C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  T::operator int () const ambiguous with T::operator Handle ( Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Tim Clacy Guest Posted: Fri Apr 29, 2005 4:45 pm    Post subject: T::operator int () const ambiguous with T::operator Handle ( Please illuminate; what operator of class 'Event' will get matched for these two cases [see Event outline later]: Event ev1; Event ev2; // Case 1 // if (ev1) ; // Case 2 // if (ev1 || ev2) ; I would have thought 'operator int ()' is the only obvious match, but my compiler generates errors: "ambiguous 3-way choice of conversion from 'struct Event' in Boolean context All I'm trying to do is wrap an OS event in a class that can be treated like a Boolean; can anyone explain what the ambiguity is and how it can be resolved (preferable without casts)? Best regards Tim typedef struct tagHandle { }* Handle; bool eventIsSignalled(Handle hEvent); struct Event { : bool IsSignalled() const { return eventIsSignalled(*this); } operator int () const { return IsSignalled(); } operator Handle () const { return handle; } private: Handle handle; }; Back to top Shezan Baig Guest Posted: Fri Apr 29, 2005 4:59 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Tim Clacy wrote: Quote: typedef struct tagHandle { }* Handle; bool eventIsSignalled(Handle hEvent); struct Event { : bool IsSignalled() const { return eventIsSignalled(*this); } operator int () const { return IsSignalled(); } operator Handle () const { return handle; } ints *and* pointers are convertible to boolean (with the same priority). So it is ambigious. Quote: private: Handle handle; }; Back to top abecedarian@spambob.com Guest Posted: Fri Apr 29, 2005 8:28 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Shezan Baig wrote: Quote: Tim Clacy wrote: typedef struct tagHandle { }* Handle; try instead: struct Handle { SomeType* realHandle; }; Quote: bool eventIsSignalled(Handle hEvent); struct Event { : bool IsSignalled() const { return eventIsSignalled(*this); } operator int () const { return IsSignalled(); } operator Handle () const { return handle; } ints *and* pointers are convertible to boolean (with the same priority). So it is ambigious. Back to top ben Guest Posted: Sat Apr 30, 2005 7:18 am    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Quote: All I'm trying to do is wrap an OS event in a class that can be treated like a Boolean; can anyone explain what the ambiguity is and how it can be resolved (preferable without casts)? ... typedef struct tagHandle { }* Handle; bool eventIsSignalled(Handle hEvent); struct Event { : bool IsSignalled() const { return eventIsSignalled(*this); } operator int () const { return IsSignalled(); } operator Handle () const { return handle; } private: Handle handle; }; If Event::IsSignalled does fine, why use operator int? Event itself is not a handle, why provide Handle operator? To simplify: struct Event { private: Handle handle; public: Handle GetHandle() const { return handle; } operator int() const { return eventIsSignalled(); } //... }; Back to top Tim Guest Posted: Sat Apr 30, 2005 1:55 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand <abecedarian (AT) spambob (DOT) com> wrote Quote: Shezan Baig wrote: Tim Clacy wrote: typedef struct tagHandle { }* Handle; try instead: struct Handle { SomeType* realHandle; }; Shezen, Hi. I can't try right now but how would that help? Quote: ints *and* pointers are convertible to boolean (with the same priority). So it is ambigious. Hmm, that means that no C++ class can be used in an 'if' statement if it has more than one operator that returns a POD type... doesn't it? Back to top Tim Guest Posted: Sat Apr 30, 2005 2:04 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand "ben" <benhongh (AT) hotmail (DOT) com> wrote Quote: All I'm trying to do is wrap an OS event in a class that can be treated like a Boolean; can anyone explain what the ambiguity is and how it can be resolved (preferable without casts)? ... typedef struct tagHandle { }* Handle; bool eventIsSignalled(Handle hEvent); struct Event { : bool IsSignalled() const { return eventIsSignalled(*this); } operator int () const { return IsSignalled(); } operator Handle () const { return handle; } private: Handle handle; }; If Event::IsSignalled does fine, why use operator int? Event itself is not a handle, why provide Handle operator? To simplify: struct Event { private: Handle handle; public: Handle GetHandle() const { return handle; } operator int() const { return eventIsSignalled(); } //... }; Ben, Hi. It looks like I'll have to run with your suggestion. 'operator int'; is convenient in 'if' and || expressions for exactly the same reason that you would prefer this: if (a || b || c) ; to this: if ((a != false) || (b != false) || (c != false)) ; I'm a little disappointed that 'operator in' isn't a better match that 'operator void*' in an 'if' expression. This is surely a case of 'C' crippling the power of C++ operator overloading isn't it? Regards Tim Back to top Julián Albo Guest Posted: Sat Apr 30, 2005 2:07 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Tim wrote: Quote: ints *and* pointers are convertible to boolean (with the same priority). So it is ambigious. Hmm, that means that no C++ class can be used in an 'if' statement if it has more than one operator that returns a POD type... doesn't it? Just define a conversion to bool for that class. -- Salu2 Back to top Pete Becker Guest Posted: Sat Apr 30, 2005 2:27 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Tim wrote: Quote: Hmm, that means that no C++ class can be used in an 'if' statement if it has more than one operator that returns a POD type... doesn't it? No. POD types aren't inherently convertible to bool. Some of them (in particular, builtin types) are. