Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c++ > #3619

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

Followups directed to: comp.lang.c++

Show all headers | View raw

Kai-Uwe Bux wrote:

> 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.

Seems, it was the third option:

  [24.1/5] Iterators can also have singular values that are not associated 
  with any container. [Example: After the declaration of an uninitialized 
  pointer x (as with int* x;), x must always be assumed to have a singular 
  value of a pointer. ] Results of most expressions are undefined for 
  singular values; the only exception is an assignment of a non-singular 
  value to an iterator that holds a singular value. In this case the 
  singular value is overwritten the same way as any other value. 
  Dereferenceable values are always non-singular.

Hence, vec.begin()+1 is not UB (as vec.begin() is non-singular), but 
assigning the resulting singular value to the iterator variable iter is UB.


Best,

Kai-Uwe Bux

Back to comp.lang.c++ | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Initializing iterators to one past end() in for loops Nikos Chantziaras <realnc@arcor.de> - 2011-04-14 00:38 +0300
  Re: Initializing iterators to one past end() in for loops Leigh Johnston <leigh@i42.co.uk> - 2011-04-13 22:46 +0100
  Re: Initializing iterators to one past end() in for loops Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-04-13 23:48 +0200
    Re: Initializing iterators to one past end() in for loops Leigh Johnston <leigh@i42.co.uk> - 2011-04-13 22:50 +0100
      Re: Initializing iterators to one past end() in for loops Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-04-14 00:23 +0200
        Re: Initializing iterators to one past end() in for loops Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-04-14 00:32 +0200
          Re: Initializing iterators to one past end() in for loops Noah Roberts <dont@email.me> - 2011-04-13 17:16 -0700
            Re: Initializing iterators to one past end() in for loops Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-04-14 05:57 +0200
              Re: Initializing iterators to one past end() in for loops Leigh Johnston <leigh@i42.co.uk> - 2011-04-14 14:29 +0100
                Re: Initializing iterators to one past end() in for loops Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-04-14 17:57 +0200
                Re: Initializing iterators to one past end() in for loops Leigh Johnston <leigh@i42.co.uk> - 2011-04-14 17:56 +0100
                Re: Initializing iterators to one past end() in for loops Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-04-14 19:33 +0200
              Re: Initializing iterators to one past end() in for loops Noah Roberts <dont@email.me> - 2011-04-14 09:54 -0700
                Re: Initializing iterators to one past end() in for loops Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-04-14 20:08 +0200
  Re: Initializing iterators to one past end() in for loops gwowen <gwowen@gmail.com> - 2011-04-13 14:49 -0700
  Re: Initializing iterators to one past end() in for loops Sam <sam@email-scan.com> - 2011-04-13 16:58 -0500
  Re: Initializing iterators to one past end() in for loops Nikos Chantziaras <realnc@arcor.de> - 2011-04-14 01:07 +0300
    Re: Initializing iterators to one past end() in for loops Ian Collins <ian-news@hotmail.com> - 2011-04-14 10:12 +1200
    Re: Initializing iterators to one past end() in for loops gwowen <gwowen@gmail.com> - 2011-04-13 15:24 -0700
    Re: Initializing iterators to one past end() in for loops Paavo Helde <myfirstname@osa.pri.ee> - 2011-04-13 23:05 -0500
      Re: Initializing iterators to one past end() in for loops Ian Collins <ian-news@hotmail.com> - 2011-04-14 16:20 +1200
        Re: Initializing iterators to one past end() in for loops Paavo Helde <myfirstname@osa.pri.ee> - 2011-04-13 23:52 -0500
        Re: Initializing iterators to one past end() in for loops Jeff Flinn <TriumphSprint2000@hotmail.com> - 2011-04-14 07:49 -0400
      Re: Initializing iterators to one past end() in for loops Nikos Chantziaras <realnc@arcor.de> - 2011-04-14 21:45 +0300

csiph-web