C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How to do this in c++          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message john.burton@jbmail.com Guest Posted: Thu Sep 29, 2005 10:17 am    Post subject: How to do this in c++ I used to use c++ a lot but recently I've been using java and pythons and I'm finding it a lot harder to think of the best way to do something in C++ Imagine I want a function that returns a "list" of selected users from a big list of users, or a database or something based on some selection. In python I might use a generator function which yielded the next relevent user each time it was called. In java I'd probably return an Iterator which iterated through the selected users probably using an instance of an inner class to implement the iterator that just iterated through the items I wanted. It's not so clear how I'd best do this in C++? C++ iterators are totally different things to java iterators and it doesn't seem at all elegent to define a new type of iterator which only iterates though selected items. I could create a new list and return that by value but that seems somewhat inefficient so say the least. Returning any kind of reference to an existing list seems fraught with memory management issues over how to create and delete the memory. What's the best way to do this in C++? I hope this makes some sense... Back to top Bob Hairgrove Guest Posted: Thu Sep 29, 2005 10:34 am    Post subject: Re: How to do this in c++ On 29 Sep 2005 03:17:50 -0700, [email]john.burton (AT) jbmail (DOT) com[/email] wrote: Quote: I could create a new list and return that by value but that seems somewhat inefficient so say the least. Returning any kind of reference to an existing list seems fraught with memory management issues over how to create and delete the memory. Use std::vector<> and let it manage the memory for you. Create an empty list or vector and pass a non-const reference to it to your function that selects the names, etc. from the database or file or whatever, and let the function fill up the vector. You are right that returning the vector or list by value would most likely be too inefficient. std::list or std::map might be more effcient than a vector if you need to insert and delete lots of objects. std::vector is better for sequential or random access. And returning a reference to any object locally allocated within a function (i.e. not created with "new", but a stack-based or automatic variable, whatever you want to call it) is a big no-no because that memory ceases to exist when the function returns...thus invalidating the reference. -- Bob Hairgrove [email]NoSpamPlease (AT) Home (DOT) com[/email] Back to top Michael Guest Posted: Thu Sep 29, 2005 10:34 am    Post subject: Re: How to do this in c++ [email]john.burton (AT) jbmail (DOT) com[/email] wrote: Quote: I used to use c++ a lot but recently I've been using java and pythons and I'm finding it a lot harder to think of the best way to do something in C++ Imagine I want a function that returns a "list" of selected users from a big list of users, or a database or something based on some selection. In python I might use a generator function which yielded the next relevent user each time it was called. In java I'd probably return an Iterator which iterated through the selected users probably using an instance of an inner class to implement the iterator that just iterated through the items I wanted. It's not so clear how I'd best do this in C++? C++ iterators are totally different things to java iterators and it doesn't seem at all elegent to define a new type of iterator which only iterates though selected items. I could create a new list and return that by value but that seems somewhat inefficient so say the least. Returning any kind of reference to an existing list seems fraught with memory management issues over how to create and delete the memory. What's the best way to do this in C++? I hope this makes some sense... What do you have the "big list" as ? how about returning a list of pointers to the relevant objects? Mike Back to top Giancarlo Niccolai Guest Posted: Thu Sep 29, 2005 11:15 am    Post subject: Re: How to do this in c++ [email]john.burton (AT) jbmail (DOT) com[/email] wrote: Quote: I used to use c++ a lot but recently I've been using java and pythons and I'm finding it a lot harder to think of the best way to do something in C++ Imagine I want a function that returns a "list" of selected users from a big list of users, or a database or something based on some selection. In python I might use a generator function which yielded the next relevent user each time it was called. In java I'd probably return an Iterator which iterated through the selected users probably using an instance of an inner class to implement the iterator that just iterated through the items I wanted. It's not so clear how I'd best do this in C++? C++ iterators are totally different things to java iterators and it doesn't seem at all elegent to define a new type of