C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  why reture "const &"?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message zealotcat@gmail.com Guest Posted: Thu Dec 30, 2004 3:50 am    Post subject: why reture "const &"? template <class T> inline T const& max (T const& a, T const& b) { // if a < b then use b else use a return a } thanks very much!! Back to top Jonathan Mcdougall Guest Posted: Thu Dec 30, 2004 6:26 am    Post subject: Re: why reture "const &"? [email]zealotcat (AT) gmail (DOT) com[/email] wrote: Quote: why reture "const &"? template inline T const& max (T const& a, T const& b) { // if a < b then use b else use a return a } To prevent atrocities like std::max(a, b) = 2; and to allow passing and returning of rvalues. Jonathan Back to top David White Guest Posted: Thu Dec 30, 2004 8:25 am    Post subject: Re: why reture "const &"? "Jonathan Mcdougall" <jonathanmcdougall (AT) DELyahoo (DOT) ca> wrote Quote: zealotcat (AT) gmail (DOT) com wrote: why reture "const &"? template inline T const& max (T const& a, T const& b) { // if a < b then use b else use a return a } To prevent atrocities like std::max(a, b) = 2; Why is that an atrocity? You might want to assign the max of a and b to 2. DW Back to top Niels Dekker - no reply a Guest Posted: Thu Dec 30, 2004 9:09 am    Post subject: Re: why return "const &"? zealotcat at gmail.com wrote: Quote: template <class T inline T const& max (T const& a, T const& b) { // if a < b then use b else use a return a } If you return by value (T) instead of by const reference (T const&), you get a carbon copy of the maximum, not the original. When returning by (const) reference, the class T doesn't need to have a copy contructor. BTW, the "const" is necessary because your parameters are const references as well. You might consider having a non-const version as well: template inline T& max (T& a, T& b) { return a } See also http://www.aristeia.com/Papers/C++ReportColumns/jan95.pdf Kind regards, Niels Dekker http://www.xs4all.nl/~nd/dekkerware Back to top simont Guest Posted: Thu Dec 30, 2004 4:00 pm    Post subject: Re: why reture "const &"? David White wrote: Quote: "Jonathan Mcdougall" <jonathanmcdougall (AT) DELyahoo (DOT) ca> wrote in message news:KcNAd.30793$%k.2409376 (AT) weber (DOT) videotron.net... [email]zealotcat (AT) gmail (DOT) com[/email] wrote: why reture "const &"? template <class T inline T const& max (T const& a, T const& b) { // if a < b then use b else use a return a } To prevent atrocities like std::max(a, b) = 2; Why is that an atrocity? You might want to assign the max of a and b to 2. You /can't/ "assign the max of a and b to 2" in any meaningful sense. Writing Quote: max(5,7) = 2; doesn't "assign the max of 5 and 7", because you can't change that. What would it mean to do so? That I want globally to switch integer arithmetic to modulo 5? The function 'max' is usually considered to be returning the larger of two values, rather than performing side-effects or anything else. Unless you meant something like Quote: template T& larger_of( T& a, T& b ) { return a>b ? a : b; } where Quote: int a(5); int b(7); // change the larger integer for some reason ... larger_of(a,b) /= 2; might make sense. That isn't what max means, though. Quote: DW Back to top Niels Dekker - no reply a Guest Posted: Thu Dec 30, 2004 5:01 pm    Post subject: Re: why reture "const &"? zealotcat at gmail.com asked: Quote: why return "const &"? Jonathan Mcdougall replied: Quote: To prevent atrocities like std::max(a, b) = 2; What about calling some other non-const member function? Suppose I'm programming a GUI, and I have an ordered group of buttons on a window, so that I can say button A < button B. (E.g., based on tab order.) I might want to do: void MyFunc(Button & a, Button & b) { // Setting the "default property" (or whatever!) // of the button that has the maximum value. max(a, b).SetDefault(true); } Typically Button::SetDefault is a non-const member function. So I need a max(a, b) that returns a non-const reference! Kind regards, Niels Dekker http://www.xs4all.nl/~nd/dekkerware Back to top jeffc Guest Posted: Thu Dec 30, 2004 5:17 pm    Post subject: Re: why reture "const &"? "David White" <no (AT) email (DOT) provided> wrote Quote: To prevent atrocities like std::max(a, b) = 2; Why is that an atrocity? You might want to assign the max of a and b to 2. Maybe so, but that code won't do it. max is a function that returns a value, not a variable. Back to top Jonathan Mcdougall Guest Posted: Thu Dec 30, 2004 5:25 pm    Post subject: Re: why reture "const &"? jeffc wrote: Quote: "David White" <no (AT) email (DOT) provided> wrote in message news:1_OAd.2401$i6.24225 (AT) nasal (DOT) pacific.net.au... To prevent atrocities like std::max(a, b) = 2; Why is that an atrocity? You might want to assign the max of a and b to 2. Maybe so, but that code won't do it. max is a function that returns a value, not a variable. std::max is a function that returns a const reference, not a value nor a "variable". Jonathan Back to top Jonathan Mcdougall Guest Posted: Thu Dec 30, 2004 5:27 pm    Post subject: Re: why reture "const &"? Niels Dekker - no reply address wrote: Quote: zealotcat at gmail.com asked: why return "const &"? Jonathan Mcdougall replied: To prevent atrocities like std::max(a, b) = 2; What about calling some other non-const member function? That is not what std::max was designed to do. Quote: Suppose I'm programming a GUI, and I have an