Re: indirect / index sorting with the STL

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Fri, 20 May 2011 19:24:32 CST
Message-ID:
<ir50it$peh$1@dont-email.me>
On 2011-05-20 00:41, Joshua Lehrer wrote:

On May 18, 4:49 pm, Kai-Uwe Bux<jkherci...@gmx.net> wrote:

Joshua Lehrer wrote:

I was challenged to come up with a way to use existing STL algorithms
to sort one vector but given sorting precedence in a separate vector.
An additional constraint is that only O(1) constant external storage
may be used.


to be modified and the first should stay the same. The following does sort
_both_ vectors based upon the comparison from the first. It is faking the
reference that you need as the return type for operator*.


Kai-Uwe, your solution is the closest, and I went this route as well.
However, there is still a problem. Operator* must return a true
reference, you can not return a temporary object.

swap(*a,*b)

is the problem. swap takes two non-const references. If *a returns a
temporary, not a true reference, then it can't bind this temporary to
a non-const reference, and it fails to compile.


I agree that the random access iterator requirements basically impose
the requirement that the expression *it (given an iterator value named
'it') is a real reference. This is a recognized design defect in the
iterator categories, because traversal and access are not controlled by
independent axes. Note that C++0x has introduced swappable requirements
and these also support proxy references because given these swappable
requirements an implementation has to ensure that swap calls are
"ADL-enabled" and there is no requirement (in general) that such swap
calls must accept lvalues. It was explicit part of the design of these
requirements to allow user-swap functions to accept rvalues and to allow
mixed swaps as well. This leads to the grey zone, where the a strict
interpretation of the random access iterator requirements meets with the
more general swap requirements. In this case I would expect that a
modern C++0x-enabled library implementation just accepts random access
iterator types, where the expression *it does not return a real
reference, because the design problem of iterators is well-known and
will hopefully be fixed in the future.

I ran into this problem as well trying to implement it myself:

I ran into trouble when I had to implement operator* on my pointer class because I had
no real object to return a reference to


This shouldn't be so hard, but I continue to claim that it is not
possible with std::sort.


It should be possible in a C++0x-enabled library implementation,
especially since C++0x sort is required to respect the Swappable
requirements on the *caller* of swap (i.e. to be enforced to activate ADL).

HTH & Greetings from Bremen,

Daniel Kr?gler

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