C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why does T need to be copy constructible for std::min?          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Matthias Hofmann Guest Posted: Sat Oct 09, 2004 6:58 pm    Post subject: Why does T need to be copy constructible for std::min? Hello! I got a question which I have recently posted on comp.lang.c++.moderated without getting an answer, so I'm trying to find one here... Section 25.3.7 is about the requirements of T in std::min (and std::max), which is that T be LessThanComparable and CopyConstructible. As far as I know, std::min is defined and usually implemented like so: template <class T> inline const T& min( const T& a, const T& b ) { return a < b ? a : b; } As everyone can see, the objects of type T are passed by reference, and a reference is returned as well. So I wonder: where does the compiler need to make a copy of a or b, so that the requirement of T being CopyConstructible is justified? Best regards, Matthias Hofmann --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Victor Bazarov Guest Posted: Mon Oct 11, 2004 6:20 pm    Post subject: Re: Why does T need to be copy constructible for std::min? Matthias Hofmann wrote: Quote: I got a question which I have recently posted on comp.lang.c++.moderated without getting an answer, so I'm trying to find one here... Section 25.3.7 is about the requirements of T in std::min (and std::max), which is that T be LessThanComparable and CopyConstructible. As far as I know, std::min is defined and usually implemented like so: template <class T> inline const T& min( const T& a, const T& b ) { return a < b ? a : b; } As everyone can see, the objects of type T are passed by reference, and a reference is returned as well. So I wonder: where does the compiler need to make a copy of a or b, so that the requirement of T being CopyConstructible is justified? I think the reason is in the possibility to use a temporary object as either of the arguments to 'min'. The Standard says that when binding a reference to a temporary is happening, another temporary may be necessary. Even if the compiler can avoid creating the other temporary, the ability has to exist. See 12.2 and related clauses. Victor --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top David Abrahams Guest Posted: Tue Oct 12, 2004 9:57 pm    Post subject: Re: Why does T need to be copy constructible for std::min? [email]v.Abazarov (AT) comAcast (DOT) net[/email] (Victor Bazarov) writes: Quote: Matthias Hofmann wrote: I got a question which I have recently posted on comp.lang.c++.moderated without getting an answer, so I'm trying to find one here... Section 25.3.7 is about the requirements of T in std::min (and std::max), which is that T be LessThanComparable and CopyConstructible. As far as I know, std::min is defined and usually implemented like so: template <class T> inline const T& min( const T& a, const T& b ) { return a < b ? a : b; } As everyone can see, the objects of type T are passed by reference, and a reference is returned as well. So I wonder: where does the compiler need to make a copy of a or b, so that the requirement of T being CopyConstructible is justified? I think the reason is in the possibility to use a temporary object as either of the arguments to 'min'. The Standard says that when binding a reference to a temporary is happening, another temporary may be necessary. Even if the compiler can avoid creating the other temporary, the ability has to exist. See 12.2 and related clauses. I thought about that, but it doesn't seem like a good enough reason to require a copy ctor. After all, the argument could be an lvalue. I think this is a defect or at least an overly restrictive specification. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Victor Bazarov Guest Posted: Wed Oct 13, 2004 4:03 am    Post subject: Re: Why does T need to be copy constructible for std::min? David Abrahams wrote: Quote: v.Abazarov (AT) comAcast (DOT) net (Victor Bazarov) writes: Matthias Hofmann wrote: I got a question which I have recently posted on comp.lang.c++.moderated without getting an answer, so I'm trying to find one here... Section 25.3.7 is about the requirements of T in std::min (and std::max), which is that T be LessThanComparable and CopyConstructible. As far as I know, std::min is defined and usually implemented like so: template <class T> inline const T& min( const T& a, const T& b ) { return a < b ? a : b; } As everyone can see, the objects of type T are passed by reference, and a reference is returned as well. So I wonder: where does the compiler need to make a copy of a or b, so that the requirement of T being CopyConstructible is justified? I think the reason is in the possibility to use a temporary object as either of the arguments to 'min'. The Standard says that when binding a reference to a temporary is happening, another temporary may be necessary. Even if the compiler can avoid creating the other temporary, the ability has to exist. See 12.2 and related clauses. I thought about that, but it doesn't seem like a good enough reason to require a copy ctor. After all, the argument could be an lvalue. I think this is a defect or at least an overly restrictive specification. Then you probably should request clarification on a particular clause of the Standard or the rationale behind a particular decision. So far, the Standard is what it is, and whatever I or you think doesn't really matter. I'd start another thread with the request (for clarification). V --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top David Abrahams Guest Posted: Wed Oct 13, 2004 7:01 pm    Post subject: Re: Why does T need to be copy constructible for std::min? [email]v.Abazarov (AT) comAcast (DOT) net[/email] (Victor Bazarov) writes: Quote: David Abrahams wrote: [email]v.Abazarov (AT) comAcast (DOT) net[/email] (Victor Bazarov) writes: Matthias Hofmann wrote: I got a question which I have recently posted on comp.lang.c++.moderated without getting an answer, so I'm trying to find one here... Section 25.3.7 is about the requirements of T in std::min (and std::max), which is that T be LessThanComparable and CopyConstructible. As far as I know, std::min is defined and usually implemented like so: template <class T> inline const T& min( const T& a, const T& b ) { return a < b ? a : b; } As everyone can see, the objects of type T are passed by reference, and a reference is returned as well. So I wonder: where does the compiler need to make a copy of a or b, so that the requirement of T being CopyConstructible is justified? I think the reason is in the possibility to use a temporary object as either of the arguments to 'min'. The Standard says that when binding a reference to a temporary is happening, another temporary may be necessary. Even if the compiler can avoid creating the other temporary, the ability has to exist. See 12.2 and related clauses. I thought about that, but it doesn't seem like a good enough reason to require a copy ctor. After all, the argument could be an lvalue. I think this is a defect or at least an overly restrictive specification. Then you probably should request clarification on a particular clause of the Standard or the rationale behind a particular decision. Why me? You can do it if you want something clarified. Quote: So far, the Standard is what it is, and whatever I or you think doesn't really matter. I think I have a pretty good idea how the process works, thanks ;-) Quote: I'd start another thread with the request (for clarification). Then please do. I have already forwarded the thread to the LWG chair so we could get it on the issues list. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Victor Bazarov Guest Posted: Thu Oct 14, 2004 4:49 am    Post subject: Re: Why does T need to be copy constructible for std::min? David Abrahams wrote: Quote: v.Abazarov (AT) comAcast (DOT) net (Victor Bazarov) writes: David Abrahams wrote: I thought about that, but it doesn't seem like a good enough reason to require a copy ctor. After all, the argument could be an lvalue. I think this is a defect or at least an overly restrictive specification. Then you probably should request clarification on a particular clause of the Standard or the rationale behind a particular decision. Why me? You can do it if you want something clarified. Wasn't it you who said "I think this is a defect or at least an overly restrictive specification". I don't think it's a defect, nor do I need anything clarified. You seemed to need clarification, that's why I suggested that you should request it. Quote: So far, the Standard is what it is, and whatever I or you think doesn't really matter. I think I have a pretty good idea how the process works, thanks ;-) I'd start another thread with the request (for clarification). Then please do. I have already forwarded the thread to the LWG chair so we could get it on the issues list. If you have forwarded it, then I don't need to do anything. V --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top David Abrahams Guest Posted: Thu Oct 14, 2004 8:25 pm    Post subject: Re: Why does T need to be copy constructible for std::min? [email]v.Abazarov (AT) comAcast (DOT) net[/email] (Victor Bazarov) writes: Quote: David Abrahams wrote: [email]v.Abazarov (AT) comAcast (DOT) net[/email] (Victor Bazarov) writes: David Abrahams wrote: I thought about that, but it doesn't seem like a good enough reason to require a copy ctor. After all, the argument could be an lvalue. I think this is a defect or at least an overly restrictive specification. Then you probably should request clarification on a particular clause of the Standard or the rationale behind a particular decision. Why me? You can do it if you want something clarified. Wasn't it you who said "I think this is a defect or at least an overly restrictive specification". I don't think it's a defect, nor do I need anything clarified. You seemed to need clarification, that's why I suggested that you should request it. I didn't, but thanks. I have formed an opinion and I don't need anyone to clarify anything. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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