RE: [boost] boost CVS changes

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2001-06-07 03:13:44

John E. Potter wrote:
> There is no void& and you can't do anything with a void*.  You can't
> read an output_iterator; so, it doesn't have a value_type => void.
[snip]

I don't think that's that simple. First of all, the standard explicitly says
that notion of value type is valid for _all_ iterators: 

<quote>
24.1 [lib.iterator.requirements] para 1:
All iterators i support the expression *i, resulting in a value of some
class, enumeration, or built-in type T, called the value type of the
iterator.
</quote>

Second, the following two quotes imply that value type of an output iterator
is _not_ void, otherwise the cited requirements table's line is meaningless:

<quote>
24.1 [lib.iterator.requirements] para 9: In the following sections, a and b
denote values of X, n denotes a value of the difference type Distance, u,
tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of
value type T.
</quote>

<quote>
24.1.2 [lib.output.iterators] para 1: A class or a built-in type X satisfies
the requirements of an output iterator if X is an Assignable type (23.1) and
also the following expressions are valid, as shown in Table 73:

Table 73-Output iterator requirements

expression 	return type		...
...
*a = t		result is not used
...
</quote>

Now, in the light of the above, let's look what 24.3.1
[lib.iterator.traits], para 1 says:

<quote>
To implement algorithms only in terms of iterators, it is often necessary to
determine the value and difference types that correspond to a particular
iterator type. Accordingly, it is required that if Iterator is the type of
an iterator, the types

        iterator_traits<Iterator>::difference_type
        iterator_traits<Iterator>::value_type
        iterator_traits<Iterator>::iterator_category

be defined as the iterator's difference type, value type and iterator
category, respectively. In the case of an output iterator, the types

        iterator_traits<Iterator>::difference_type
        iterator_traits<Iterator>::value_type

are both defined as void.
</quote>

So it _does not_ really say that value type of an output iterator is void.
What it says is that the "iterator_traits<Iterator>::value_type" typedef is
a synonym for void. 

IMO it's kind of odd - value type of an output iterator is not void, but
"iterator_traits<Iterator>::value_type" is required to be a synonym for
void. That's even more strange, it seems like this is an intentional
decision, so I wonder if anybody knows a rationale (if the above analysis is
correct). 

Hmm.. just noticed that the discussion more and more drifts to something
more appropriate for comp.std.c++ than for this list :).

Aleksey