C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Spec. question: STL and default constructors          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Garrett Kajmowicz Guest Posted: Tue Jan 04, 2005 1:18 pm    Post subject: Spec. question: STL and default constructors Quick question. If I create a std::vector<T>, is T required to have a default constructor? I know that the spec requires an assignment operator or copy constructor, but I can't find anything about a default constructor. Please point out reference in standard if there isn't one. If not, may I ask the reasoning behind not requiring default constructors. I can tell you that it is causing me a bit of pain (I'm going an independent implementation of the C++ library for embedded systems: cxx.uclibc.org) Thank you for the information. - Garrett Kajmowicz --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top James Dennett Guest Posted: Tue Jan 04, 2005 2:17 pm    Post subject: Re: Spec. question: STL and default constructors Garrett Kajmowicz wrote: Quote: Quick question. If I create a std::vector<T>, is T required to have a default constructor? Only if no initial value is specified as a constructor argument to std::vector<T>. E.g. a default constructor for T is required if I write std::vector<T> uses_default_ctor(5); but not if I write T initial_value(42); std::vector<T> copies_only(5, initial_value); Quote: I know that the spec requires an assignment operator or copy constructor, but I can't find anything about a default constructor. Please point out reference in standard if there isn't one. There is not one, so I can't point it out. Quote: If not, may I ask the reasoning behind not requiring default constructors. It makes std::vector more generic, without any perceived serious difficulty to implementers. Quote: I can tell you that it is causing me a bit of pain (I'm going an independent implementation of the C++ library for embedded systems: cxx.uclibc.org) Is there a reason why you can't do the normal thing of copy constructing, from a default-constructed instance if none is supplied? -- James --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top kanze@gabi-soft.fr Guest Posted: Wed Jan 05, 2005 3:48 pm    Post subject: Re: Spec. question: STL and default constructors Garrett Kajmowicz wrote: Quote: Quick question. If I create a std::vector<T>, is T required to have a default constructor? No. Quote: I know that the spec requires an assignment operator or copy constructor, but I can't find anything about a default constructor. Please point out reference in standard if there isn't one. That's going to be difficult. Pointing out the reference when there isn't one, I mean. And that's exactly the situation. There is no requirement that T have a default constructor. So obviously, we cannot point out where this requirement doesn't appear. Quote: If not, may I ask the reasoning behind not requiring default constructors. Because it isn't needed, and there was no desire to impose artificial constraints. Quote: I can tell you that it is causing me a bit of pain. (I'm going an independent implementation of the C++ library for embedded systems: cxx.uclibc.org) I'm not sure how this could be a pain. You're allowed to provide a default constructor for T, if you want one. The standard library just won't normally use it, that's all. You do have to provide copy and assignment. And that can be a pain. When it is, of course, the answer is such types don't belong in standard containers. (Often, in such cases, what you really want is a container of pointers to the objects.) -- James Kanze GABI Software http://www.gabi-soft.fr 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 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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