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

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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

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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

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