[Boost-users] Re: metaprogramming exercise 2-0

From: David Abrahams (dave_at_[hidden])
Date: 2005-02-16 11:38:20

Ian McCulloch <ianmcc_at_[hidden]> writes:

> Chris Goller wrote:
>
>> David-
>> 
>> First, thanks for the response.  BTW, I really like the book.
>> 
>> Okay, I understand what you are saying about add_const and how it works.
>> 
>> My next question is why was it implemented in that way?  In other words,
>> was there a specific reason not to make a special case
>> for references such that add_const<int&> would be int const &?
>
> FWIW, you will soon be able to get that effect without a metafunction.  
> from http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#106
> [example]
> int i;
> typedef int& RI;
> RI& r = i;          // r has the type int&
> const RI& r = i;    // r has the type const int&
> [/example]
>
> I'm not sure I like it, because a 'RI const' is 'int&', but a reference to a
> 'RI const' is 'int const&'.  But maybe its useful that way.

Oh, that's very disturbing.  What is the rationale for moving the
const inside the reference?

Ah, I see it in N1245:

  template<class T> class X { f(const T&) ; /* ... */ };

  X<int&> x;  // X<int&>::f has the argument type const int&

I'm not convinced that special rule is worth the breath and text
needed to explain it.  With any well-behaved class X, if f was written
to work on a "const X&" argument, it will work equally well and
correctly on a "X&" argument.

It's probably too late for objections now, though.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com