[boost] Re: any interest in a foreach looping construct?

From: Eric Niebler (eric_at_[hidden])
Date: 2003-10-11 16:40:31

Daniel Wallin wrote:
>   1. It operates on containers rather than iterators.

Yes, containers, arrays and null-terminated strings. What is your 
objection exactly? If you are saying that iterating over a container is 
wrong *in principle*, then I disagree -- it's often exactly what you 
want. If you are saying that there should be a way to iterate over a 
sub-sequence, then I agree. I just uploaded a new version that lets you 
iterate over an iterator range like this:

using boost::for_each::in_range;
BOOST_FOREACH( int i, in_range( iter1, iter2 ) )
{
    ...
}

>   2. It reevaluates the container expression on every iteration.

True, but the "container expression" must be an lvalue, which makes it 
difficult to use incorrectly. The container expression cannot be a 
function call that returns a temporary, for instance.

You may be wondering about in_range() above -- it is a function that 
returns a temporary, yet it is being used as a container expression. 
in_range() returns a special type, and BOOST_FOREACH accepts it even 
though it is not an lvalue. However, the *arguments* to in_range() must 
be iterator lvalues and not temporaries returned by functions.

I understand your concerns, however.  There is still the danger for this:

BOOST_FOREACH( int i, in_range( ++iter1, iter2 ) )
                                 ^^^^^^^ oops!

I'm not sure how to guard against that.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com