C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  what is "validate"          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message NewToTemplates Guest Posted: Fri Jun 24, 2005 7:18 am    Post subject: what is "validate" I was told that there is difference between "list++" and "++list". What is the difference between these two? list; what does the above statement do? I was told that it validates the object. What does it mean? is validate used only in Templates? Thanks. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ferdi Smit Guest Posted: Fri Jun 24, 2005 11:25 am    Post subject: Re: what is "validate" "NewToTemplates" <hetzme (AT) yahoo (DOT) com> wrote Quote: I was told that there is difference between "list++" and "++list". What is the difference between these two? list; what does the above statement do? I was told that it validates the object. What does it mean? is validate used only in Templates? I think you mean list::iterator. i++ is postifx increment, en ++i is prefix increment. The difference? i++ will first fetch i, and then increment it; ++i will first increment i and then fetch it. Consider the following: f(int x) {cout << x;} int i=0; f(i++); // outputs 0, i=1 f(++i); // outputs 2, i=2 I don't know what you mean by validate? I think you must mean invalidate, and then in a different context... A deque erasure for example invalidates all iterators...so you can't do this: (this goes for vector and list etc as well) std::deque // fill d for (std::deque<int>::iterator i = d.begin(); i != d.end(); ++i) { // prefix ++i is faster, because it doesn't need to copy i if (needs_remove(*i)) d.erase(i); // BAD, can't increment i anymore after the erasure, as it is invalidated. } -- Regards, Ferdi Smit smit xs4all nl [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Fri Jun 24, 2005 12:39 pm    Post subject: Re: what is "validate" NewToTemplates wrote: Quote: I was told that there is difference between "list++" and "++list". What is the difference between these two? Presuming that by list, you really mean an iterator into a list, the difference is the results of the expression: list++ results in the value before the incrementation, and ++list the value after. If you don't use the results, there's really no difference. Quote: list; what does the above statement do? Absolutely nothing. Quote: I was told that it validates the object. What does it mean? No idea. You'll have to ask whoever told you, because I've never heard the term applied to a language construct. (Obviously, you validate all input, and you validate parameters to functions, and a lot of other things. But, at least in C++, it always refers to code you have to write.) Quote: is validate used only in Templates? Again, in what sense. I've never heard the term used with regards to templates. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mark Van Peteghem Guest Posted: Mon Jun 27, 2005 10:37 am    Post subject: Re: what is "validate" NewToTemplates wrote: Quote: I was told that there is difference between "list++" and "++list". What is the difference between these two? list; what does the above statement do? I was told that it validates the object. What does it mean? I suppose you're using a chained list (in the pre-STL way). You then have a class like class ListNode { public: ListNode(const Data &theData): data(theData), next(0) {} Data data; ListNode *next; }; A list is then simply an object of type ListNode, that has the first element of the list, and a pointer to the second element, which has a pointer to the third, etc. It ends with a ListNode object in which next is NULL. You can traverse such a list with for (ListNode *list=myList; list; ++list) { ... // use list } The for-loop ends when list; evaluates to NULL, which means it's at the end of the loop. These days it's better to use std::list. Quote: is validate used only in Templates? No, it can be used in non-template lists, as in the example above, but most likely it will be used in templates. -- Mark dot Van dot Peteghem at q-mentum dot com http://www.q-mentum.com -- easier and more powerful unit testing [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top L.Suresh Guest Posted: Tue Jun 28, 2005 12:06 pm    Post subject: Re: what is "validate" the post increment creates a temporary and it should be done only if it is needed.. [email]kanze (AT) gabi-soft (DOT) fr[/email] wrote: Quote: NewToTemplates wrote: I was told that there is difference between "list++" and "++list". What is the difference between these two? Presuming that by list, you really mean an iterator into a list, the difference is the results of the expression: list++ results in the value before the incrementation, and ++list the value after. If you don't use the results, there's really no difference. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Wed Jun 29, 2005 11:20 am    Post subject: Re: what is "validate" L.Suresh wrote: Quote: kanze (AT) gabi-soft (DOT) fr wrote: NewToTemplates wrote: I was told that there is difference between "list++" and "++list". What is the difference between these two? Presuming that by list, you really mean an iterator into a list, the difference is the results of the expression: list++ results in the value before the incrementation, and ++list the value after. If you don't use the results, there's really no difference. the post increment creates a temporary and it should be done only if it is needed.. The creation of the temporary or not should have no observable effect on the program, so who cares. About the only difference it could possibly make (unless you write very weird ++ operators) is in performance, but when I actually benchmarked, there was no measurable difference there either. If you're starting a project from scratch, with no existing code, you can choose one or the other -- you might as well choose prefix, since that seems to be the in thing today. Most of the time, however, the correct form to use is simply the one that is in use in the rest of the code. Since it doesn't make a difference, be consistent. