C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  template for all derived classes          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Tony Guest Posted: Tue Jan 20, 2004 10:59 pm    Post subject: template for all derived classes Hi, C++ Gurus, I am considering the following problem. struct Scalar { double s; Scalar(double d):s(d) { } }; struct Length:public Scalar { static double convertor; Length(double l):Scalar(l) { } }; struct Area:public Scalar { static double convertor; Area(double l):Scalar(l) { } }; template<class S> S operator+(const S &s1, const S &s2) { S s(s1.s + s2.s); return s; } In the template, I want S to be derived class of Scalar, nothing else. How can I do it? If I allow S to be other types, operator+ may be called for some types accidently. This is what I want to avoid. Regards Tony [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jonathan Turkanis Guest Posted: Wed Jan 21, 2004 9:16 am    Post subject: Re: template for all derived classes "Tony" <tongru_huo (AT) netzero (DOT) com> wrote Quote: Hi, C++ Gurus, I am considering the following problem. struct Scalar { double s; Scalar(double d):s(d) { } }; struct Length:public Scalar { static double convertor; Length(double l):Scalar(l) { } }; struct Area:public Scalar { static double convertor; Area(double l):Scalar(l) { } }; template<class S> S operator+(const S &s1, const S &s2) { S s(s1.s + s2.s); return s; } In the template, I want S to be derived class of Scalar, nothing else. How can I do it? There are at least two ways. If there are no other overloads of operator+ in the picture (I'm being intentionally vague), you can do a static assert within the body of the function: template<class S> S operator+(const S &s1, const S &s2) { BOOST_STATIC_ASSERT(( is_base_and_derived<Scalar, S>::value )); S s(s1.s + s2.s); return s; } This causes a compile-time error is operator+ is called with S not a derived class of Scalar. (See http://www.boost.org/libs/static_assert/static_assert.htm and http://www.boost.org/libs/type_traits/index.htm for documentation.) If there are other overloads of operator+ around, you can use the enable_if utility described in the June 2003 issue of CUJ, and soon to be part of boost: template<class S> typename boost::enable_if< boost::is_base_and_derived S>::type operator+(const S &s1, const S &s2); This prevents overload resolution from selecting the above overload of operator+ unless S is derived from Scalar. For documentation, see CUJ, http://www.esva.net/~beman/boost_1_31_0_rc1.zip, or soon, http://www.boost.org/libs/utility/enable_if.html. HTH Jonathan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mario Guest Posted: Wed Jan 21, 2004 3:44 pm    Post subject: Re: template for all derived classes [email]tongru_huo (AT) netzero (DOT) com[/email] (Tony) wrote in message > Quote: template<class S> S operator+(const S &s1, const S &s2) { S s(s1.s + s2.s); return s; } In the template, I want S to be derived class of Scalar, nothing else. How can I do it? Hi, you can use the boost library Use "::boost::is_base_and_derived<T,U>::value" from boost/type_traits.hpp to get the relation between the classes and use "mpl::if_< >" from boosts mpl library to make a decision at compile time. You can get a structure failing to compile if the relations are not valid. hth Mario [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dan McLeran Guest Posted: Wed Jan 21, 2004 7:50 pm    Post subject: Re: template for all derived classes Quote: In the template, I want S to be derived class of Scalar, nothing else. How can I do it? If I allow S to be other types, operator+ may be called for some types accidently. This is what I want to avoid. See boost::type_traits, http://www.boost.org/libs/type_traits/index.htm, and boost::static_assert, http://www.boost.org/libs/static_assert/static_assert.htm. Here is an example: #include <iostream> #include <boost/type_traits.hpp> #include <boost/static_assert.hpp> using std::cout; using std::endl; struct Thing { }; struct Something : public Thing { void say() const { cout<<"I am a thing!"< } }; struct NotAThing { void say() const { cout<<"I am not a thing!"< } }; template void SaySomething(U const& arg) { using namespace boost; BOOST_STATIC_ASSERT((is_base_and_derived<Thing,U>::value)); arg.say(); } int main() { Something a; NotAThing b; SaySomething(a); //SaySomething(b);//will cause a compiler error return 0; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Peter Dimov Guest Posted: Fri Jan 23, 2004 10:26 am    Post subject: Re: template for all derived classes [email]tongru_huo (AT) netzero (DOT) com[/email] (Tony) wrote in message news:<9396fe12.0401200902.645a38a4 (AT) posting (DOT) google.com>... Quote: Hi, C++ Gurus, I am considering the following problem. struct Scalar { double s; Scalar(double d):s(d) { } }; struct Length:public Scalar { static double convertor; Length(double l):Scalar(l) { } }; struct Area:public Scalar { static double convertor; Area(double l):Scalar(l) { } }; template<class S> S operator+(const S &s1, const S &s2) { S s(s1.s + s2.s); return s; } In the template, I want S to be derived class of Scalar, nothing else. How can I do it? Without repeating what others already said, here's one more possibility: template<class Derived> struct Scalar { double s; Scalar(double d):s(d) { } }; struct Length:public Scalar<Length> { static double convertor; Length(double l):Scalar<Length>(l) { } }; struct Area:public Scalar<Area> { static double convertor; Area(double l):Scalar<Area>(l) { } }; template<class S> Scalar<S> operator+(const Scalar<S> &s1, const Scalar<S> &s2) { Scalar<S> s(s1.s + s2.s); return s; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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