Re: [boost] [parameter] Tutorial checked in

From: David Abrahams (dave_at_[hidden])
Date: 2005-07-22 09:13:02

Rob Stewart <stewart_at_[hidden]> writes:

> From: David Abrahams <dave_at_[hidden]>
>> David Abrahams <dave_at_[hidden]> writes:
>> 
>> > and the example taking a non-const reference is confusing.  I'll
>> > shuffle things around.
>> 
>> Done.
>
> Much better.
>
>    On the other hand\x{2014}as you may recall from the parameter
>    table\x{2014}``color_map`` is an \x{201C}out\x{201D}
>    parameter, so it really should be passed by non-const
>    reference.
>
> Note that "color_map" and "out" are quoted significantly
> differently with no apparent reason.

No, ``color_map`` isn't quotation; it's markup for "code font."  Look
at the HTML.

>    A keyword object has a pair of operator= overloads that ensure
>    we can pass anything by name, but when an \x{201C}out\x{201D}
>    parameter is passed positionally, that's no help: in this
>    case, core::depth_first_search would end up with a const
>    reference to the color_map and compilation will fail when
>    mutating operations are used on it.
>
> That's a long sentence and did you really mean to say "we can
> pass *anything*" (emphasis mine)?

Yep.

> Perhaps this would be better:
>
>    A keyword object has a pair of operator = overloads that
>    ensure we can pass references to const or non-const objects by
>    name, 

You don't get to pass references to anything in C++.  You pass the
object referred to by a name or expression and the callee decides
whether it goes by reference.

Also, the point is that you can pass both lvalues and rvalues, or
temporaries and non-temporaries, not that the objects can be const or
non-const.  You can do that just fine with a single templated function
taking a T& (T will be deduced as U const when the object is constant).

>    but when an "out" parameter is passed positionally,
>    that's no help.  In that case, core:;depth_first_search would
>    end up with a...."

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