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top NewToTemplates Guest Posted: Sun Jul 24, 2005 6:06 pm    Post subject: Re: what is "validate" Thanks for all the replies. I lost this thread, so could not check for a while. I am going to summarize everything that I understood from all the above postings and also add more comments. Please let me know if I am on the same page with u all. --------------------------------------------------------------------------------- Ferdi Smit said "i++ will first fetch i, and then increment it; ++i will first increment i and then fetch it." How does the compiler interpret the following code? i++; ++i; Is the following interpretation correct? i++; ----- i; i = i + 1; i; ++i; ----- i = i + 1; i; --------------------------------------------------------------------------------- L.Suresh said "the post increment creates a temporary and it should be done only if it is needed.. " Suresh, does it call the constructor when it creates the temporary? --------------------------------------------------------------------------------- James Kanze said "the only difference it could possibly make is in performance .. " James, did u mean that in the post-increment operation we execute more instructions than pre-increment? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Jul 25, 2005 1:22 pm    Post subject: Re: what is "validate" NewToTemplates wrote: Quote: --------------------------------------------------------------------------------- Ferdi Smit said "i++ will first fetch i, and then increment it; ++i will first increment i and then fetch it." How does the compiler interpret the following code? i++; ++i; Is the following interpretation correct? i++; ----- i; i = i + 1; i; Rather: tmp = i ; i = i + 1 ; tmp ; Quote: ++i; ----- i = i + 1; i; --------------------------------------------------------------------------------- L.Suresh said "the post increment creates a temporary and it should be done only if it is needed.. " Suresh, does it call the constructor when it creates the temporary? If the type in question has a constructor. Of course, typically, the compiler is able to see through all this, and will generate the same code, or very similar code, regardless of the form. Quote: --------------------------------------------------------------------------------- James Kanze said "the only difference it could possibly make is in performance .. " Somethings missing in the context there. (I was responding to a specific claim, not making a general statement.) The results of ++ i and i ++ are different. If you use the results, it very much makes a difference. And which one you choose depends on which results you want to use. If you don't use the results, the only possible difference it could possibly make is in performance. And even that should be qualified with something along the lines of "supposing reasonable definitions of the operator." A user defined operator can do just about anything, and a user could, plausibly, define prefix to read from a data base, and postfix to write to it. I (and I suspect most others) would not consider that a reasonable use of ++. Quote: James, did u mean that in the post-increment operation we execute more instructions than pre-increment? On a built-in type, I doubt that there is a compiler in the world which would generate different code, even without optimization, and supposing that the results were not used. For a user defined type, the "classic" implementation of post-fix requires constructing an additional variable. If the operator is simple and inlined, compilers seem to be able to detect that this variable is not used, and suppress it. If the compiler doesn't suppress it, however, then the post-increment form will pay the cost of creating the additional variable. In the past, some very emminant experts have suggested that because of this, you should always use prefix if you don't use the results. My own benchmarks show that in fact, the difference isn't measurable, so I would recommend a much watered down form of the rule: if you are starting a new project, with no existing code, use prefix by default, because it will never be slower. If, however, you have existing code, and it is consistant, continue doing whatever it does, because the difference in performance almost certainly won't be measurable, so the change isn't worth the bother. (Of course, if in your code, it turns out that the difference is measurable, you then have to weigh the cost of changine all of the existing code against that of being inconsistant. If the inconsistency is limited to one critical loop, it can easily be handled with a comment, but if it is wider spread, the best choice may not be obvious.) -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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