C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  User-defined manipulators that take arguments          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Dave Guest Posted: Sun Jan 23, 2005 3:02 am    Post subject: User-defined manipulators that take arguments Hello all, It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments. Please consider below which, for purposes of illustration, implements a manipulator similar to setw(). Presumably, I have broken a rule here and this program is non-conforming. However, I cannot see what rule that would be. May I request those who are so inclined to try and poke holes in it? Thank! Dave #include <iostream> #include <ostream> #include <string> using namespace std; template<typename T, typename C> class ManipInfra { public: ManipInfra( basic_ostream<C> &(*pFun)(basic_ostream<C> &, T), T val ): manipFun_(pFun), val_(val) { } void operator()(basic_ostream<C> &os) const { manipFun_(os, val_); } private: T val_; basic_ostream<C> &(*manipFun_)(basic_ostream<C> &, T); }; template<typename T, typename C> basic_ostream<C> &operator<<( basic_ostream const ManipInfra<T, C> &manip ) { manip(os); return os; } ostream &setTheWidth(ostream &os, int n) { os.width(n); return os; } ManipInfra<int, char> setWidth(int n) { return ManipInfra<int, char>(setTheWidth, n); } int main() { cout << setWidth(10) << right << "foo" << endl; } [ 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 Jan 23, 2005 10:15 am    Post subject: Re: User-defined manipulators that take arguments In article <10v62e72h95jh56 (AT) news (DOT) supernews.com>, Dave <better_cs_now (AT) yahoo (DOT) com> wrote: Quote: Hello all, It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments. Please consider below which, for purposes of illustration, implements a manipulator similar to setw(). Presumably, I have broken a rule here and this program is non-conforming. However, I cannot see what rule that would be. May I request those who are so inclined to try and poke holes in it? There are many portable ways to do this. The simplest is to write a class whose constructor takes the arguments of your proposed manipulator. and a member function say std::basic_ostream & do_it(std::ostream &); finally you want to overload operator << to 'output' your manimpulator and a function to do template type deduction for your class. something like the following is portable. #include #include <ostream> template <class T,class C,class CT> class ManipulatorClass { T val_; typedef std::basic_ostream<C,CT> stream_type; typedef stream_type & (*func_type)(stream_type &,T); func_type fun_; public: ManipulatorClass(func_type pFun,T val):fun_(pFun),val_(val){} stream_type & do_it (stream_type &os) const { fun_(os,val); return os; } }; template <class T,class C,class CT> inline ManipulatorClass<T,C,CT> Manipulator(std::basic_ostream<C,CT>(*pf)(T),T val) { return ManipulationClass<T,C,CT> Manipulator(pf,val); } template <class T,class C,class CT> inline std::basic_ostream<C,CT> & operator << (std::basic_ostream const ManipulatorClass<T,C,CT> &manip) { return mainip.do_it(os); } // example usuage std::cout << 5 << Manipulator(some_fun,20) << 26 << 'n'; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top dtmoore Guest Posted: Mon Jan 24, 2005 11:42 am    Post subject: Re: User-defined manipulators that take arguments Dave wrote: Quote: Hello all, It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments. Please consider below which, for purposes of illustration, implements a manipulator similar to setw(). Presumably, I have broken a rule here and this program is non-conforming. However, I cannot see what rule that would be. May I request those who are so inclined to try and poke holes in it? What happens if I try to use your manip on an ostream where I have used a non default char_traits? I am pretty sure that it will (at best) generate a compile time error. However, it is (probably) easy to fix that problem by adding another template parameter to your ManipInfra class to represent the char_traits of the basic_ostream. Dave Moore [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Jan 24, 2005 10:36 pm    Post subject: Re: User-defined manipulators that take arguments Dave wrote: Quote: It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments. Why do you say that? Why wouldn't the implémentation of, say, std::setw() be portable? -- 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 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Guest Posted: Tue Jan 25, 2005 10:47 am    Post subject: Re: User-defined manipulators that take arguments Quote: It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments. Why do you say that? Why wouldn't the implémentation of, say, std::setw() be portable? Josuttis, "The C++ Standard Library", page 613. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Tue Jan 25, 2005 7:59 pm    Post subject: Re: User-defined manipulators that take arguments In article <10vbkn4ip9q6o05 (AT) news (DOT) supernews.com>, Dave <better_cs_now (AT) yahoo (DOT) com> writes Quote: It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments. Why do you say that? Why wouldn't the implémentation of, say, std::setw() be portable? Josuttis, "The C++ Standard Library", page 613. Where does Josuttis say that it is not possible to write portable manipulators with parameters? AFAICS he does not touch on the subject. If you want to know about them try 'Standard C++ IOStreams & Locales' by Langer and Kreft (in particular, pages 179-185) -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael Brooks Guest Posted: Wed Jan 26, 2005 7:14 pm    Post subject: Re: User-defined manipulators that take arguments In article <0FmC$mVChm9BFwR2 (AT) robinton (DOT) demon.co.uk>, Francis Glassborow wrote: Quote: In article <10vbkn4ip9q6o05 (AT) news (DOT) supernews.com>, Dave [email]better_cs_now (AT) yahoo (DOT) com[/email]> writes Josuttis, "The C++ Standard Library", page 613. Where does Josuttis say that it is not