C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Concepts and conversions          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Guest Posted: Fri Apr 20, 2007 11:07 am    Post subject: Concepts and conversions I have a question about how function parameter conversions will work when the function signature is declared in a concept. Say you have the following types (specifically defined for the conversions between them): struct X {}; struct Y { Y(); Y(X); }; struct Z { Z(); Z(Y); }; and a concept which deals with the 'middle' type: concept Pootle<class T> { void f(T, Y); } If I understand correctly, the following type can model this concept: struct S {}; void f(const S&, Z); concept_map Pootle<S> {} since Z is convertible to Y. My question is, if a constrained function template uses Pootle as follows: template <Pootle P> void g(P&& p) { f(p, X()); } will this work with S as the template parameter? In particular, is a double conversion performed (from X->Y->Z)? Thanks 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.comeaucomputing.com/csc/faq.html ] Back to top Pedro Lamarão Guest Posted: Fri Apr 20, 2007 5:34 pm    Post subject: Re: Concepts and conversions On 20 abr, 08:07, tasjae...@gmail.com wrote: Quote: My question is, if a constrained function template uses Pootle as follows: template <Pootle P void g(P&& p) { f(p, X()); } will this work with S as the template parameter? In particular, is a double conversion performed (from X->Y->Z)? For this piece of code: // begin code struct X { }; struct Y { Y(); Y(X); }; struct Z { Z(); Z(Y); }; struct S { }; void f (S const&, Z); template <typename T> void g (T const& t) { f(t, X()); } void h () { g(S()); } //end code Visual Studio 2005 gives me the error: ------ Build started: Project: tmp2, Configuration: Debug Win32 ------ Compiling... test.cpp c:\documents and settings\psilva\meus documentos\visual studio 2005\projects\remoting\tmp2\test.cpp(12) : error C2664: 'f' : cannot convert parameter 2 from 'X' to 'Z' No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called c:\documents and settings\psilva\meus documentos\visual studio 2005\projects\remoting\tmp2\test.cpp(15) : see reference to function template instantiation 'void g<S>(const T &)' being compiled with [ T=S ] Build log was saved at "file://c:\Documents and Settings\psilva\Meus documentos\Visual Studio 2005\Projects\Remoting\tmp2\Debug \BuildLog.htm" tmp2 - 1 error(s), 0 warning(s) ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ========== I believe the reason is just that only one user-defined conversion is allowed. This rule should not be affected by the use of concept maps. -- Pedro Lamarão --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Guest Posted: Tue Apr 24, 2007 4:56 am    Post subject: Re: Concepts and conversions On Apr 20, 6:34 pm, Pedro Lamarão <pedro.lama...@gmail.com> wrote: Quote: On 20 abr, 08:07, tasjae...@gmail.com wrote: My question is, if a constrained function template uses Pootle as follows: template <Pootle P void g(P&& p) { f(p, X()); } will this work with S as the template parameter? In particular, is a double conversion performed (from X->Y->Z)? Visual Studio 2005 gives me the error: ------ Build started: Project: tmp2, Configuration: Debug Win32 ------ Compiling... test.cpp c:\documents and settings\psilva\meus documentos\visual studio 2005\projects\remoting\tmp2\test.cpp(12) : error C2664: 'f' : cannot convert parameter 2 from 'X' to 'Z' No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called snip I believe the reason is just that only one user-defined conversion is allowed. Yes, only a single conversion is allowed in the current language. Quote: This rule should not be affected by the use of concept maps. ConceptGCC also rejects my example code due to the double conversion, so that would seem to agree with you. But isn't there something broken about it? My S models Pootle. My g is using Pootle correctly within a constrained template. Result: syntax error. I think a double conversion is correct in this situation. If you look at N2081 (paragraph 3.3.3), it says that, since concept_map Pootle<S> {} does not provide a definition for f, a forwarding function is generated. Something like: concept_map Pootle<S> { void f(const S& s, Y y) { f(s, y); }} So there are effectively two functions, each one performing one conversion. 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.comeaucomputing.com/csc/faq.html ] Back to top Pedro Lamarão Guest Posted: Tue Apr 24, 2007 6:15 pm    Post subject: Re: Concepts and conversions On 24 abr, 01:56, tasjae...@gmail.com wrote: Quote: I think a double conversion is correct in this situation. If you look at N2081 (paragraph 3.3.3), it says that, since concept_map Pootle<S> {} does not provide a definition for f, a forwarding function is generated. Something like: concept_map Pootle<S> { void f(const S& s, Y y) { f(s, y); }} So there are effectively two functions, each one performing one conversion. I see your point. The function f in scope satisfies the concept but doesn't have the exact signature required, and the language in 3.3.3 tells us this exact f should be implicitly generated. But this seems strange. I would expect that the concept Pootle is fully satisfied by the type S -- because there is a proper f in scope. The language on section 3 seems to require that every concept_map provide this function f for S either explicitly or through an implicit (forwarding) definition, no matter what. So what should happen in this case? struct P { }; void f (P, Z) { } concept_map Pottle<P> { }; Should this fail because the concept_map will implicitly generate a new, ambiguous, f? Am I missing something? I admit I haven't studied N2081 in full detail. -- Pedro Lamarão --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Douglas Gregor Guest Posted: Wed Apr 25, 2007 5:47 pm    Post subject: Re: Concepts and conversions On Apr 20, 7:07 am, tasjae...@gmail.com wrote: Quote: I have a question about how function parameter conversions will work when the function signature is declared in a concept. [snip My question is, if a constrained function template uses Pootle as follows: template <Pootle P void g(P&& p) { f(p, X()); } will this work with S as the template parameter? In particular, is a double conversion performed (from X->Y->Z)? Yes, there is a double-conversion here. The first conversion is performed inside the g(), to turn the X into a Y when calling Pootle<P>::f. The second conversion occurs inside the concept map Pootle<S>. ConceptGCC does not properly compile this code, but the code is correct. - Doug --- [ 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.comeaucomputing.com/csc/faq.html ] Back to top Sylvester Hesp Guest Posted: Fri May 04, 2007 9:11 pm    Post subject: Re: Concepts and conversions "Douglas Gregor" <doug.gregor (AT) gmail (DOT) com> wrote in message news:1177513356.654845.101050 (AT) r3g2000prh (DOT) googlegroups.com... Quote: On Apr 20, 7:07 am, tasjae...@gmail.com wrote: I have a question about how function parameter conversions will work when the function signature is declared in a concept. [snip My question is, if a constrained function template uses Pootle as follows: template <Pootle P void g(P&& p) { f(p, X()); } will this work with S as the template parameter? In particular, is a double conversion performed (from X->Y->Z)? Yes, there is a double-conversion here. The first conversion is performed inside the g(), to turn the X into a Y when calling Pootle<P>::f. The second conversion occurs inside the concept map Pootle<S>. Is such a double conversion always performed, even if the 1st and 3rd types (X and Z in the example) are equal? E.g. struct X { X(Y); }; struct Y { Y(X); }; concept MyConcept<class T> { void f(T, Y); } struct MyStruct { }; void f(MyStruct, X); concept_map MyConcept<MyStruct> { }; template<MyConcept T> void foo(T t) { f(t, X()); // #1 MyConcept<T>::f(t, X()); // #2 }; Would #1 convert X to Y, and then back to X? Or would it first try to convert X directly to X, skipping the conversion that was introduced by the signature in the concept, and only if that fails do a double-conversion? Or would it simply find f(MyStruct,X) in the global scope and use that instead, bypassing the concept altogether? But in the latter case, what about #2? N2081 is a bit unclear about this. - Sylvester Hesp --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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