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

From: Scott Meyers (usenet_at_[hidden])
Date: 2007-07-22 15:55:29

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.

> 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.  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.  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.

For the record, this compiles:

   find_if(b2, e2, const_parameters(_1 == 'x'));

Thanks for the explanation.

Scott