C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  refrence from return value ?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Nils O. Selåsdal Guest Posted: Wed Sep 29, 2004 1:11 pm    Post subject: refrence from return value ? given a prototye: string foo(); What would be the diffrence in calling it, and assigning the return value in string a = foo(); vs string &a = foo(); (is the last one even legal?) Back to top Nicolas Pavlidis Guest Posted: Wed Sep 29, 2004 1:20 pm    Post subject: Re: refrence from return value ? "Nils O. Selåsdal" <NOS (AT) Utel (DOT) no> writes: Quote: given a prototye: string foo(); What would be the diffrence in calling it, and assigning the return value in string a = foo(); vs string &a = foo(); AFAIK the last version will lead to an segfault on the next try to read from it. foo() returns a string, which is copied befor it is retured, so a temporary object will be assignet to a reference, but the temporary object will be dead after the assignement, and so the reference show to anyway. I did not try it out, but I think it will no work, but I'm no expert, maybe my interpretation is wrong. Kid regards, Nicolas -- Quote: Nicolas Pavlidis | Elvis Presly: | |__ | Student of SE & KM | "Into the goto" | |__| | [email]pavnic (AT) sbox (DOT) tugraz.at[/email] | ICQ #320057056 | | -------------------University of Technology, Graz----------------| Back to top Karl Heinz Buchegger Guest Posted: Wed Sep 29, 2004 1:51 pm    Post subject: Re: refrence from return value ? Nicolas Pavlidis wrote: Quote: "Nils O. Selåsdal" <NOS (AT) Utel (DOT) no> writes: given a prototye: string foo(); What would be the diffrence in calling it, and assigning the return value in string a = foo(); vs string &a = foo(); AFAIK the last version will lead to an segfault on the next try to read from it. foo() returns a string, which is copied befor it is retured, so a temporary object will be assignet to a reference, but the temporary object will be dead after the assignement, and so the reference show to anyway. I did not try it out, but I think it will no work, but I'm no expert, maybe my interpretation is wrong. Well. Yes. The guys writing the standard took provisions for that. The rule is: The temporary lives as long as the reference to it lives. But honestly: I wouldn't count on all compilers to implement this correctly in all cases. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Peter van Merkerk Guest Posted: Wed Sep 29, 2004 2:13 pm    Post subject: Re: refrence from return value ? Nicolas Pavlidis wrote: Quote: "Nils O. Selåsdal" <NOS (AT) Utel (DOT) no> writes: given a prototye: string foo(); What would be the diffrence in calling it, and assigning the return value in string a = foo(); vs string &a = foo(); AFAIK the last version will lead to an segfault on the next try to read from it. foo() returns a string, which is copied befor it is retured, so a temporary object will be assignet to a reference, but the temporary object will be dead after the assignement, and so the reference show to anyway. In this particular example the temporary object bound to the reference will persist for the lifetime of the reference (see also the C++ Standard chapter 12.2). So no problem here. Note that when a temporary object is passed by reference to a function or constructor, the temporary object will only exist during the call. Once the call has completed the temporary object will be destroyed, even if the function or constructor has bound the temporary object to another reference. -- Peter van Merkerk peter.van.merkerk(at)dse.nl Back to top John Harrison Guest Posted: Wed Sep 29, 2004 2:51 pm    Post subject: Re: refrence from return value ? "Nicolas Pavlidis" <pavnic (AT) sbox (DOT) tugraz.at> wrote Quote: "Nils O. Selåsdal" <NOS (AT) Utel (DOT) no> writes: given a prototye: string foo(); What would be the diffrence in calling it, and assigning the return value in string a = foo(); vs string &a = foo(); AFAIK the last version will lead to an segfault on the next try to read from it. foo() returns a string, which is copied befor it is retured, so a temporary object will be assignet to a reference, but the temporary object will be dead after the assignement, and so the reference show to anyway. I did not try it out, but I think it will no work, but I'm no expert, maybe my interpretation is wrong. The code is not legal because a temporary cannot be bound to a non-const reference. But there are no temporary lifetime issues here I think so. const string &a = foo(); would be perfectly legal and OK. john Back to top Sumit Rajan Guest Posted: Wed Sep 29, 2004 3:59 pm    Post subject: Re: refrence from return value ? Karl Heinz Buchegger wrote: Quote: Well. Yes. The guys writing the standard took provisions for that. The rule is: The temporary lives as long as the reference to it lives. Don't you think that would contradict 12.2/2. I'm quoting the relevant line only (the whole thing sounds terribly confusing anyway -- like everything else in the Standard): A temporary bound to the returned value in a function return statement (6.6.3) persists until the function exits. Regards, Sumit. -- Sumit Rajan <sumit DOT RAJAN AT gmail DOT com> Back to top Sumit Rajan Guest Posted: Wed Sep 29, 2004 4:03 pm    Post subject: Re: refrence from return value ? Sumit Rajan wrote: Quote: Karl Heinz Buchegger wrote: Well. Yes. The guys writing the standard took provisions for that. The rule is: The temporary lives as long as the reference to it lives. Don't you think that would contradict 12.2/2. I'm quoting the relevant ^^^^^^^ Oops, sorry. That was 12.2/5 and not 12.2/2. -- Sumit Rajan <sumit DOT RAJAN AT gmail DOT com> Back to top Karl Heinz Buchegger Guest Posted: Wed Sep 29, 2004 4:16 pm    Post subject: Re: refrence from return value ? Sumit Rajan wrote: Quote: Karl Heinz Buchegger wrote: Well. Yes. The guys writing the standard took provisions for that. The rule is: The temporary lives as long as the reference to it lives. Don't you think that would contradict 12.2/2. I'm quoting the relevant line only (the whole thing sounds terribly confusing anyway -- like everything else in the Standard): A temporary bound to the returned value in a function return statement ************************* Quote: (6.6.3) persists until the function exits. That's a different case. We are not talking about what happens during the execution of the return statement. The case under discussion happens after the function has exited. The above applies in situations like this std::string foo() { return std::string(); } Here a temporary string object is created for the return value. That temporary exists until the function exits (has terminated). But that is very different to what you actually do with the returned object: const std::string& a = foo(); In foo()'s return statment a temporary was constructed. The return type of foo() specifies that foo returns 'by value' thus a copy of that temporary is to be made. And since that second temporary is bound to a reference, it will live on as long as the reference exists. But the temporary created during execution of the return statement is not the same temporary that gets bound to the reference. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Sumit Rajan Guest Posted: Wed Sep 29, 2004 4:36 pm    Post subject: Re: refrence from return value ? Karl Heinz Buchegger wrote: Quote: That's a different case. We are not talking about what happens during the execution of the return statement. The case under discussion happens after the function has exited. The above applies in situations like this std::string foo() { return std::string(); } Here a temporary string object is created for the return value. That temporary exists until the function exits (has terminated). But that is very different to what you actually do with the returned object: const std::string& a = foo(); In foo()'s return statment a temporary was constructed. The return type of foo() specifies that foo returns 'by value' thus a copy of that temporary is to be made. And since that second temporary is bound to a reference, it will live on as long as the reference exists. But the temporary created during execution of the return statement is not the same temporary that gets bound to the reference. I guess I had misunderstood your initial statement. Sorry for the trouble. -- Sumit Rajan <sumit DOT rajan AT gmail DOT com> 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 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.