Re: Constness for user-defined by-reference types

From:
SG <s.gesemann@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Sun, 3 Jan 2010 16:15:00 CST
Message-ID:
<6061c4e0-8bfb-45c6-9223-2697c9a99b2f@m3g2000yqf.googlegroups.com>
On 3 Jan., 17:54, Florian Weimer wrote:

Balog Pal wrote:

"Florian Weimer" <f...@deneb.enyo.de>

What is a good way to express constness for user-defined by-reference
types? Plain const does not work because you can easily make a
non-const object,

 void foo (const T& t) {
   T t1(t);
   // non-const path to t via t1


No, that creates a copy of t.


Not really, it's just a shallow copy if T has reference-like
semantics, like an iterator.


Even if T is an iterator type t1 will be a copy of t -- with whatever
semantics T's copy ctor has. It seems you confuse

   U const * // Type1

with

   U * const // Type2

Top-level constness of an iterator should only affect the iterator
itself, not the object it refers to. At least that's the idea as
iterators are a generalization of pointers. Don't use Type2 if you
want Type1. In your case it means: The use of two different user-
defined types may be the best solution -- just like containers offer
associated types "iterator" and "const_iterator". Note: "const"
doesn't refer to the top level constness in this case which might be
one reason for the confusion. So, "const iterator" and
"const_iterator" are different types.

Cheers,
SG

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