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

Ian McCulloch 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 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 x; // X::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