* Re: Strange behavior with templates and G++ [not found] <201102171203.53710.pascal@francq.info> @ 2011-02-17 13:09 ` Jonathan Wakely 2011-02-17 14:05 ` Axel Freyn 0 siblings, 1 reply; 3+ messages in thread From: Jonathan Wakely @ 2011-02-17 13:09 UTC (permalink / raw) To: Pascal Francq; +Cc: gcc-help [Moved from the gcc list] On 17 February 2011 11:03, Pascal Francq wrote: > Hi, > While compiling the following code, I got an error : > > > template<class C> class Super > { > public: > Super(void) {} > void Test(C*) {} > }; > > class A > { > public: > A(void) {} > }; > > class A1 : public A > { > public: > A1(void) : A() {} > }; > > class A2 : public A > { > public: > A2(void) : A() {} > }; > > class Super2 : public Super<A1>, public Super<A2> > { > public: > Super2(void) {} > }; > > void Test(void) > { > Super2 T; > A1* ptr; > T.Test(ptr); > } > > > The compiler gives me the following error for the Test() function: > error: request for member ‘Test’ is ambiguous > error: candidates are: void Super<C>::Test(C*) [with C = A2] > error: void Super<C>::Test(C*) [with C = A1] > But here the call refers clearly to the second method. The error still appears > if A1 and A2 do not inherit from a same root class. If I replace the code with > an explicit call it works: > T.Super<A1>::Test(ptr) > But this make the code less cleaner. > > Is this a problem related to a misunderstood concept from me, a wrong > implementation or a technical problem of g++ ? The ambiguity has nothing to do with templates, the code can be reduced to: class A1 { public: void Test(A1*) {} }; class A2 { public: void Test(A2*) {} }; class Super2 : public A1, public A2 { }; void Test(void) { Super2 T; A1* ptr; T.Test(ptr); } This is covered by 10.2 [class.member.lookup] in the C++ standard, which says that a name is ambiguous if found in more than one base class. That makes the lookup ill-formed, before overload resolution is attempted. ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Strange behavior with templates and G++ 2011-02-17 13:09 ` Strange behavior with templates and G++ Jonathan Wakely @ 2011-02-17 14:05 ` Axel Freyn 2011-02-17 16:30 ` Jonathan Wakely 0 siblings, 1 reply; 3+ messages in thread From: Axel Freyn @ 2011-02-17 14:05 UTC (permalink / raw) To: gcc-help On Thu, Feb 17, 2011 at 12:22:53PM +0000, Jonathan Wakely wrote: > [Moved from the gcc list] > > On 17 February 2011 11:03, Pascal Francq wrote: > > Hi, > > While compiling the following code, I got an error : > > > > > > template<class C> class Super > > { > > public: > > Â Â Â Â Super(void) {} > > Â Â Â Â void Test(C*) {} > > }; > > > > class A > > { > > public: > > Â Â Â Â A(void) {} > > }; > > > > class A1 : public A > > { > > public: > > Â Â Â Â A1(void) : A() {} > > }; > > > > class A2 : public A > > { > > public: > > Â Â Â Â A2(void) : A() {} > > }; > > > > class Super2 : public Super<A1>, public Super<A2> > > { > > public: > > Â Â Â Â Super2(void) {} > > }; > > > > void Test(void) > > { > > Â Â Â Â Super2 T; > > Â Â Â Â A1* ptr; > > Â Â Â Â T.Test(ptr); > > } > > > > > > The compiler gives me the following error for the Test() function: > > Â Â Â Â error: request for member âTestâ is ambiguous > > Â Â Â Â error: candidates are: Â void Super<C>::Test(C*) [with C = A2] > > Â Â Â Â error: Â Â Â Â Â Â Â Â Â Â Â Â Â void Super<C>::Test(C*) [with C = A1] > > But here the call refers clearly to the second method. The error still appears > > if A1 and A2 do not inherit from a same root class. If I replace the code with > > an explicit call it works: > > Â Â Â Â T.Super<A1>::Test(ptr) > > But this make the code less cleaner. > > > > Is this a problem related to a misunderstood concept from me, a wrong > > implementation or a technical problem of g++ ? > > > The ambiguity has nothing to do with templates, the code can be reduced to: > > class A1 > { > public: > void Test(A1*) {} > }; > > class A2 > { > public: > void Test(A2*) {} > }; > > class Super2 : public A1, public A2 > { > }; > > void Test(void) > { > Super2 T; > A1* ptr; > T.Test(ptr); > } > > This is covered by 10.2 [class.member.lookup] in the C++ standard, > which says that a name is ambiguous if found in more than one base > class. That makes the lookup ill-formed, before overload resolution > is attempted. In addition: it is not necessary to use everywhere an explicit call. You can import the function by adding "using"-definitions. With: class Super2 : public A1, public A2 { public: using A1::Test; using A2::Test; }; you can call T.Test(ptr) directly, and C++ will do correct overload resolution -- both with and without templates HTH, Axel ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Strange behavior with templates and G++ 2011-02-17 14:05 ` Axel Freyn @ 2011-02-17 16:30 ` Jonathan Wakely 0 siblings, 0 replies; 3+ messages in thread From: Jonathan Wakely @ 2011-02-17 16:30 UTC (permalink / raw) To: gcc-help; +Cc: Axel Freyn On 17 February 2011 13:16, Axel Freyn wrote: > On Thu, Feb 17, 2011 at 12:22:53PM +0000, Jonathan Wakely wrote: >> [Moved from the gcc list] >> >> On 17 February 2011 11:03, Pascal Francq wrote: >> > Hi, >> > While compiling the following code, I got an error : >> > >> > >> > template<class C> class Super >> > { >> > public: >> > Super(void) {} >> > void Test(C*) {} >> > }; >> > >> > class A >> > { >> > public: >> > A(void) {} >> > }; >> > >> > class A1 : public A >> > { >> > public: >> > A1(void) : A() {} >> > }; >> > >> > class A2 : public A >> > { >> > public: >> > A2(void) : A() {} >> > }; >> > >> > class Super2 : public Super<A1>, public Super<A2> >> > { >> > public: >> > Super2(void) {} >> > }; >> > >> > void Test(void) >> > { >> > Super2 T; >> > A1* ptr; >> > T.Test(ptr); >> > } >> > >> > >> > The compiler gives me the following error for the Test() function: >> > error: request for member ‘Test’ is ambiguous >> > error: candidates are: void Super<C>::Test(C*) [with C = A2] >> > error: void Super<C>::Test(C*) [with C = A1] >> > But here the call refers clearly to the second method. The error still appears >> > if A1 and A2 do not inherit from a same root class. If I replace the code with >> > an explicit call it works: >> > T.Super<A1>::Test(ptr) >> > But this make the code less cleaner. >> > >> > Is this a problem related to a misunderstood concept from me, a wrong >> > implementation or a technical problem of g++ ? >> >> >> The ambiguity has nothing to do with templates, the code can be reduced to: >> >> class A1 >> { >> public: >> void Test(A1*) {} >> }; >> >> class A2 >> { >> public: >> void Test(A2*) {} >> }; >> >> class Super2 : public A1, public A2 >> { >> }; >> >> void Test(void) >> { >> Super2 T; >> A1* ptr; >> T.Test(ptr); >> } >> >> This is covered by 10.2 [class.member.lookup] in the C++ standard, >> which says that a name is ambiguous if found in more than one base >> class. That makes the lookup ill-formed, before overload resolution >> is attempted. > > In addition: it is not necessary to use everywhere an explicit call. > You can import the function by adding "using"-definitions. With: > class Super2 : public A1, public A2 > { > public: > using A1::Test; > using A2::Test; > }; > you can call T.Test(ptr) directly, and C++ will do correct overload > resolution -- both with and without templates Right, in that case name lookup finds two overloads of "Test" without looking in any base classes, then overload resolution selects the right one. ^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2011-02-17 14:08 UTC | newest] Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- [not found] <201102171203.53710.pascal@francq.info> 2011-02-17 13:09 ` Strange behavior with templates and G++ Jonathan Wakely 2011-02-17 14:05 ` Axel Freyn 2011-02-17 16:30 ` Jonathan Wakely
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).