm not sure.

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