C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Dangling Reference          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message kaede Guest Posted: Mon Oct 27, 2003 5:32 pm    Post subject: Dangling Reference Hi all, I have a quick question regarding binding temporary to const reference. I wrote a little toy program to test it, but I don't seems to understand the behaviour. Consider the following code snipplet: // Data class, also overload operator<< class Data { ... }; // Holds the data class DataHolder { public: DataHolder(Data data_): m_data(data_) {} // 1 void dump() const { cout << m_data << "n"; } private: const Data& m_data; }; Scenerio A) int main() { DataHolder holder( Data(....) ); // this line will dump out some garbage. My understanding is that // The member variable m_data binds to a temporary copied of Data // and when the DataHolder constructor completes, the temporary is // destroyed that, the m_data reference is no longer valid holder.dump(); return 1; } Scenerio B) int main() { Data data( .. ); DataHolder holder( data ); // this line successfully dump the contents of data. I am not sure why? // shouldn't it behaves the same Scenerio A? holder.dump(); return 1; } Scenerio C) Change the DataHolder constructor to the following DataHolder(Data& data_): m_data(data_) {} or DataHolder(const Data& data_): m_data(data_) {} int main() { Data data( .. ); DataHolder holder( data ); // this line successfully dump the contents of data. I am not sure why? holder.dump(); return 1; } Scenerio D) Now, rather than using Data as a class, we change data to the following: enum Data { LONG, FLOAT, INT }; When re-running scenerio A - C, the contents of data seems to be dumping out correctly. I am not sure why? Can anyone enlighten me? Your help is appreciated. Thanks!!!! Kaede Back to top Ron Natalie Guest Posted: Mon Oct 27, 2003 5:45 pm    Post subject: Re: Dangling Reference "kaede" <kaedeRukawa_jp (AT) hotmail (DOT) com> wrote Quote: When re-running scenerio A - C, the contents of data seems to be dumping out correctly. I am not sure why? Because one of the insidious features of undefined behavior is that things might appear to work normally (only to fail later). In Scenario A, you have it right. The temporary ceases to be as soon as the constructor finishes. In Scenario B, you have undefined behavior. What happens if you change the member of data after holder is constructed but before you print? I suspect either that you are benefiting from the eliding of the temporary or somehow the reference ends up seated at the variable in main. In either case it's just a conincidence. Scenario C, you don't have undefined behavior. There is no temporary. The reference in holder refers to the variable data in main. It lives longer than holder does in fact. In Scenario D, you just get more coincidental behavior, but you've changed the data object's size so that it's really small and may end up being parked in a register. Back to top kaede Guest Posted: Mon Oct 27, 2003 10:53 pm    Post subject: Re: Dangling Reference "Ron Natalie" <ron (AT) sensor (DOT) com> wrote Quote: "kaede" <kaedeRukawa_jp (AT) hotmail (DOT) com> wrote When re-running scenerio A - C, the contents of data seems to be dumping out correctly. I am not sure why? Because one of the insidious features of undefined behavior is that things might appear to work normally (only to fail later). In Scenario A, you have it right. The temporary ceases to be as soon as the constructor finishes. In Scenario B, you have undefined behavior. What happens if you change the member of data after holder is constructed but before you print? I suspect either that you are benefiting from the eliding of the temporary or somehow the reference ends up seated at the variable in main. In either case it's just a conincidence. Scenario C, you don't have undefined behavior. There is no temporary. The reference in holder refers to the variable data in main. It lives longer than holder does in fact. In Scenario D, you just get more coincidental behavior, but you've changed the data object's size so that it's really small and may end up being parked in a register. Thanks ... I think I see what is happening. Regards, Kaede Back to top kaede Guest Posted: Tue Oct 28, 2003 9:51 pm    Post subject: Re: Dangling Reference "Ron Natalie" <ron (AT) sensor (DOT) com> wrote Quote: "kaede" <kaedeRukawa_jp (AT) hotmail (DOT) com> wrote When re-running scenerio A - C, the contents of data seems to be dumping out correctly. I am not sure why? Because one of the insidious features of undefined behavior is that things might appear to work normally (only to fail later). In Scenario A, you have it right. The temporary ceases to be as soon as the constructor finishes. In Scenario B, you have undefined behavior. What happens if you change the member of data after holder is constructed but before you print? I suspect either that you are benefiting from the eliding of the temporary or somehow the reference ends up seated at the variable in main. In either case it's just a conincidence. Scenario C, you don't have undefined behavior. There is no temporary. The reference in holder refers to the variable data in main. It lives longer than holder does in fact. In Scenario D, you just get more coincidental behavior, but you've changed the data object's size so that it's really small and may end up being parked in a register. I was thinking about it this morning and what would happened if I modified Scenerio C to the following: int main() { DataHolder* p = 0; { Data data( ... ); p = new DataHolder(data); } // (2) // would this cause undefined behaviour since data already get destroyed // after (2). or will the const& m_data prolongs the scope of data? p->getType(); } Similarily, what if it is in different translational unit: // a.cxx void A::registerData(Data& data_) { DataHolder* p = new DataHolder(data_); C::registerDataHolder(p); } // b.cxx void B::someFunc() { Data data(..); A.registerData(data); } Also, I read something about when temporary binds to const&, the const& that binds to the temporary will prolongs the life of the temporary. That is the temporary will be destroyed when the const& is destoyed. Is that correct, I don't think I understand the meaning and the usefulless of it. Can anyone explain it to me. Thanks =) Kaede Back to top lilburne Guest Posted: Tue Oct 28, 2003 9:52 pm    Post subject: Re: Dangling Reference kaede wrote: Quote: I was thinking about it this morning and what would happened if I modified Scenerio C to the following: In both cases you'll have undefined behaviour. 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