BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  operator ++ doubts Goto page 1, 2, 3  Next          BorlandTalk.com Forum Index -> C++ Builder (Language C++) View previous topic :: View next topic   Author Message Petar Popara Guest Posted: Wed Mar 09, 2005 3:50 pm    Post subject: operator ++ doubts I have this case: int i=0; int result = (i++) + (i++); and result is 0. I would expect that after first "i++", "i" is incremented and result would be 1, but it seems that "i" is incremented only after whole line of code is executed (line: int result = (i++) + (i++)? Back to top Wiljo Guest Posted: Wed Mar 09, 2005 4:15 pm    Post subject: Re: operator ++ doubts Petar Popara wrote: Quote: I have this case: int i=0; int result = (i++) + (i++); and result is 0. I would expect that after first "i++", "i" is incremented and result would be 1, but it seems that "i" is incremented only after whole line of code is executed (line: int result = (i++) + (i++)? Hello, This ++ operator isn't called post operator for nothing. It increments the associated variable after the expression has been evaluated. One could use the pre operator instead, thus ++i. The increment will then take place before the expression is evaluated: int i = 0; int result = (++i) + (++i); Here i becomes 2 and result would be 4 (2 + 2). Needless to say that writing this kind of expressions is curious at best. Better write seperate statements, so everyone is clear about what is happening: int i = 0; ++i; ++i; int result = i + i; This takes more lines, but everyone will know exactly what is going on. I might add that using these operators with a macro will be even worse. Say for instance we have a MAX macro: #define MAX( a, b ) ((a) > (b) ? (a) : (b)) Do you know the value of result after the next bit of code: int i = 0; int result = MAX( i++, ++i ); How many times will i be incremented here? The line would translate to: int result = ((i++) > (++i) ? (i++) : (++i)); The variable i will be incremented three times, as opposed to the two appearing with the call of the MAX macro. Thus i equals to 3, and result? I don't know! My guess is 2. Wiljo. Back to top Danzer Guest Posted: Wed Mar 09, 2005 4:15 pm    Post subject: Re: operator ++ doubts Petar Popara wrote: Quote: I have this case: int i=0; int result = (i++) + (i++); and result is 0. I would expect that after first "i++", "i" is incremented and result would be 1, but it seems that "i" is incremented only after whole line of code is executed (line: int result = (i++) + (i++)? Your results will vary between brands of C/C++ compilers. Rule of thumb: do not increment/decrement a variable more that once in an expression. To get an exact reason for this, I suggest you post your question to comp.lang.c++.moderated. In that moderated group, you will probably get an extensive answer to your question. Back to top Alan Bellingham Guest Posted: Wed Mar 09, 2005 4:24 pm    Post subject: Re: operator ++ doubts "Petar Popara" <none> wrote: Quote: I have this case: int i=0; int result = (i++) + (i++); and result is 0. Yes, that looks likely. Not guaranteed, but likely. Quote: I would expect that after first "i++", "i" is incremented and result would be 1, but it seems that "i" is incremented only after whole line of code is executed (line: int result = (i++) + (i++)? Ah, Grasshopper, you are not yet one with C++. What you've encountered is a case of Undefined Behaviour. In this case, you are modifying the same value twice: "Between two sequence points, an object is modified more than once, or is modified and the prior value is read other than to determine the value to be stored" (C Standard) In this case, you apply ++ twice, and attempt to read the prior value twice. The brackets don't help. Alan