Re: [boost] Suggested change to type_traits/function_traits

Subject: Re: [boost] Suggested change to type_traits/function_traits
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-07-14 07:44:20

Joseph Garvin wrote:
>
> I only just now subscribed to this list so forgive me if this has been
> suggested and rejected before.

There are searchable archives.

> Currently function_traits<F> exposes the argument types for F as
> individual typedefs, arg1_type, arg2_type, etc. I think this could be
> made more powerful with two changes:
>
> 1. First, change it to be 0 indexed, arg0_type should be first. This
> is more consistent with the rest of boost and C++ in general.

That would be a breaking change, so untenable.

> 2. Have an inner arg struct, templated over an integer indicating
> which argument you want. So instead of:
>
> typedef some_type arg0_type;
> typedef some_other_type arg1_type;
> //...
>
> You would have:
>
> template<int i>
> struct arg
>
> template<>
> struct arg<0>
> {
>    typedef some_type type;
> }
>
> template<>
> struct arg<1>
> {
>    typedef some_other_type type;
> }
>
> // etc.
>
> I think this would make function_traits more usable for
> metaprogramming. As is, to access the different argN_types you have to
> resort to using boost preprocessor. For backwards compatibility the
> old interface could co-exist with the new one.

Adding the new interface is not unreasonable.  Whether the two should have different starting indices is arguable.  std::get<0>(t) returns the first element of the tuple t, for example, but I think arguments are typically addressed with one-based indices, and using different starting indices would be potentially confusing in the same trait.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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