C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Relationship between pointer and iterator associated types Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Yuriy Solodkyy Guest Posted: Wed Sep 07, 2005 7:09 pm    Post subject: Relationship between pointer and iterator associated types Hi guys, I don't see any requirement for Container::pointer associated type in the standard, though it is mentioned on SGI's STL website (however const_pointer is not mentioned there either). My question is what is the relationship between pointer and iterator associated types? What's the difference between them? When to use one and when the other? Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? Thanks in advance, Yuriy [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Thu Sep 08, 2005 9:16 am    Post subject: Re: Relationship between pointer and iterator associated typ Yuriy Solodkyy wrote: Quote: I don't see any requirement for Container::pointer associated type in the standard, though it is mentioned on SGI's STL website (however const_pointer is not mentioned there either). My question is what is the relationship between pointer and iterator associated types? None, AFAICS. 'pointer' is a typedef from 'Allocator::pointer', where 'Allocator' is the template argument. Quote: What's the difference between them? They are different. IOW, they are not the same. Quote: When to use one and when the other? Many standard algorithms are designed to use iterators. Use iterators with those. Use pointers when pointers are expected. Quote: Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? Dereferencing 'Container::iterator' gives you 'Container::reference', probably, and if you use '&' on that, you're going to probably get the same type as 'Container::pointer'. V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Thu Sep 08, 2005 9:23 am    Post subject: Re: Relationship between pointer and iterator associated typ In article <dfncfa$b48$1 (AT) news (DOT) tamu.edu>, Yuriy Solodkyy <solodon (AT) tamudotedu (DOT) invalid> wrote: Quote: Hi guys, I don't see any requirement for Container::pointer associated type in the standard, though it is mentioned on SGI's STL website (however const_pointer is not mentioned there either). My question is what is the relationship between pointer and iterator associated types? What's the difference between them? When to use one and when the other? Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? There is no requirement for Container::pointer. All I could guess its something like typedef Container::value_type *pointer; and not useful as if you really need a type pointer to container element, then define one yourself where its needed. for a forward,bidirectional or random access iterator with value_type V. typedef reference &V ; // or const &V for a constant iterator; typedef pointer *V; // or const *V for a constant iterator. for input iterators they are iterator dependent but often as above. for output iterators the types are not needed. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Thu Sep 08, 2005 1:45 pm    Post subject: Re: Relationship between pointer and iterator associated typ Victor Bazarov wrote: Quote: Yuriy Solodkyy wrote: I don't see any requirement for Container::pointer associated type in the standard, though it is mentioned on SGI's STL website (however const_pointer is not mentioned there either). My question is what is the relationship between pointer and iterator associated types? None, AFAICS. 'pointer' is a typedef from 'Allocator::pointer', where 'Allocator' is the template argument. What's the difference between them? They are different. IOW, they are not the same. They do bear one simularity: both can be used to access a specific element in the container, by means of the * operator. Quote: When to use one and when the other? Many standard algorithms are designed to use iterators. Use iterators with those. Use pointers when pointers are expected. Attention: a pointer in C++ is more than just a pointer. It is also an iterator into a C style array. Be very careful when using pointers that the function being called doesn't expect an iterator into a C style array. Quote: Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? Dereferencing 'Container::iterator' gives you 'Container::reference', probably, and if you use '&' on that, you're going to probably get the same type as 'Container::pointer'. Probably. But is it guaranteed ? -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Fri Sep 09, 2005 3:12 pm    Post subject: Re: Relationship between pointer and iterator associated typ kanze wrote: Quote: Victor Bazarov wrote: Dereferencing 'Container::iterator' gives you 'Container::reference', probably, and if you use '&' on that, you're going to probably get the same type as 'Container::pointer'. Probably. But is it guaranteed ? Nothing is guaranteed AFA real world is concerned. V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Yuriy Solodkyy Guest Posted: Sat Sep 10, 2005 1:35 pm    Post subject: Re: Relationship between pointer and iterator associated typ Quote: They do bear one simularity: both can be used to access a specific element in the container, by means of the * operator. If I understand it right, the main difference of pointer and iterator is that iterator goes only one way: by dereferencing iterator you get a reference to an element. Taking address of that reference won't return you iterator back but a pointer. This is different from built-in pointers as by taking address of dereferenced reference you'll get the same pointer back. This also raises a question whether pointer type should implement other things implemented by iterator e.g. operator++() or should it just behave as a trivial iterator? Is this defined somewhere? Quote: Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? Dereferencing 