Re: Initializing iterators to one past end() in for loops

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 14 Apr 2011 00:23:24 +0200

Message-ID:

<io57ov$rjf$1@hoshi.visyn.net>

Leigh Johnston wrote:

On 13/04/2011 22:48, Kai-Uwe Bux wrote:

Nikos Chantziaras wrote:

Is there some simple way to convert this:

    std::vector<int> vec;
    // ...
    for (size_t i = 1; i< vec.size(); ++i)
        // do something with vec[i]

to a for-loop that uses iterators? I've tried:

    typedef std::vector<int>::iterator it_t;
    for (it_t i = vec.begin() + 1; i != vec.end(); ++i)
        // do something with *i

but I'm not surprised that it doesn't work.

Iterators for std::vector<> are random access. Hence, they
are<-comparable and you can add integers to them. So, you should be able
to translate one- to-one:

   for ( std::vector<int>::iterator iter = vec.begin() + 1;
         iter< vec.end(); ++ iter ) {
     // do something with *iter
   }

If the vector is empty then surely vec.begin() + 1 is UB?

Hm, my first impulse was to say: "oh, true; that kills it". However, upon
looking up table 76 in the standard, it appears that

r += n

has no pre-condition (like r is dereferencable). Now, r+n is defined in
terms of r += n. Also, the dereferencability precondition is mentioned at
other places (e.g., for ++r in table 74 about forward iterators). So, maybe
vec.begin()+1 is not undefined behavior after all. Of course, it could also
be an omission in the standard, or it could be that I am too tired to figure
it out correctly.

Best,

Kai-Uwe Bux