Bellingham -- ACCU Conference 2005 - 20-23 April, Randolph Hotel, Oxford, UK Back to top Wiljo Guest Posted: Wed Mar 09, 2005 4:41 pm    Post subject: Re: operator ++ doubts Alan Bellingham wrote: Quote: "Petar Popara" <none> wrote: I have this case: int i=0; int result = (i++) + (i++); and result is 0. Yes, that looks likely. Not guaranteed, but likely. Why not guaranteed? Who is the Grasshopper now? The ++ operator is in this case a post increment, thus i will be incremented twice after the expression is evaluated. Therefore result==0 and i==2. And this is guaranteed, across every C compiler. Quote: I would expect that after first "i++", "i" is incremented and result would be 1, but it seems that "i" is incremented only after whole line of code is executed (line: int result = (i++) + (i++)? That is correct. Ah, Grasshopper, you are not yet one with C++. With this line of code, everyone is a Grasshopper. And this line is just normal C-code. It is not yet even C++. Quote: What you've encountered is a case of Undefined Behaviour. In this case, you are modifying the same value twice: There is *no* Undefined Behaviour here, although it looks like it. This is just the interpretation of the viewer. I can see what happens, but not everyone is clear about this. Better avoid these constructions as much as possible. Quote: "Between two sequence points, an object is modified more than once, or is modified and the prior value is read other than to determine the value to be stored" (C Standard) In this case, you apply ++ twice, and attempt to read the prior value twice. The attempt will succeed, and it is not an attempt. The prior or initial value is 0, as result shows. Quote: The brackets don't help. They will not indeed. Alan Bellingham Be clear on this: no insult is intended. Everyone can make mistakes. See also my other post on this matter. Wiljo. Back to top Alan Bellingham Guest Posted: Wed Mar 09, 2005 4:50 pm    Post subject: Re: operator ++ doubts Wiljo <invalid (AT) invalid (DOT) address> wrote: Quote: int i = 0; int result = (++i) + (++i); Here i becomes 2 and result would be 4 (2 + 2). _MIGHT_ be 4. But the expression is allowed to generate 4128423 if it feels like it, since you've invoked undefined behaviour. Highly unlikely, but the language at this point takes the point of view that it will not define the effect of modifying the same variable twice and also trying to read it 'between two sequence points'. In effect, it's refusing to say that the modified value has been written out until it gets to the terminating semi-colon, and that therefore the expression could increment 0 twice. Other languages make other decisions, but C (and thus C++) tries to put as few limitations on the underlying code as possible. Alan Bellingham -- ACCU Conference 2005 - 20-23 April, Randolph Hotel, Oxford, UK Back to top Gillmer J. Derge [TeamB] Guest Posted: Wed Mar 09, 2005 4:50 pm    Post subject: Re: operator ++ doubts Wiljo wrote: Quote: There is *no* Undefined Behaviour here, although it looks like it. This is just the interpretation of the viewer. I can see what happens, but not everyone is clear about this. Better avoid these constructions as much as possible. Alan is right on this. See question 3.2 here for details: http://www.faqs.org/faqs/C-faq/faq -- Gillmer J. Derge [TeamB] Back to top Alan Bellingham Guest Posted: Wed Mar 09, 2005 4:55 pm    Post subject: Re: operator ++ doubts Wiljo <invalid (AT) invalid (DOT) address> wrote: Quote: Be clear on this: no insult is intended. Everyone can make mistakes. As indeed you have, I'm afraid. Quote: See also my other post on this matter. Which is sadly also erroneous. Go find the standard. Read it. Understand it. Go talk to the gurus. Or check the