C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Output iterator          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Anders Guest Posted: Sun Jun 29, 2003 10:00 pm    Post subject: Output iterator Hello there. Can anyone tell me why the ++-operator is defined for ostream_iterator ? I can't see the difference between: ostream_iterator<int> test(cout); test++ = 1; test++ = 2; test++ = 3; test++ = 4; and: ostream_iterator<int> test(cout); test = 1; test = 2; test = 3; test = 4; //Anders Back to top Anders Guest Posted: Sun Jun 29, 2003 10:06 pm    Post subject: Re: Output iterator And yes, I know that I should be using preincrement instead - it's faster: --- test = 1; ++test = 2; ++test = 3; ++test = 4; --- //Anders Back to top Howard Hinnant Guest Posted: Sun Jun 29, 2003 10:19 pm    Post subject: Re: Output iterator In article <3eff61ac$0$32546$edfadb0f (AT) dread16 (DOT) news.tele.dk>, Anders <gregersen (AT) adslhome (DOT) dk> wrote: Quote: Hello there. Can anyone tell me why the ++-operator is defined for ostream_iterator ? I can't see the difference between: ostream_iterator<int> test(cout); test++ = 1; test++ = 2; test++ = 3; test++ = 4; and: ostream_iterator<int> test(cout); test = 1; test = 2; test = 3; test = 4; It is so that generic code will work with ostream_iterator. For example: template <class InputIterator, class OutputIterator> OutputIterator copy(InputIterator first, InputIterator last, OutputIterator result) { for (; first != last; ++first, ++result) *result = *first; return result; } If you put an ostream_iterator into copy as the OutputIterator, you really do want it to compile and "do the right thing", which in this case is nothing for operator++(). -- Howard Hinnant Metrowerks Back to top Rolf Magnus Guest Posted: Sun Jun 29, 2003 10:38 pm    Post subject: Re: Output iterator Howard Hinnant wrote: Quote: In article <3eff61ac$0$32546$edfadb0f (AT) dread16 (DOT) news.tele.dk>, Anders [email]gregersen (AT) adslhome (DOT) dk[/email]> wrote: | Hello there. | | Can anyone tell me why the ++-operator is defined for | ostream_iterator ? I can't see the difference between: | | ostream_iterator<int> test(cout); | test++ = 1; | test++ = 2; | test++ = 3; | test++ = 4; | | and: | | ostream_iterator<int> test(cout); | test = 1; | test = 2; | test = 3; | test = 4; It is so that generic code will work with ostream_iterator. For example: template OutputIterator copy(InputIterator first, InputIterator last, OutputIterator result) { for (; first != last; ++first, ++result) *result = *first; return result; } If you put an ostream_iterator into copy as the OutputIterator, you really do want it to compile and "do the right thing", which in this case is nothing for operator++(). Actually, you must use operator++ to increment the iterator for each read/write operation, since the implementation might choose to actually rely on this. Back to top Rolf Magnus Guest Posted: Sun Jun 29, 2003 11:54 pm    Post subject: Re: Output iterator Howard Hinnant wrote: Quote: In article <bdnq3o$unr$01$1 (AT) news (DOT) t-online.com>, Rolf Magnus [email]ramagnus (AT) t-online (DOT) de[/email]> wrote: | Howard Hinnant wrote: | | > If you put an ostream_iterator into copy as the OutputIterator, | > you really do want it to compile and "do the right thing", which | > in this case is nothing for operator++(). | | Actually, you must use operator++ to increment the iterator for each | read/write operation, since the implementation might choose to | actually rely on this. nod> It might. But the C++ standard says in section 24.5.2.2 - ostream_iterator operations, paragraph 3: | ostream_iterator& operator++(); | ostream_iterator& operatot++(int); | | -3- Returns: *this (i.e. do nothing) Does that mean the same as if it specifically said that the operator does nothing? Quote: I certainly would never write code that used ostream_iterator in an unconventional manner that took advantage of this detail. And I'm not suggesting anybody should. But if you want to get picky, ostream_iterator::operator++() is defined by the C++ standard to do nothing but return *this. I didn't read in the standard, but in TC++PL3, which says: "The ++ operation might trigger an actual output operation, or it might have no effect. Different implementations will use different implementation strategies. Consequently, for code to be portable, a ++ must occur between every two assignments to an ostream_iterator." Quote: And the main reason that ostream_iterator has all of these "extra" operations that do nothing is so that it can be used seamlessly as any other iterator that supports the semantics required by output_iterator_tag (i.e. output, forward, bidirectional and random access iterators). Ack. Back to top Howard Hinnant Guest Posted: Mon Jun 30, 2003 12:52 am    Post subject: Re: Output iterator In article <bdnuim$20g$01$1 (AT) news (DOT) t-online.com>, Rolf Magnus <ramagnus (AT) t-online (DOT) de> wrote: Quote: Howard Hinnant wrote: nod> It might. But the C++ standard says in section 24.5.2.2 - ostream_iterator operations, paragraph 3: | ostream_iterator& operator++(); | ostream_iterator& operatot++(int); | | -3- Returns: *this (i.e. do nothing) Does that mean the same as if it specifically said that the operator does nothing? Hmm... no, I think you're right. It could have other effects, though I'm having trouble coming up with a quick realistic example. Maybe some kind of debugging info? Quote: I certainly would never write code that used ostream_iterator in an unconventional manner that took advantage of this detail. And I'm not suggesting anybody should. But if you want to get picky, ostream_iterator::operator++() is defined by the C++ standard to do nothing but return *this. I didn't read in the standard, but in TC++PL3, which says: "The ++ operation might trigger an actual output operation, or it might have no effect. Different implementations will use different implementation strategies. Consequently, for code to be portable, a ++ must occur between every two assignments to an ostream_iterator." Well, the standard explicitly says that the output will occur under op=(const T&) operator: Quote: ostream_iterator& operator=(const T& value ); -1- Effects: *out_stream << value; if(delim != 0) * out_stream << delim; return (*this); So I don't see how the op++ could also trigger output without really messing things up. Quote: Ack. Bill? Bill The Cat? Is that you?!! 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