From: David Abrahams (dave_at_[hidden])
Date: 2005-05-11 08:09:54
Daniel Wallin <dalwan01_at_[hidden]> writes:
> David Abrahams wrote:
>> Daniel Wallin <dalwan01_at_[hidden]> writes:
>>>David Abrahams wrote:
>>>>Daniel Wallin <dalwan01_at_[hidden]> writes:
>> Done now. I hate the name index_result, though. I was thinking maybe
>> parameter::binding<p, key, default>::type
> Sure. Maybe we should have two, so people who want to do metaprogramming
> doesn't have to remove_reference<>?
> binding<p, key, default>::type -> T | default
> binding_result<p, key, default>::type -> T& | default
I don't understand why you'd want the one without the reference
type... and -- I'm sure it's not what you want me to focus on -- but I
don't like the name "binding_result" one bit.
I'm inclined to vote "no" on this one. It isn't a flexibility that's
known to be needed.
>>>>I think we might be able to make it a bit safer by detecting when
>>>>the default is a non-const rvalue and having p[...]
>>>Can we really detect that?
>> I'm pretty sure that for all practical purposes, we can.
>> struct fu
>> template <class T>
>> int operator|(T& x) const;
>> template <class T>
>> char* operator|(T) const volatile;
>> int a = 1;
>> int x = fu() | a;
>> char* y = fu() | 999;
>> Clearly you can also use a free function with enable_if to avoid the
>> volatile interaction if you really care.
> OK, cool.
>>>There's only so much we can do. This case seems hard to get wrong for
>> Well I guess ultimately you want a macro (ick, in this case) with
>> binding<p, k, decltype(some_expression)>::type x = p[x | some_expression];
> Ultimately I would want to write:
> auto y = p[x | some_expression];
> and have p[...] detect rvalues and return by value when appropriate. ;)
Well, _ultimately_ p[...] can return an rvalue reference to
some_expression and avoid a copy ;-)
Anyway, I'm inclined that we not try to do this by-value return
thing. Your thoughts?
1. for the lazy case we should probably use result_of<F()>::type on the
compilers that support it?
2. We probably also need lazy_binding<p, k, f>::type, right?
-- Dave Abrahams Boost Consulting www.boost-consulting.com
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