possible to write portable manipulators with parameters? Reading the last paragraph of that page in my copy (10th printing) gave me that impression, too. Quote: AFAICS he does not touch on the subject. If you want to know about them try 'Standard C++ IOStreams & Locales' by Langer and Kreft (in particular, pages 179-185) I don't have this text - my first attempt would be to try a functor with operator() that could be called as required by the manipulator interface, and takes the parameters in the constructor. Is that anything like the way it should be done? If so, I don't see what is non-standard - am I just reading Josuttis' argument wrongly? -- Mick Brooks [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Thu Jan 27, 2005 9:19 pm    Post subject: Re: User-defined manipulators that take arguments Michael Brooks wrote: [concerning manipulators with parameters...] Quote: I don't have this text - my first attempt would be to try a functor with operator() that could be called as required by the manipulator interface, and takes the parameters in the constructor. It doesn't even have to be a functor. Any class for which you define a << or a >> can do the trick. The constructor takes the parameters and memorizes them, and in << or >>, you do whatever you have to set the flags according to the parameters. The standard defines the manipulators with parameters as functions which return an unspecified type of object, whose << and >> operators did the trick; early version of CFront had some macros, I think, to facilitate this. But personally, I don't see what it buys compared to just constructing the object explicitly. (Well, it does allow you to conform to a naming convention which requires manipulators and functions to start with small letters, and classes with caps. But I'd favor just making an exception to the convention, and say that the names of classes which act as manipulators should start with a small letter.) The function approach may be justified if some of the manipulator's parameters are characters (say for fill); in this case, the function allows for automatic type deduction, where as using the class directly requires the user to explicitly specify the character type. Quote: Is that anything like the way it should be done? If so, I don't see what is non-standard - am I just reading Josuttis' argument wrongly? Without knowing what you think Josuttis was complaining about, it's impossible to tell. As far as I know, the approach above is fully compliant with the standard, and portable to all hosted implementations. -- James Kanze GABI Software http://www.gabi-soft.fr Conseils en informatique orient=E9e objet/ Beratung in objektorientierter Datenverarbeitung 9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, 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 Michael Brooks Guest Posted: Sat Jan 29, 2005 4:28 am    Post subject: Re: User-defined manipulators that take arguments James Kanze wrote: Hi James, Quote: Michael Brooks wrote: [concerning manipulators with parameters...] I don't have this text - my first attempt would be to try a functor with operator() that could be called as required by the manipulator interface, and takes the parameters in the constructor. It doesn't even have to be a functor. Any class for which you define a << or a >> can do the trick. The constructor takes the parameters and memorizes them, and in << or >>, you do whatever you have to set the flags according to the parameters. I have to admit that I haven't looked into this too closely - the question about user defined manipulators with parameters not being standard caught my attention, and I looked up the page reference in Josuttis, and came to the same conclusion as the OP. Francis' assertion that this was wrong left me confused. Your explanation goes beyond the one in Josuttis, and I can't quite follow it at the moment - but I won't ask more questions until I've studied the problem better. Quote: Is that anything like the way it should be done? If so, I don't see what is non-standard - am I just reading Josuttis' argument wrongly? Without knowing what you think Josuttis was complaining about, it's impossible to tell. Well, I'm not sure what Josuttis was complaining about, either, but he seems pretty clear that there is a problem. A paragraph from section 13.6.1 - How Manipulators Work - has this: "The C++ standard library also contains manipulators with arguments. How these manipulators work exactly is implementation dependent, and there is no standard way to implement user-defined manipulators with arguments." Quote: As far as I know, the approach above is fully compliant with the standard, and portable to all hosted implementations. Thanks. I'll study it. -- Mick Brooks [email]michael.brooks (AT) physics (DOT) ox.ac.uk[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Jan 31, 2005 11:42 am    Post subject: Re: User-defined manipulators that take arguments Michael Brooks wrote: [...] Quote: Well, I'm not sure what Josuttis was complaining about, either, but he seems pretty clear that there is a problem. A paragraph from section 13.6.1 - How Manipulators Work - has this: "The C++ standard library also contains manipulators with arguments. How these manipulators work exactly is implementation dependent, and there is no standard way to implement user-defined manipulators with arguments." Which is perfectly correct, if you take the second use of standard in its general sense. The C++ standard library contains manipulators with arguments, but (as is the case with most of the library) only defines the effects, not what the implementation must do to achieve them. And there is no standard way to do this -- different implementations use different techniques. There are several standard conforming ways to do this, but none has become the standard way. -- 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 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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