why template version is prefered in this case
in [C++]
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From: Ike Naar on 11 Aug 2010 16:37 In article <48423290-31d8-4e62-bdd1-9db86fa61033(a)v15g2000yqe.googlegroups.com>, junvi <junvi.byhh(a)gmail.com> wrote: > [...] >template <typename T> >void f(const T a) { > cout<<"Template version."<<endl; >} >void f(const int* a) { > cout<<"Plain version."<<endl; >} >int main() { > int *a=0; > f(a); > return 0; >} > >the output is Plain version., I don't understand why is the case Calling plain ``f'' with ``a'' as an argument requires an implicit conversion from ``int*'' to ``const int*'', while no conversion is required for the templated f, so templated f is a better match. Change the declaration of ``a'' in main to ``const int *a = 0;'' then plain ``f'' will be chosen. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Jia-sen on 11 Aug 2010 16:31 On Aug 10, 3:41 pm, junvi <junvi.b...(a)gmail.com> wrote: > #include <iostream> > using namespace std; > > template <typename T> > void f(const T a) { > cout<<"Template version."<<endl; > > } > > void f(const int* a) { > cout<<"Plain version."<<endl; > > } > > int main() { > > int *a=0; > > f(a); > > return 0; > > } > > the output is Plain version., I don't understand why is the case { quoted clc++m banner removed; please do it yourself. -mod } The point is that non-const to const conversion is still a conversion. The argument 'a' is non-const, so template version matches better than the plain one. Try define 'a' as 'const int* a=0' and see which version is called. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: red floyd on 11 Aug 2010 22:15 On Aug 10, 12:41 pm, junvi <junvi.b...(a)gmail.com> wrote: > #include <iostream> > using namespace std; > > template <typename T> > void f(const T a) { > cout<<"Template version."<<endl; > > } > > void f(const int* a) { > cout<<"Plain version."<<endl; > > } > > int main() { > > int *a=0; > > f(a); > > return 0; > > } > > the output is Plain version., I don't understand why is the case Your question doesn't make sense. You ask "Why template version is preferred". it's not. You get the non-template version, as you should. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Christian Broom on 11 Aug 2010 22:15 The code doesn't make much sense in general. As is, the specification says that a pointer to null results in undefined behavior. My compiler (VC++08) calls the template function instead. To be sure which function you call, give 'a' a proper reference and use the form: f<T>(a) // Specifically call the template function with a type T. To be really sure which function is called, don't be silly enough to declare two functions with the same name. It is usually a sign of bad design and merits refactoring. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Florian on 12 Aug 2010 08:25
On 12 ao�t, 15:15, Christian Broom <soulbea...(a)gmail.com> wrote: > The code doesn't make much sense in general. As is, the specification > says that a pointer to null results in undefined behavior. My compiler > (VC++08) calls the template function instead. To be sure which > function you call, give 'a' a proper reference and use the form: Use a NULL pointer is a UB, but here the pointer isn't use (juste define), so there isn't UB, just normal behavior. > To be really sure which function is called, don't be silly enough to > declare two functions with the same name. It is usually a sign of bad > design and merits refactoring. Overload template function is a good solution to solve the problem of partial specialization, so have two functions with the same name could be good (and I thing that overload is better than specialization for template function, but I don't find a good source which explains that, perhaps the book of Alexandrescu and Sutter). -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |