C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Problems initializing a static reference member variable          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Soulshift Guest Posted: Sun Oct 26, 2003 5:00 am    Post subject: Problems initializing a static reference member variable Hi, Given the following: class A { public: A(){} private: A(A&){} }; class B { private: static A& staticMember; }; A& B::staticMember = A(); MSVC 6's compiler complains that it needs to access A's copy constructor: error C2248: 'A::A' : cannot access private member declared in class 'A' However, when I make the copy constructor public, it never gets called anyway. Also, there doesn't seem to be a requirement for a copy constructor in this case. Am I somehow doing this wrongly or is this yet another MSVC6 compiler bug? Thanks, Linus Tan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Sun Oct 26, 2003 11:08 am    Post subject: Re: Problems initializing a static reference member variable "Soulshift" <ltan (AT) do (DOT) not.spam.epicentrix.net> wrote in message Quote: class A { public: A(){} private: A(A&){} }; class B { private: static A& staticMember; }; A& B::staticMember = A(); MSVC 6's compiler complains that it needs to access A's copy constructor: error C2248: 'A::A' : cannot access private member declared in class 'A' However, when I make the copy constructor public, it never gets called anyway. Also, there doesn't seem to be a requirement for a copy constructor in this case. Am I somehow doing this wrongly or is this yet another MSVC6 compiler bug? Is this one of those cases where the copy constructor is required to exist, though the compiler may optimize away calling it? Anyway, you create a temporary A object, store the reference of it. Remember that at the semicolon the program destroys the temporary A object. So your reference is invalid. -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Christoph Schulz Guest Posted: Sun Oct 26, 2003 3:04 pm    Post subject: Re: Problems initializing a static reference member variable Hello! Soulshift <ltan (AT) do (DOT) not.spam.epicentrix.net> wrote: Quote: Hi, Given the following: class A { public: A(){} private: A(A&){} }; class B { private: static A& staticMember; }; A& B::staticMember = A(); MSVC 6's compiler complains that it needs to access A's copy constructor: error C2248: 'A::A' : cannot access private member declared in class 'A' However, when I make the copy constructor public, it never gets called anyway. Also, there doesn't seem to be a requirement for a copy constructor in this case. Am I somehow doing this wrongly or is this yet another MSVC6 compiler bug? First, initializing a non-const reference with a temporary is not valid C++. Because the expression "A()" creates an (unnamed) temporary object, this object can only be bound to a "const A &" (in your example). Second, the copy constructor needs to be accessible because of paragraph 8.5.3/5 in the standard. For details, see the thread http://groups.google.com/groups?selm=1B097B7096652942BBF2F1ED436CC4E2052F77C8%40MSPMAILV2.retek.int Basically, if your member was of type "const A &" and the copy- constructor accepted a "const A &" parameter, a C++ implementation would be allowed to bind "B::staticMember" during initialization to a temporary which is copy-constructed from the initializer expression. That means that "A()" would be first copied (by calling the copy-constructor) and then "staticMember" would be bind to that copy. The standard does not mandate the copying, but it doesn't prohibit it either, so the copy-constructor must be accessible at the point of reference binding. Again, this all applies *only* to binding to const references, and not to non-const references, as in your example. Regards, Christoph [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Sun Oct 26, 2003 7:12 pm    Post subject: Re: Problems initializing a static reference member variable On 26 Oct 2003 01:00:02 -0400, "Soulshift" <ltan (AT) do (DOT) not.spam.epicentrix.net> wrote: Quote: class A { public: A(){} private: A(A&){} }; class B { private: static A& staticMember; }; A& B::staticMember = A(); MSVC 6's compiler complains that it needs to access A's copy constructor: error C2248: 'A::A' : cannot access private member declared in class 'A' However, when I make the copy constructor public, it never gets called anyway. Also, there doesn't seem to be a requirement for a copy constructor in this case. Am I somehow doing this wrongly or is this yet another MSVC6 compiler bug? This is amusing. The code as written is invalid because an rvalue, A(), may not be bound to a non-const reference. It seems that the compiler is accepting this for way backward compatibility. If we change it to A const& in both places, then it is valid to bind the rvalue to a const reference, but a copy constructor is required to be callable. This is the error you are seeing. Making the copy ctor public removes this problem, but introduces another binding to a non-const reference because your copy ctor does not take a reference to const. In addition to making the ctor public, it must also take a const&. The call may be optimized out as you saw, but it must be callable. You could also use the rvalue to lvalue conversion of operator= to remove the error keeping the copy ctor private and taking a reference to non-const. A& B::staticMember = A() = A(); Note that the first symbol '=' is the initializer syntax while the second is the compiler generated operator=. This is equivalent to A& B::staticMember((A() = A())); The extra set of parens are needed to change the function declaration to a variable definition. Now we construct two A objects, copy assign the right one to the left one returning an A& from operator= which is then bound to the staticMember. Now both A objects are destructed leaving a dangling reference. But, there is no syntax error. ;) John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Sun Oct 26, 2003 7:13 pm    Post subject: Re: Problems initializing a static reference member variable On 26 Oct 2003 06:08:43 -0500, "Siemel Naran" <SiemelNaran (AT) REMOVE (DOT) att.net> wrote: Quote: Is this one of those cases where the copy constructor is required to exist, though the compiler may optimize away calling it? Yes, iff you accept the old concept of binding a non-const reference to an rvalue. Quote: Anyway, you create a temporary A object, store the reference of it. Remember that at the semicolon the program destroys the temporary A object. So your reference is invalid. I suspect that the backward compiler also extends the lifetime of the temporary in the same way that it would for a const reference. It will likely work on that compiler only and fail to compile on the later releases or other vendors. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Soulshift Guest Posted: Sun Oct 26, 2003 7:13 pm    Post subject: Re: Problems initializing a static reference member variable "Siemel Naran" <SiemelNaran (AT) REMOVE (DOT) att.net> wrote Quote: "Soulshift" <ltan (AT) do (DOT) not.spam.epicentrix.net> wrote in message class A { public: A(){} private: A(A&){} }; class B { private: static A& staticMember; }; A& B::staticMember = A(); MSVC 6's compiler complains that it needs to access A's copy constructor: error C2248: 'A::A' : cannot access private member declared in class 'A' However, when I make the copy constructor public, it never gets called anyway. Also, there doesn't seem to be a requirement for a copy constructor in this case. Am I somehow doing this wrongly or is this yet another MSVC6 compiler bug? Is this one of those cases where the copy constructor is required to exist, though the compiler may optimize away calling it? Seems highly possible. I'm compiling with optimizations off though, so I'm not sure if the compiler would do that particular optimization. Quote: Anyway, you create a temporary A object, store the reference of it. Remember that at the semicolon the program destroys the temporary A object. So your reference is invalid. I thought it might be as well, but oddly enough it works. In fact, I verified that the destructor for A gets called when the program exits. So... What's the right way to do this? - Linus Tan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Sun Oct 26, 2003 11:58 pm    Post subject: Re: Problems initializing a static reference member variable On 26 Oct 2003 14:13:52 -0500, "Soulshift" <ltan (AT) do (DOT) not.spam.epicentrix.net> wrote: Quote: So... What's the right way to do this? Why do you want a static reference non-object refering to an object? The compiler will likely optimize it out since references are not objects. You may as well have an object. static A staticMember; .... A B::staticMember; // is default constructed John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Mon Oct 27, 2003 12:02 am    Post subject: Re: Problems initializing a static reference member variable "Soulshift" <ltan (AT) do (DOT) not.spam.epicentrix.net> wrote in message Quote: So your reference is invalid. I thought it might be as well, but oddly enough it works. In fact, I verified that the destructor for A gets called when the program exits. See response by John that explains why it works. Quote: So... What's the right way to do this? I'd venture for, (1) Make a default constructed A object in an unnamed namespace. (2) Make B::staticMember point/refer to the object. // B.cpp #include "B.hpp" namespace { A a; } A& B::staticMember = a; -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Glen Low Guest Posted: Tue Oct 28, 2003 5:30 am    Post subject: Re: Problems initializing a static reference member variable Quote: Anyway, you create a temporary A object, store the reference of it. Remember that at the semicolon the program destroys the temporary A object. So your reference is invalid. I thought it might be as well, but oddly enough it works. In fact, I verified that the destructor for A gets called when the program exits. Hmm? According to 12.2/5, you are binding a reference to a temporary (even though strictly speaking it should be a const reference), so it persists for the lifetime of the reference, which in your case is until the end of the program. Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Soulshift Guest Posted: Wed Oct 29, 2003 9:30 am    Post subject: Re: Problems initializing a static reference member variable "John Potter" <jpotter (AT) falcon (DOT) lhup.edu> wrote Quote: On 26 Oct 2003 14:13:52 -0500, "Soulshift" [email]ltan (AT) do (DOT) not.spam.epicentrix.net[/email]> wrote: So... What's the right way to do this? Why do you want a static reference non-object refering to an object? The compiler will likely optimize it out since references are not objects. You may as well have an object. static A staticMember; ... A B::staticMember; // is default constructed John Ach, that was so blindingly obvious I completely failed to realize it. Thanks for your other informative responses as well :) - Linus Tan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] 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 (Moderated) All times are GMT Page 1 of 1   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.