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top Ian Guest Posted: Sun May 01, 2005 3:01 am    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Julián Albo wrote: Quote: Tim wrote: ints *and* pointers are convertible to boolean (with the same priority). So it is ambigious. Hmm, that means that no C++ class can be used in an 'if' statement if it has more than one operator that returns a POD type... doesn't it? Just define a conversion to bool for that class. And then live with all the bizarre run time errors such an operator will give you! Use a bool member like isOK(), the extra typing is worth it. Ian Back to top Julián Albo Guest Posted: Sun May 01, 2005 4:25 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Ian wrote: Quote: Julián Albo wrote: Tim wrote: ints *and* pointers are convertible to boolean (with the same priority). So it is ambigious. Hmm, that means that no C++ class can be used in an 'if' statement if it has more than one operator that returns a POD type... doesn't it? Just define a conversion to bool for that class. And then live with all the bizarre run time errors such an operator will give you! Use a bool member like isOK(), the extra typing is worth it. I do. But the question was if that can be done. -- Salu2 Back to top Tim Guest Posted: Mon May 02, 2005 6:46 am    Post subject: Re: T::operator int () const ambiguous with T::operator Hand "Julián Albo" <JULIANALBO (AT) terra (DOT) es> wrote Quote: Tim wrote: ints *and* pointers are convertible to boolean (with the same priority). So it is ambigious. Hmm, that means that no C++ class can be used in an 'if' statement if it has more than one operator that returns a POD type... doesn't it? Just define a conversion to bool for that class. -- Salu2 Julián, Hi. Are you saying that 'operator bool' would be matched in preference to 'operator int' for an 'if' expression? Back to top Julián Albo Guest Posted: Mon May 02, 2005 3:00 pm    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Tim wrote: Quote: Hi. Are you saying that 'operator bool' would be matched in preference to 'operator int' for an 'if' expression? Don't checked the standard but gcc accepts it: // boolconv.cpp #include <iostream> class Convertible { public: operator int () { return 0; } operator long () { return 0; } operator unsigned int () { return 0; } operator unsigned long () { return 0; } operator bool () { return true; } }; int main () { Convertible c; if (c) std::cout << "Hello, world" << std::endl; } $ g++ -Wall -pedantic -std=c++98 -o boolconv boolconv.cpp $ ./boolconv Hello, world But as Ian remarked, it's better to avoid that. -- Salu2 Back to top Tim Clacy Guest Posted: Tue May 03, 2005 7:52 am    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Julián Albo wrote: Quote: Tim wrote: Hi. Are you saying that 'operator bool' would be matched in preference to 'operator int' for an 'if' expression? Don't checked the standard but gcc accepts it: // boolconv.cpp #include class Convertible { public: operator int () { return 0; } operator long () { return 0; } operator unsigned int () { return 0; } operator unsigned long () { return 0; } operator bool () { return true; } }; int main () { Convertible c; if (c) std::cout << "Hello, world" << std::endl; } $ g++ -Wall -pedantic -std=c++98 -o boolconv boolconv.cpp $ ./boolconv Hello, world But as Ian remarked, it's better to avoid that. Julián, Cheers. Adding the 'operator bool' has fixed the issue; an 'Event' can used as Boolean and a Handle now. Why is it better to avoid 'operator bool'? Back to top Karl Heinz Buchegger Guest Posted: Tue May 03, 2005 8:33 am    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Tim Clacy wrote: Quote: Cheers. Adding the 'operator bool' has fixed the issue; an 'Event' can used as Boolean and a Handle now. Why is it better to avoid 'operator bool'? The problem with all those conversion operators is that it enables the compiler to use them even in situations where you don't want them -> The compiler will use those operators to synthesize expressions which logically make no sense just to find a way to compile your code at all. Eg. if a class has an operator bool() class Test { operator bool() { ... } }; then Test t; int i = t + 5; becomes a valid expression (because false -> 0, true -> 1) That is exactly why eg. the standard stream classes don't have an operator bool() -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Tim Clacy Guest Posted: Tue May 03, 2005 10:24 am    Post subject: Re: T::operator int () const ambiguous with T::operator Hand Karl Heinz Buchegger wrote: Quote: Tim Clacy wrote: Cheers. Adding the 'operator bool' has fixed the issue; an 'Event' can used as Boolean and a Handle now. Why is it better to avoid 'operator bool'? The problem with all those conversion operators is that it enables the compiler to use them even in situations where you don't want them -> The compiler will use those operators to synthesize expressions which logically make no sense just to find a way to compile your code at all. Eg. if a class has an operator bool() class Test { operator bool() { ... } }; then Test t; int i = t + 5; becomes a valid expression (because false -> 0, true -> 1) That is exactly why eg. the standard stream classes don't have an operator bool() Hmm, that's pretty disgusting... but couldn't you prevent this kind of anarchy by defining private operators (e.g. operator + (Event const &, int), operator + (Event const&, unsigned), etc.). In fact, couldn't you do all the dirty work in a template? template<typename T> class DisableStupidImplicitConversions { int operator + (int rhs); : }; class Event : DisableStupidImplicitConversions<T> { : }; Are there any other reasons for not defining an 'operator bool'? It seems like one of the most useful operators for bi-stable objects (like Event, Interrupt, Timer...). Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.