ordered group of buttons on a window, so that I can say button A < button B. (E.g., based on tab order.) I might want to do: void MyFunc(Button & a, Button & b) { // Setting the "default property" (or whatever!) // of the button that has the maximum value. max(a, b).SetDefault(true); } That's quite an ugly function, imho. std::max will do the correct thing if operator< is implemented correctly, but getting the maximum or minimum button makes no sense, whether they are order by their text of tab stop order. That's a misuse of std::max. Quote: Typically Button::SetDefault is a non-const member function. So I need a max(a, b) that returns a non-const reference! Roll you own. Jonathan Back to top Rolf Magnus Guest Posted: Thu Dec 30, 2004 7:58 pm    Post subject: Re: why reture "const &"? simont wrote: Quote: Writing | max(5,7) = 2; doesn't "assign the max of 5 and 7", because you can't change that. That wouldn't work anyway, neither with a max that returns (and takes) references (no matter whether they're const or not), nor with one that returns by value. So that wouldn't be a problem anyway. Quote: What would it mean to do so? An error. Quote: The function 'max' is usually considered to be returning the larger of two values, rather than performing side-effects or anything else. However, even std::max doesn't do this. It returns a const reference, not a value. Quote: Unless you meant something like | template <typename T | T& larger_of( T& a, T& b ) { return a>b ? a : b; } where | int a(5); | int b(7); | // change the larger integer for some reason ... | larger_of(a,b) /= 2; might make sense. That isn't what max means, though. max means whatever its programmer makes it mean. I don't see a need to call it larger_of instead of max. Back to top David White Guest Posted: Thu Dec 30, 2004 8:05 pm    Post subject: Re: why reture "const &"? "simont" <s_j_turner (AT) yahoo (DOT) co.uk> wrote Quote: David White wrote: "Jonathan Mcdougall" <jonathanmcdougall (AT) DELyahoo (DOT) ca> wrote in message std::max(a, b) = 2; Why is that an atrocity? You might want to assign the max of a and b to 2. You /can't/ "assign the max of a and b to 2" in any meaningful sense. Writing | max(5,7) = 2; doesn't "assign the max of 5 and 7", because you can't change that. What would it mean to do so? That I want globally to switch integer arithmetic to modulo 5? Of course not. I wasn't referring to constants. I was referring to a and b, and you could have an l-value version of the template that would work in that case. I'm not suggesting that such a template should be provided. I'm just disagreeing that "std::max(a, b) = 2;" is an atrocity. It looks quite neat and elegant to me, even if hardly anyone would ever need it. DW Back to top Vladimir Guest Posted: Thu Dec 30, 2004 9:13 pm    Post subject: Re: why reture "const &"? simont wrote: Quote: You /can't/ "assign the max of a and b to 2" in any meaningful sense. Writing | max(5,7) = 2; doesn't "assign the max of 5 and 7", because you can't change that. You can't assign 5 to 2 in any meaningful sense either: 5 = 2; Does this mean we have to get rid of assignment operator?? Writing max(a, b) = c makes perfect sense either in math or in programming At least for those who knows what references are used for -- Vladimir Back to top Dave Rahardja Guest Posted: Fri Dec 31, 2004 12:42 am    Post subject: Re: why reture "const &"? [email]zealotcat (AT) gmail (DOT) com[/email] wrote: Quote: template inline T const& max (T const& a, T const& b) { // if a < b then use b else use a return a } thanks very much!! Because the input variables are const references. You shouldn't return a non-const reference to a const object. By the way, why is everyone on this thread assuming that the original poster is talking about std::max? I do like the idea of max returning an lvalue. The expression max(a, b) = 5; is succint, clear, and unambiguous. -dr Back to top Mike Hewson Guest Posted: Fri Dec 31, 2004 6:55 am    Post subject: Re: why reture "const &"? [email]zealotcat (AT) gmail (DOT) com[/email] wrote: Quote: template inline T const& max (T const& a, T const& b) { // if a < b then use b else use a return a } thanks very much!! The parameters when the function 'max' is called have type 'T const&' - meaning 'a' and 'b' are T's ( whatever they are ) that cannot be altered in this context ( the 'const' ), and reference semantics are used ( possibly to avoid pointer stuff, but there are other reasons ). For the type 'T' one hopes that the '<' operator is satisfactorily and sensibly defined with regard to the meaning and usage of type 'T'. The expression 'a the conditional-expression operator as applied ( 'a represent 'a' if 'a The function thus returns one of 'a' or 'b' according to that test, but with the type 'T const&'. Presumably, this is because if the 'max' function ought not alter 'a' or 'b' ( the 'const' in the argument list suggests this intention ), then neither should the function that called 'max' ( to which either 'a' or 'b' will be returned ). Assigning to max(a,b) would seem to violate that intent, that is: max(a,b) = Whatever; // Whatever is of type 'T'. would be illegal as it stands. Some other function named 'max', but with a different signature, could be used however - a non-const version for instance. -- Cheers -- Hewson::Mike "This letter is longer than usual because I lack the time to make it shorter" - Blaise Pascal 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.