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List:       cfe-dev
Subject:    Re: [cfe-dev] const correctness of iterator methods
From:       Sebastian Redl <sebastian.redl () getdesigned ! at>
Date:       2013-06-26 23:09:15
Message-ID: 492CA6DF-33BA-4E72-BA33-11496C80EC04 () getdesigned ! at
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On 27.06.2013, at 00:38, Shriramana Sharma wrote:

> Hi -- again a few questions about the libc++ container templates.
> 
> I note that __list_iterator::operator*() returns a non-const reference
> but is marked as const. The operator-> next to it returns a non-const
> pointer but is also marked const. OTOH in the list class, the begin()
> and end() methods returning non-const iterators are NOT marked const
> and only those returning const iterators are marked const.

Note that none of this is in any way particular to libc++; that's how the standard \
specifies these things to work.

> 
> I thought a method should be const if it does not modify the contents
> of the object by itself, and not based on whether the value it returns
> could be used to modify the object.

This is an oversimplification that is leading you astray.

The correct rule is, "a const method should not give the caller a way of modifying \
the logical state of the object". What is part of the logical state and what is not \
is the class designer's decision.

The logical state of an iterator is "I point at some element of a sequence". The \
value of that element is not part of its logical state. Therefore, operator* and \
operator-> are const: they only allow changing the value of the element pointed at, \
not what element the iterator points at.

The logical state of a container is "all the logical states of my elements". \
Modifying an element's logical state is thus a modification of the container's \
logical state. Therefore, the versions of begin() and end() that return non-const \
iterators cannot be const, since they offer a way to modify the container's logical \
state.

Does that make sense?

Sebastian
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