Re: [Boost-users] [Lambda] Trouble using istreambuf_iterators

From: David Abrahams (dave_at_[hidden])
Date: 2007-07-23 00:04:36

on Sun Jul 22 2007, Scott Meyers <usenet-AT-aristeia.com> wrote:

> David Abrahams wrote:
>
>> The issue is that istreambuf_iterator is an input iterator, not a
>> forward iterator, and thus is allowed to return from operator* by
>> value instead of reference (which it does).
>
> Of course, one has to know this.  Note that the code works with
> istream_iterators, and they are also input iterators.

Yes.  Note "allowed," not "required."

Incidentally, that allowed variation is why "input iterator" can't be
an implicitly deduced concept.  If vector<T>::insert were to
mistakenly deduce an input iterator (single-pass) to be a (multipass)
forward iterator, the library could crash or worse.

>> http://tinyurl.com/37v98x#lambda.actual_arguments_to_lambda_functors
>> explains the problem and contains a link to a section about
>> workarounds.
>
> Technically, yes, it explains the problem, but it makes no mention
> of the implications for users of standard library iterator types.

True.

> I, for one, did not know that operator* for istreambuf_iterator may
> return an rvalue, and I question whether I should be expected to
> know this.  

I don't know how to judge whether anyone should be expected to know
that.  It's just a standard property of input iterators, like that
they don't support operator+, but maybe it falls below some threshold
of reasonable knowledge.

> It would be nice if
> http://www.boost.org/doc/html/lambda/using_library.html#lambda.actual_arguments_to_lambda_functors
> mentioned the conditions under which using Lambda with standard
> iterator types might not compile and called out istreambuf_iterator
> by name.  At least then my Googling for information would have been
> likely to help.

Yes, it would be nice.  IIUC not much effort is going into lambda
stuff because it's supposed to be replaced by phoenix 2... but if you
want to report a bug, please enter a ticket into the Boost trac at
http://svn.boost.org; at least it will stand a chance of being
addressed.

> For the record, this compiles:
>
>    find_if(b2, e2, const_parameters(_1 == 'x'));
>
> Thanks for the explanation.

Glad it helped.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
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