iterator which only iterates though selected items. Actually, you can and it is not inelegant at all. The already proposed solution of the function receiving a non-const reference to a vector or other container, and then filling it, is one of the best. Let me just to remember also the possibility to return a pointer to a new() allocated structure within the function (or 0 in case of failure); this won't fragment your memory as much as a garbage collector would, so as long as you delete the object properly, providing a correct destructor if needed, you don't have to worry about it. Let me remember also the possibility to build a class to manage your input source, so that the data structure is a member of the class, and the functions filling it are methods. Bests, Giancarlo Niccolai Back to top Alf P. Steinbach Guest Posted: Thu Sep 29, 2005 11:16 am    Post subject: Re: How to do this in c++ * [email]john.burton (AT) jbmail (DOT) com[/email]: Quote: I used to use c++ a lot but recently I've been using java and pythons and I'm finding it a lot harder to think of the best way to do something in C++ Imagine I want a function that returns a "list" of selected users from a big list of users, or a database or something based on some selection. In python I might use a generator function which yielded the next relevent user each time it was called. In java I'd probably return an Iterator which iterated through the selected users probably using an instance of an inner class to implement the iterator that just iterated through the items I wanted. It's not so clear how I'd best do this in C++? The root of this problem is not with iterators (yes, they are awkward in C++) but with lifetime and copying issues for the object you return. To solve that use a smart-pointer to a dynamically allocated object -- which is essentially what you have in Java and Python. The simplest kind of smart-pointer is std::auto_ptr, which just transfers ownership, and perhaps that will be sufficient for you. Otherwise, consider e.g. boost::shared_ptr. For that you'll have to install the boost library. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? Back to top John Burton Guest Posted: Thu Sep 29, 2005 12:00 pm    Post subject: Re: How to do this in c++ Bob Hairgrove wrote: Quote: On 29 Sep 2005 03:17:50 -0700, [email]john.burton (AT) jbmail (DOT) com[/email] wrote: I could create a new list and return that by value but that seems somewhat inefficient so say the least. Returning any kind of reference to an existing list seems fraught with memory management issues over how to create and delete the memory. Use std::vector<> and let it manage the memory for you. Create an empty list or vector and pass a non-const reference to it to your function that selects the names, etc. from the database or file or whatever, and let the function fill up the vector. You are right that returning the vector or list by value would most likely be too inefficient. Of course. I used to know all this. I've been doing java and python too long! Thanks for this suggestion and the others who posted! Back to top Shezan Baig Guest Posted: Thu Sep 29, 2005 12:21 pm    Post subject: Re: How to do this in c++ [email]john.burton (AT) jbmail (DOT) com[/email] wrote: Quote: I used to use c++ a lot but recently I've been using java and pythons and I'm finding it a lot harder to think of the best way to do something in C++ Imagine I want a function that returns a "list" of selected users from a big list of users, or a database or something based on some selection. If you would like to keep as close as possible to Java semantics, then the function should return shared_ptr< list, where you allocate the list inside your function. And the shared_ptr will delete it automatically. This is semantically similar to what you would do in Java. Hope this helps, -shez- Back to top john.burton@jbmail.com Guest Posted: Thu Sep 29, 2005 12:34 pm    Post subject: Re: How to do this in c++ Shezan Baig wrote: Quote: john.burton (AT) jbmail (DOT) com wrote: I used to use c++ a lot but recently I've been using java and pythons and I'm finding it a lot harder to think of the best way to do something in C++ Imagine I want a function that returns a "list" of selected users from a big list of users, or a database or something based on some selection. If you would like to keep as close as possible to Java semantics, then the function should return shared_ptr< list, where you allocate the list inside your function. And the shared_ptr will delete it automatically. This is semantically similar to what you would do in Java. Thanks for the suggestion. I'm not particularly interested in replicating the java solution, I just couldn't remember how this was done in c++ after a year or so of doing very little with it! I think that passing in an empty list or vector to fill in is the best solution for what I wanted. Back to top persenaama Guest Posted: Thu Sep 29, 2005 1:06 pm    Post subject: Re: How to do this in c++ Actually, it's not too difficult