Re: std::for_each + break

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 20 Nov 2007 04:13:31 -0800

Message-ID:

<fhuj23$9a$1@murdoch.acc.Virginia.EDU>

terminator wrote:

On Nov 20, 12:45 pm, Kai-Uwe Bux <jkherci...@gmx.net> wrote:

Kira Yamato wrote:

On 2007-11-19 07:12:29 -0500, yurec <Yurij.Zha...@materialise.kiev.ua>
said:

Hi

I wanna give template method of library class with some predicate,
which I put to std::for_each in that method.How to break the
std::for_each with predicate?(exceptions are used olny in exceptional
situations, as i know)

Thanks

I would think throwing an exception is a simple and elegant solution
here.

Simple and elegant? in this case?

Compare

  try {
    std::for_each( seq.begin(), seq.end(), throwing_predicate_and_action
    );
  }
  catch ( whatever ) {}

to

  for ( iterator_type iter = seq.begin();
        iter != seq.end() && ! break_condition( *iter );
        ++iter ) {
    some_action( *iter );
  }

I cannot say that I find the try-throw-catch version easier to grok or
more elegant.

Just because we have called a mechanism 'exception' and gave it the
connotation that it is for error-handling purposes, does not mean that
we can only use the mechanism as such and nothing else.

I wholeheartly agree to that. I prefer to refer to the mechanism as
try-throw-catch or stack-unwinding to avoid connotations. Sometimes, when
someone says "exception are only to be thrown is exceptional
circumstances", I reply (only half-jokingly): "ok. so throw something
else".

Ok, so some runtime cost is needed to setup try-catch blocks, but I'm
not sure scanning the list twice with find_if/for_each combo is
necessarily faster.

No, but a simple for loop might be (and it also might convey intend
better than any of the alternatives).

what`s wrong with std::find_if when breaking is needed?

It depends: std::find_if conveys the intend of searching. It goes against
expectations to use it with a predicate whose evaluation leaves
side-effects on the container elements. On the other hand, std::for_each
carries no such connotation (although some people feel otherwise on the
ground that the standard classifies it as non-mutating). So, std::find_if
is not a good substitute for std::for_each in the case where you just want
a loop that performs some action on an initial segment of the list.

I guess, the way to do this idiomatically in STLeese is something like:

std::for_each( begin, std::find_if( begin, end, condition ), action );

Now, I see that the OP did not specify whether there is a need for
performing an action on the elements of the sequence or whether the task at
hand is simply to find the first where a condition holds. In the later
case, std::find_if is of course perfectly fine.

Best

Kai-Uwe Bux