C FAQ on this very subject: http://www.eskimo.com/~scs/C-faq/q3.2.html Alan Bellingham -- ACCU Conference 2005 - 20-23 April, Randolph Hotel, Oxford, UK Back to top Alan Bellingham Guest Posted: Wed Mar 09, 2005 5:00 pm    Post subject: Re: operator ++ doubts "Gillmer J. Derge [TeamB]" <spam (AT) gillmerderge (DOT) com> wrote: [snip] Alan is right [snip] But of course. I'm always right. Even when I'm wrong <grin> Alan Bellingham -- ACCU Conference 2005 - 20-23 April, Randolph Hotel, Oxford, UK Back to top Duane Hebert Guest Posted: Wed Mar 09, 2005 5:19 pm    Post subject: Re: operator ++ doubts "Wiljo" <invalid (AT) invalid (DOT) address> wrote Quote: Why not guaranteed? Who is the Grasshopper now? The ++ operator is in this case a post increment, thus i will be incremented twice after the expression is evaluated. Therefore result==0 and i==2. And this is guaranteed, across every C compiler. I would expect that after first "i++", "i" is incremented and result would be 1, but it seems that "i" is incremented only after whole line of code is executed (line: int result = (i++) + (i++)? The problem is that the order of evaluation is undefined. Stroustrup has a link at his FAQ that may interest you: http://www.research.att.com/~bs/bs_faq2.html#evaluation-order Back to top Alan Bellingham Guest Posted: Wed Mar 09, 2005 5:23 pm    Post subject: Re: operator ++ doubts "Duane Hebert" <spoo (AT) flarn (DOT) com> wrote: Quote: The problem is that the order of evaluation is undefined. Stroustrup has a link at his FAQ that may interest you: http://www.research.att.com/~bs/bs_faq2.html#evaluation-order Oh, now that's unfair, claiming Bjarne Stroustrup as an authority. What's he know about C++ ... Alan Bellingham -- ACCU Conference 2005 - 20-23 April, Randolph Hotel, Oxford, UK Back to top Bob Gonder Guest Posted: Wed Mar 09, 2005 5:51 pm    Post subject: Re: operator ++ doubts Alan Bellingham wrote: Quote: Wiljo <invalid (AT) invalid (DOT) address> wrote: See also my other post on this matter. Which is sadly also erroneous. True. Quote: Go find the standard. Read it. Understand it. Go talk to the gurus. Or check the C FAQ on this very subject: http://www.eskimo.com/~scs/C-faq/q3.2.html Alan Bellingham Also, if you compile the code on BC++3.1, with optimizations turned off, you get the behaviour the OP was looking for (1). Turn on optimization, and you get 0. I too was mad at Borland for breaking my fine code until a year or so later when I read the reason for it in another NG. char*BCDpack(char*dest,char*src,int n) { while(n--) *(dest++) = ((*(src++)&0xf)<<4) | (*(src++)&0xf); return dest; } So elegant, and it worked until I turned on optimization. The only timing guarantee about post++ I can remember is it will hit sometime before the code after ";" If you know what "sequence points" are (";" is one), that's when they hit (some time before that, anytime before that, that's what makes it UB). Back to top Remy Lebeau (TeamB) Guest Posted: Wed Mar 09, 2005 5:57 pm    Post subject: Re: operator ++ doubts "Wiljo" <invalid (AT) invalid (DOT) address> wrote Quote: int i = 0; int result = (++i) + (++i); Here i becomes 2 and result would be 4 (2 + 2). More likely, it will become 3 ((1 + 2) or (2 + 1)). Gambit Back to top Duane Hebert Guest Posted: Wed Mar 09, 2005 6:05 pm    Post subject: Re: operator ++ doubts "Alan Bellingham" <alanb (AT) episys (DOT) com> wrote Quote: "Duane Hebert" <spoo (AT) flarn (DOT) com> wrote: The problem is that the order of evaluation is undefined. Stroustrup has a link at his FAQ that may interest you: http://www.research.att.com/~bs/bs_faq2.html#evaluation-order Oh, now that's unfair, claiming Bjarne Stroustrup as an authority. What's he know about C++ ... C++? I thought he was