to write a filter which returns a new iterator for each iteration. The usage pattern could boil down to something similiar to this: container::iterator i = container_instance.begin(); for ( ; i != container_instance.end(); ++i ) { if ( filter(*i) ) { // object passes the inspection, what now boss? } } Looks straightforward to me, iterate and inspect.. if your goal is to immediately act it might be a good idea to do it inside the if-block. If you really want to keep record around, it depends.. you might want to return std::vector<yourtype*>, or you might want to copy objects then have std::vector<yourtype> if you intend to pass the new container around for some other processing. It really depends what you have in mind after you do the filtering.. I cannot go and assume what you have in mind, I design code to solve a problem and in your case there isn't any.. just return whatever you want as long as it gets the job done. How optimal in memory, time, etc.. you can and want solution to be depends on how you going to access the results.. I just don't know that. Ofcourse, if I knew what this is used for might result in a completely different answer. :) Back to top Ali Çehreli Guest Posted: Thu Sep 29, 2005 8:37 pm    Post subject: Re: How to do this in c++ <john.burton (AT) jbmail (DOT) com> wrote Quote: Imagine I want a function that returns a "list" of selected users from a big list of users, or a database or something based on some selection. It's not so clear how I'd best do this in C++? The best way would be to get yourself familiar with the C++ standard library and use its features. In this case, you need the copy_if algorithm, which unfortunately in not part of the standard (There is some story to that.) copy_if may be present as an extension in your library, you can find it on the web, or you can write it yourself. I found one at http://www.bauklimatik-dresden.de/privat/nicolai/html/en/cpp.html#algorithms template <typename InputIt, typename OutputIt, typename Predicate> OutputIt copy_if (InputIt first, InputIt last, OutputIt result, Predicate pred) { while (first != last) { if (pred(*first)) *result++ = *first; ++first; } return result; } With that, what you need is very simple: typedef list<User> Users; // The original users Users const users = get_users_from_somewhere(); Users selected_ones; copy_if(users.begin(), users.end(), back_inserter(selected_ones), my_predicate); my_predicate is either a function that returns bool depending on whether a User meets your criteria: bool my_predicate(User const & user) { /* ... */ } or it is a function object: class MyPredicate { /* ... */ bool operator() (User const & user) { /* ... */ } }; In that case, the usage might be: copy_if(users.begin(), users.end(), back_inserter(selected_ones), MyPredicate(/* parameters to help the predicate */)); If you wrap copy_if in one of your functions, then you end up with a nice little function: Users get_selected_ones(Users const & users) { Users selected_ones; copy_if(users.begin(), users.end(), back_inserter(selected_ones), my_predicate); return selected_ones; } And yes, return by value! It is a myth that returning by value is slow. Usually it is the named-return-value optimization (NRVO) that takes care of the seeming cost of a copy. Or as in most cases, even if the copy cannot be elided, the cost is non-detectable by the users of the program. Quote: C++ iterators are totally different things to java iterators and it doesn't seem at all elegent to define a new type of iterator which only iterates though selected items. Use function templates like copy_if to avoid typedefing or defining iterators. Quote: I could create a new list and return that by value but that seems somewhat inefficient so say the least. That is premature optimization. What you want to return is a list of users; just return them. I know that some groups use the method of receiving references for the results to be put in: int get_selected_ones(Users const & users, Users & selected_ones); What are the semantics if selected_ones is not empty? a) clear it anyway? b) append? c) return an error? It is simply not clear. Ali Back to top John Harrison Guest Posted: Thu Sep 29, 2005 9:02 pm    Post subject: Re: How to do this in c++ Quote: I know that some groups use the method of receiving references for the results to be put in: int get_selected_ones(Users const & users, Users & selected_ones); What are the semantics if selected_ones is not empty? a) clear it anyway? b) append? c) return an error? It is simply not clear. Clear it anyway or, more precisely, build up the results in a local variable and then swap the local variable with selected_ones as the last action of get_selected_ones(), this is for exception safety. Since passing in a reference is meant as a more efficient alternative to return by value options b and c do not apply since they would not be possible using return by value. The best fit with return by value is option a. john Quote: Ali Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.