'Container::iterator' gives you 'Container::reference', probably, and if you use '&' on that, you're going to probably get the same type as 'Container::pointer'. Probably. But is it guaranteed ? I talked to other people here and they said that pointer is also a type that operator-> returns on iterator and is meant to provide for cases when T* is not sufficient for implementation and some sort of proxy is needed. Basically for vector<bool> you can't write bool* p = &v[4] and should write vector<bool>::pointer p = &v[4] The reason I'm asking all this is I have to implement an iterator where I have to use proxies for both pointer and reference. Logically iterator in this case is a subclass of pointer as it contains more information then the pointer and behaves like one. Having that should it be convertible to pointer? GCC doesn't provide such conversion but should it logically be there? Yuriy [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rob Guest Posted: Sun Sep 11, 2005 3:15 pm    Post subject: Re: Relationship between pointer and iterator associated typ Yuriy Solodkyy wrote: Quote: They do bear one simularity: both can be used to access a specific element in the container, by means of the * operator. If I understand it right, the main difference of pointer and iterator is that iterator goes only one way: by dereferencing iterator you get a reference to an element. Taking address of that reference won't return you iterator back but a pointer. This is different from built-in pointers as by taking address of dereferenced reference you'll get the same pointer back. Sure. The basic relationship is that a pointer is a type of iterator, but not all iterators are pointers. Quote: This also raises a question whether pointer type should implement other things implemented by iterator e.g. operator++() or should it just behave as a trivial iterator? Is this defined somewhere? Incrementing is allowed for pointers to anything except void. Quote: Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? Dereferencing 'Container::iterator' gives you 'Container::reference', probably, and if you use '&' on that, you're going to probably get the same type as 'Container::pointer'. Probably. But is it guaranteed ? I talked to other people here and they said that pointer is also a type that operator-> returns on iterator and is meant to provide for cases when T* is not sufficient for implementation and some sort of proxy is needed. Basically for vector<bool> you can't write bool* p = &v[4] and should write vector<bool>::pointer p = &v[4] I might be out of date, but I thought though that std::vector<bool> was disallowed. Quote: The reason I'm asking all this is I have to implement an iterator where I have to use proxies for both pointer and reference. Logically iterator in this case is a subclass of pointer as it contains more information then the pointer and behaves like one. Having that should it be convertible to pointer? GCC doesn't provide such conversion but should it logically be there? The typical way to get a pointer from an iterator is to dereference the iterator (which yields a reference to the object) and take it's address. While I haven't seen the rationale for this explained specifically anywhere, I would assume the reason conversion from iterator to pointer is disallowed is, for that iterators from containers that don't store objects contiguously, that arithetic on those iterators works differently than it does for pointers. For example; #include <list> #include <vector> #include <assert.h> // C header. I'm lazy int main() { std::vector<int> x(42); std::vector<int>::iterator ix = x.begin(); int *px = &(*ix); // one way of getting pointer to first element of x ++px; ++ix; assert(&(*ix) == px); // this assertion is guaranteed to succeed // the following is like the above, with list substituted for vector std::list<int> x(42); std::list<int>::iterator ix = x.begin(); int *px = &(*ix); // one way of getting pointer to first element of x ++px; ++ix; assert(&(*ix) == px); // this assertion is NOT guaranteed to succeed } So, while a direct conversion from iterator to pointer would make some sort of sense for a vector, it would not for a list. So, while it's not necessarily wrong to support conversion from iterator to pointer, you will need to be sure that it makes sense for your container type [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Sun Sep 11, 2005 9:19 pm    Post subject: Re: Relationship between pointer and iterator associated typ In article <dft7j3$9qt$1 (AT) news (DOT) tamu.edu>, Yuriy Solodkyy <solodon (AT) tamudotedu (DOT) invalid> wrote: Quote: The reason I'm asking all this is I have to implement an iterator where I have to use proxies for both pointer and reference. Logically iterator in this case is a subclass of pointer as it contains more information then the pointer and behaves like one. Having that should it be convertible to pointer? GCC doesn't provide such conversion but should it logically be there? Be warned if your iterator provides an iterator category other than std::input_iterator_tag, or std::output_iterator_tag, it is not valid according to the standard since reference must be &value_type and pointer must be *value_type, for forward ,bidirectional or random access iterators. If you really have a need for conversion to a raw pointer, provide a member function to produce one. An iterator is not neccesarily convertable to a pointer [example std::istream_iterator<int>]. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Yuriy Solodkyy Guest Posted: Mon Sep 12, 2005 9:27 am    Post subject: Re: Relationship between pointer and iterator associated typ Quote: Sure. The basic relationship is that a pointer is a type of iterator, but not all iterators are pointers. That's not what I understood: accordingly to what I wrote above it looked to me as all iterators are pointers (not in terms of raw pointers but in terms of iterator_traits<Iterator>::pointer type) and therefore maybe there should be a conversion from iterator to its corresponding pointer type. Quote: This also raises a question whether pointer type should implement other things implemented by iterator e.g. operator++() or should it just behave as a trivial iterator? Is this defined somewhere? Incrementing is allowed for pointers to anything except void. Does it mean that iterator_traits<Iterator>::pointer type should basically duplicate all the operations that Iterator has or can they be just limited to dereferencing? If it should duplicate everything then I see no point in having reference::operator&() return pointer type but rather again iterator type. Quote: Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? Dereferencing 'Container::iterator' gives you 'Container::reference', probably, and if you use '&' on that, you're going to probably get the same type as 'Container::pointer'. Probably. But is it guaranteed ? I talked to other people here and they said that pointer is also a type that operator-> returns on iterator and is meant to provide for cases when T* is not sufficient for implementation and some sort of proxy is needed. Basically for vector<bool> you can't write bool* p = &v[4] and should write vector<bool>::pointer p = &v[4] I might be out of date, but I thought though that std::vector<bool> was disallowed. Still in the standard AFAIK Quote: While I haven't seen the rationale for this explained specifically anywhere, I would assume the reason conversion from iterator to pointer is disallowed is, for that iterators from containers that don't store objects contiguously, that arithetic on those iterators works differently than it does for pointers. I think you assume here by pointer a raw pointer type while I assume by pointer an iterator_traits<Iterator>::pointer type, which in my oppinion needn't be just a raw pointer (taking into account some other posts to this thread, I'm not sure what standard sais on this point). Specifically you write in your code: Quote: std::vector<int> x(42); std::vector<int>::iterator ix = x.begin(); int *px = &(*ix); // one way of getting pointer to first element which I think is not a fully generic code as it won't work for containers that implement pointer type differently (e.g. vector<bool>). A better version should do the following: std::vector<int>::pointer px = &(*ix); Your reasoning in code though is absolutely right if we postulate that pointer type of an iterator is guaranted to be a value_type*. Yuriy [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Yuriy Solodkyy Guest Posted: Mon Sep 12, 2005 9:28 am    Post subject: Re: Relationship between pointer and iterator associated typ Quote: Be warned if your iterator provides an iterator category other than std::input_iterator_tag, or std::output_iterator_tag, it is not valid according to the standard since reference must be &value_type and pointer must be *value_type, for forward ,bidirectional or random access iterators. Does it mean that vector<bool>::iterator is not a random access iterator? Where actually in the standard does it specify that pointer must be value_type* and reference value_type& for forward iterators and higher? Quote: If you really have a need for conversion to a raw pointer, provide a member function to produce one. An iterator is not neccesarily convertable to a pointer [example std::istream_iterator<int>]. By type pointer I didn't mean only raw pointers, though I understand that if it is defined by the standard as you said, it will be the same. I need pointer type to be a proxy as well, so it is not a raw pointer. I don't specifically have a need to provide such conversion, I was just wondering whether such should be provided for genericity. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Mon Sep 12, 2005 9:34 am    Post subject: Re: Relationship between pointer and iterator associated typ On 11 Sep 2005 17:19:01 -0400, Carl Barron <cbarron413 (AT) adelphia (DOT) net> wrote: Quote: In article <dft7j3$9qt$1 (AT) news (DOT) tamu.edu>, Yuriy Solodkyy [email]solodon (AT) tamudotedu (DOT) inva[/email]lid> wrote: The reason I'm asking all this is I have to implement an iterator where I have to use proxies for both pointer and reference. Logically iterator in this case is a subclass of pointer as it contains more information then the pointer and behaves like one. Having that should it be convertible to pointer? GCC doesn't provide such conversion but should it logically be there? Be warned if your iterator provides an iterator category other than std::input_iterator_tag, or std::output_iterator_tag, it is not valid according to the standard since reference must be &value_type I assume that you mean value_type&. I find no such requirement. It is true that *iter must return a T&; however, there is no requirement that iter::reference be anything in particular and no requirement that operator* return an iter::reference. Quote: and pointer must be *value_type, for forward ,bidirectional or random access iterators. I assume value_type*. I find no requirements on iter::pointer at all. It must be defined, but has no meaning. Proxie iterators are not allowed, so vector<bool> is invalid? I have lost track of where the standard has gone on this. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Mon Sep 12, 2005 10:34 am    Post subject: Re: Relationship between pointer and iterator associated typ Yuriy Solodkyy wrote: Quote: They do bear one simularity: both can be used to access a specific element in the container, by means of the * operator. If I understand it right, the main difference of pointer and iterator is that iterator goes only one way: by dereferencing iterator you get a reference to an element. Taking address of that reference won't return you iterator back but a pointer. This is different from built-in pointers as by taking address of dereferenced reference you'll get the same pointer back. The main difference is that you can get a pointer to any object. An iterator of a given type can only refer to objects in a specific type of container. Quote: This also raises a question whether pointer type should implement other things implemented by iterator e.g. operator++() or should it just behave as a trivial iterator? Is this defined somewhere? Arithmetic is defined on pointers, as long as the pointer (or pointers) involved designate different elements of the same C style array. Quote: Should there be a conversion from iterator to pointer (there seems to be no such in gcc implementation)? Dereferencing 'Container::iterator' gives you 'Container::reference', probably, and if you use '&' on that, you're going to probably get the same type as 'Container::pointer'. Probably. But is it guaranteed ? I talked to other people here and they said that pointer is also a type that operator-> returns on iterator and is meant to provide for cases when T* is not sufficient for implementation and some sort of proxy is needed. Basically for vector<bool> you can't write bool* p = &v[4] and should write vector<bool>::pointer p = &v[4] The reason I'm asking all this is I have to implement an iterator where I have to use proxies for both pointer and reference. There's no such thing as a proxy for a reference. You can't overload operator.(). Quote: Logically iterator in this case is a subclass of pointer No. Logically, iterators are iterators, and pointers are pointers. For historical reasons, pointers can, in certain specific cases, be used as iterators, but conceptually, the two are different, unrelated things. Quote: as it contains more information then the pointer and behaves like one. Having that should it be convertible to pointer? GCC doesn't provide such conversion but should it logically be there? No. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Mon Sep 12, 2005 2:49 pm    Post subject: Re: Relationship between pointer and iterator associated typ In article <EebVe.11217$FW1.2255 (AT) newsread3 (DOT) news.atl.earthlink.net>, John Potter <jpotter (AT) lhup (DOT) edu> wrote: Quote: Proxie iterators are not allowed, so vector<bool> is invalid? I have lost track of where the standard has gone on this. I got the punctuation backwards:) the proxy iterator problem is what the std::vector<bool> 'controversy' is about. A draft from 2003 actually shows T or const T & as result of operator *(), but it must be a typo of some kind as then *x = t does not modify the T 'pointed to by' x. The 1998 standard states T &, T == value_type. This gets confusing and is one reason the boost proposal for a separation of iteration and dereferencing [which did not make tr1] was suggested. There can not be any proxied forward iterators, to my knowledge but I could be wrong on this. the standard states that it is only neccesary to define value_type,difference_type and iterator_category for algorithms. it actually does not specify what pointer and reference should be defined as, except for the pointer specializations of std::iterator_traits< T *>, std::iterator_traits<const T *> do define pointer as a T *, and a reference as a T & and the value_type as T. Pointer and reference were added for proxies or something, but the standard [1998 standard] does not even require them to exist. going by table 74 and example 24.3.3 para 4 the meanings of are confusing and need fixing this is a bidirectional iterator by the example but not by table 74/75. Interesting. the 2003 draft I have states that reference is the result of operator *(); and pointer is the result of operator ->(). [1998 does not] the 2005 draft apparently I mis renamed it on this machine is N1804=05-0064 dated 2005--04-27. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Mon Sep 12, 2005 5:06 pm    Post subject: Re: Relationship between pointer and iterator associated typ "Rob" <nospam (AT) nonexistant (DOT) com> writes: [...] Quote: I talked to other people here and they said that pointer is also a type that operator-> returns on iterator and is meant to provide for cases when T* is not sufficient for implementation and some sort of proxy is needed. Basically for vector<bool> you can't write bool* p = &v[4] and should write vector<bool>::pointer p = &v[4] I might be out of date, but I thought though that std::vector<bool> was disallowed. Who did that? vector<bool> may be a problem is search of solution but I haven't seen the C++ standard says it is disallowed. -- Gabriel Dos Reis [email]gdr (AT) integrable-solutions (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Mon Sep 12, 2005 5:07 pm    Post subject: Re: Relationship between pointer and iterator associated typ "kanze" <kanze (AT) gabi-soft (DOT) fr> writes: [...] Quote: I talked to other people here and they said that pointer is also a type that operator-> returns on iterator and is meant to provide for cases when T* is not sufficient for implementation and some sort of proxy is needed. Basically for vector<bool> you can't write bool* p = &v[4] and should write vector<bool>::pointer p = &v[4] The reason I'm asking all this is I have to implement an iterator where I have to use proxies for both pointer and reference. There's no such thing as a proxy for a reference. You can't overload operator.(). operator(). does not make sense for all references (e.g. bool& int&, etc.). And not just because you can't overload operator(). means you can't have a proxy for a reference. As a matter of fact vector<bool>::reference is a class type. (so is bitset<N>::reference). -- Gabriel Dos Reis [email]gdr (AT) integrable-solutions (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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