an authority on Grasshoppers. Back to top Chris Uzdavinis (TeamB) Guest Posted: Wed Mar 09, 2005 6:25 pm    Post subject: Re: operator ++ doubts Wiljo <invalid (AT) invalid (DOT) address> writes: Quote: int i = 0; int result = (++i) + (++i); Two modifications to data at the same address without an intervening sequence point results in undefined behavior. -- Chris (TeamB); Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         BorlandTalk.com Forum Index -> C++ Builder (Language C++) All times are GMT Goto page 1, 2, 3  Next Page 1 of 3   Jump to: Select a forum C++ Builder Newsgroups----------------C++ BuilderC++ Builder (ActiveX)C++ Builder (Announcements)C++ Builder (Command Line Tools)C++ Builder DatabasesC++ Builder Databases (ADO)C++ Builder Databases (Desktop)C++ Builder Databases (InterBase Express)C++ Builder Databases (Multi-Tier)C++ Builder Databases (SQL Servers)C++ Builder (Graphics)C++ Builder (IDE)C++ Builder (Internet)C++ Builder (Internet Socket)C++ Builder (Internet Web)C++ Builder (Jobs)C++ Builder (Language)C++ Builder (Language C++)C++ Builder (Compatibility with MFC)C++ Builder (Native API)C++ Builder (Non-Technical)C++ Builder (Students)C++ Builder (Third Party Tools)C++ Builder (Upgrade)C++ Builder (VCL)C++ Builder (VCL Components Usage)C++ Builder (VCL Components Development)C++ Builder (VCL Usage)C++ Builder (WinAPI)C++ Builder Installation Delphi Newsgroups----------------Delphi (General)Delphi ActiveX Controls UsageDelphi ActiveX Controls DevelopmentDelphi As400Delphi DatabasesDelphi Databases (ADO)Delphi Databases (dbExpress)Delphi Databases (Desktop)Delphi Databases (InterBase Express)Delphi Databases (Multi-Tier)Delphi Databases (SQL Servers)Delphi DeploymentDelphi GraphicsDelphi IDEDelphi InternationalizationDelphi InternetDelphi Internet Isapi-WebBrokerDelphi Internet WebSnapDelphi Internet WinsockDelphi JobsDelphi Language BASMDelphi Language ObjectPascalDelphi Model Driven Architecture (General)Delphi Model Driven Architecture (Third Party)Delphi MultimediaDelphi Native APIDelphi Non-TechnicalDelphi ObjectPascalDelphi OLE AutomationDelphi OO designDelphi OpenToolsAPIDelphi Reporting-ChartingDelphi RTLDelphi StudentsDelphi TeamSourceDelphi Thirdparty ToolsDelphi Thirdparty Tools (General)Delphi Thirdparty Tools (IntraWeb)Delphi Thirdparty Tools (RaveReports)Delphi UpgradeDelphi VCLDelphi VCL ComponentsDelphi VCL Components UsingDelphi VCL Components WritingDelphi WebServices SOAPDelphi WinAPIDelphi XMLDelphi Installationcomp.lang.pascal.delphi.components.misccomp.lang.pascal.delphi.components.usagecomp.lang.pascal.delphi.components.writingcomp.lang.pascal.delphi.databasescomp.lang.pascal.delphi.misccomp.sources.delphi JBuilder Newsgroups----------------JBuilderJBuilder AnnouncementsJBuilder Applet IssuesJBuilder As400JBuilder CompilerJBuilder CORBA/RMIJBuilder DatabasesJBuilder DB SwingJBuilder DebuggerJBuilder DeploymentJBuilder DocumentationJBuilder EnterpriseJBuilder HandheldJBuilder IDEJBuilder JavaJBuilder JavaAPIJBuilder Java LanguageJBuilder Java SwingJBuilder Java BeansJBuilder Java Beans UsageJBuilder Java Beans DevelopmentJBuilder JBCLJBuilder JDataStoreJBuilder JobsJBuilder LinuxJBuilder MacintoshJBuilder & MobileSetJBuilder MultiLingual AppsJBuilder Non-TechnicalJBuilder OpenToolsJBuilder Servlets (JSP)JBuilder StudentsJBuilder Team DevelopmentJBuilder Thirdparty ToolsJBuilder UpgradeJBuilder Installation InterBase Newsgroups----------------InterBaseInterBase Bug DiscussionInterBase GeneralInterBase IBConsoleInterBase OpenSourceInterBase ReplicationInterBase SQLInterBase ThirdpartyInterBase Installation Miscellaneous Borland Newsgroups----------------AppCenterAppServerEnteraVisiBroker  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.