C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Silly returning-an-object question          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Joe Van Dyk Guest Posted: Mon May 15, 2006 11:21 pm    Post subject: Silly returning-an-object question Hi, struct Foo {}; Foo oogie_boogie() { Foo a; return a; } oogie_boogie() returns a reference to a Foo object, right? (so, when I call oogie_boogie(), only one Foo object is created?) Thanks, Joe Back to top Roy Smith Guest Posted: Mon May 15, 2006 11:21 pm    Post subject: Re: Silly returning-an-object question Joe Van Dyk <joe.vandyk (AT) boeing (DOT) com> wrote: Quote: Hi, struct Foo {}; Foo oogie_boogie() { Foo a; return a; } oogie_boogie() returns a reference to a Foo object, right? No, it returns a Foo. If you wanted it to return a reference, you would have had to declare the function: Foo& oggie_boogie(); Quote: (so, when I call oogie_boogie(), only one Foo object is created?) In theory, the "Foo a;" line will construct a Foo, and then the "return a;" will copy-construct another Foo. In practice, almost any compiler will optimize away the extra copy-construct and just create the one object. Back to top Ian Collins Guest Posted: Mon May 15, 2006 11:21 pm    Post subject: Re: Silly returning-an-object question Joe Van Dyk wrote: Quote: Hi, struct Foo {}; Foo oogie_boogie() { Foo a; return a; } oogie_boogie() returns a reference to a Foo object, right? (so, when I call oogie_boogie(), only one Foo object is created?) It returns an instance of a Foo, not a reference. The compiler will probably optimise away the extra copy. -- Ian Collins. Back to top Guest Posted: Tue May 16, 2006 2:21 am    Post subject: Re: Silly returning-an-object question Roy Smith wrote: Quote: Joe Van Dyk <joe.vandyk (AT) boeing (DOT) com> wrote: Hi, struct Foo {}; Foo oogie_boogie() { Foo a; return a; } oogie_boogie() returns a reference to a Foo object, right? No, it returns a Foo. If you wanted it to return a reference, you would have had to declare the function: Foo& oggie_boogie(); But of course we would never want to return a reference to a temporary, would we? Back to top Roy Smith Guest Posted: Tue May 16, 2006 2:21 am    Post subject: Re: Silly returning-an-object question In article <1147744148.342209.257640 (AT) v46g2000cwv (DOT) googlegroups.com>, indrawati.yahya (AT) gmail (DOT) com wrote: Quote: Roy Smith wrote: Joe Van Dyk <joe.vandyk (AT) boeing (DOT) com> wrote: Hi, struct Foo {}; Foo oogie_boogie() { Foo a; return a; } oogie_boogie() returns a reference to a Foo object, right? No, it returns a Foo. If you wanted it to return a reference, you would have had to declare the function: Foo& oggie_boogie(); But of course we would never want to return a reference to a temporary, would we? It depends. We might want to be testing our memory corruption detection system Back to top Joe Van Dyk Guest Posted: Tue May 16, 2006 2:21 am    Post subject: Re: Silly returning-an-object question Ian Collins wrote: Quote: Joe Van Dyk wrote: Hi, struct Foo {}; Foo oogie_boogie() { Foo a; return a; } oogie_boogie() returns a reference to a Foo object, right? (so, when I call oogie_boogie(), only one Foo object is created?) It returns an instance of a Foo, not a reference. The compiler will probably optimise away the extra copy. Ah, ok. That optimization is probably why I was thinking that a reference was being returned. If oogie_boogie() returned a reference to a Foo object, then would Foo &f(oogie_boogie()); be ok? Joe Back to top Roy Smith Guest Posted: Tue May 16, 2006 2:21 am    Post subject: Re: Silly returning-an-object question In article <IzC4HI.E1v (AT) news (DOT) boeing.com>, Joe Van Dyk <joe.vandyk (AT) boeing (DOT) com> wrote: Quote: Ian Collins wrote: Joe Van Dyk wrote: Hi, struct Foo {}; Foo oogie_boogie() { Foo a; return a; } oogie_boogie() returns a reference to a Foo object, right? (so, when I call oogie_boogie(), only one Foo object is created?) It returns an instance of a Foo, not a reference. The compiler will probably optimise away the extra copy. Ah, ok. That optimization is probably why I was thinking that a reference was being returned. If oogie_boogie() returned a reference to a Foo object, then would Foo &f(oogie_boogie()); be ok? Joe OK for what? I'm not sure what you're trying to do here. Back to top Old Wolf Guest Posted: Tue May 16, 2006 3:22 am    Post subject: Re: Silly returning-an-object question Joe Van Dyk wrote: Quote: Ian Collins wrote: Joe Van Dyk wrote: struct Foo {}; Foo oogie_boogie() { Foo a; return a; } If oogie_boogie() returned a reference to a Foo object, then would Foo &f(oogie_boogie()); be ok? It would compile correctly. But in order to run correctly you would have to ensure that the returned object still existed when you try and use 'f' (which is not the case in the above version of oogie_boogie). Back to top Ron Natalie Guest Posted: Sun May 21, 2006 2:21 pm    Post subject: Re: Silly returning-an-object question indrawati.yahya (AT) gmail (DOT) com wrote: Quote: But of course we would never want to return a reference to a temporary, would we? ;) It's not a temporary, just a reference to an object whose lifetime has ended. Back to top Victor Bazarov Guest Posted: Sun May 21, 2006 4:21 pm    Post subject: Re: Silly returning-an-object question Sjouke Burry wrote: Quote: Ron Natalie wrote: indrawati.yahya (AT) gmail (DOT) com wrote: But of course we would never want to return a reference to a temporary, would we? ;) It's not a temporary, just a reference to an object whose lifetime has ended. You have just now defined temporary -)-) There has to be some difference between temporary and expired... Back to top Sjouke Burry Guest Posted: Sun May 21, 2006 4:21 pm    Post subject: Re: Silly returning-an-object question Ron Natalie wrote: Quote: indrawati.yahya (AT) gmail (DOT) com wrote: But of course we would never want to return a reference to a temporary, would we? ;) It's not a temporary, just a reference to an object whose lifetime has ended. You have just now defined temporary -)-) Back to top Ron Natalie Guest Posted: Mon May 22, 2006 11:21 am    Post subject: Re: Silly returning-an-object question Sjouke Burry wrote: Quote: Ron Natalie wrote: indrawati.yahya (AT) gmail (DOT) com wrote: But of course we would never want to return a reference to a temporary, would we? ;) It's not a temporary, just a reference to an object whose lifetime has ended. You have just now defined temporary -)-) No, I haven't. A temporary object has a precise meaning in C++ which does not include automatic variables. No object in C++ has an infinite lifetime. It behooves you to understand this and when you can use them. Back to top Sjouke Burry Guest Posted: Mon May 22, 2006 9:21 pm    Post subject: Re: Silly returning-an-object question Ron Natalie wrote: Quote: Sjouke Burry wrote: Ron Natalie wrote: indrawati.yahya (AT) gmail (DOT) com wrote: But of course we would never want to return a reference to a temporary, would we? ;) It's not a temporary, just a reference to an object whose lifetime has ended. You have just now defined temporary -)-) No, I haven't. A temporary object has a precise meaning in C++ which does not include automatic variables. No object in C++ has an infinite lifetime. It behooves you to understand this and when you can use them. You can choose whatever name you want for an object which just has died(gone out of scope, or whatever),as long as you dont use it ,reference it or treat it as if it still existed. If you dont call that a temporary existence, you are welcome to any other definition,just as long at it means the same. I have seen to many applications just sort of working because the reference seemed still to be valid,depending on what you (or your interrupt software)did. Back to top Luke Meyers Guest Posted: Tue May 23, 2006 4:21 am    Post subject: Re: Silly returning-an-object question Sjouke Burry wrote: Quote: Ron Natalie wrote: Sjouke Burry wrote: Ron Natalie wrote: It's not a temporary, just a reference to an object whose lifetime has ended. You have just now defined temporary -)-) No, I haven't. A temporary object has a precise meaning in C++ which does not include automatic variables. No object in C++ has an infinite lifetime. It behooves you to understand this and when you can use them. You can choose whatever name you want for an object which just has died(gone out of scope, or whatever),as long as you dont use it ,reference it or treat it as if it still existed. If you dont call that a temporary existence, you are welcome to any other definition,just as long at it means the same. Sjouke, you're not going to get very far (except into pointless arguments) by picking fights over terminology, especially when the C++ standard is on one side and you are on the other. The sort of reasoning you just gave could be used to justify introducing pointlessly confusing alternate definitions of technical terms which already possess clear and unambiguous meanings. For example, I could rationalize using the term "public" to describe private member functions in code that exists in the public domain, but it would be perverse and misleading to do so. The standard specifies different behavior for temporaries, distinct from other kinds of variables. Obfuscatory and misleading language based on what you *want* the word to mean only serves to obscure this distinction, which is a disservice to all. Quote: I have seen to many applications just sort of working because the reference seemed still to be valid,depending on what you (or your interrupt software)did. Nobody's advocating using objects which no longer exist. A thorough understanding of object lifetime rules and their implications is crucial to writing good C++, as is clear use of the proper terminology so you can communicate with your peers and continue to advance your knowledge. Luke Back to top        C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1 Powered by phpBB © 2001, 2006 phpBB Group