C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  What Value Does Const Correctness Offer Goto page 1, 2, 3 ... 14, 15, 16  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Jessica B Guest Posted: Sat Sep 11, 2004 5:00 am    Post subject: What Value Does Const Correctness Offer I was having an argument recently with a fellow programmer. His take was that const correctness does not add significant value to your software, and, given that it is quite burdensome to maintain, he questioned whether it should be ignored. My primary argument in its favor is that it gives additional information to the compiler about your intentions, which can allow the compiler to detect bugs, however, on challenge I really couldn't remember an example of a bug that it prevented (from my own practical expereience.) I also argued that const correctness was not generally difficult to maintain, but, in all honesty, it is a significant amount of work to maintain it. Just off the top of my head I would say that about 10% of syntax errors are const related errors. (And the large majority of them only show an error in const correctness, rather than offering any productive value.) Often some of the strangest syntax errors arise from const correctness errors, and they sometimes take some detangling. Further, many external libraries one uses often have const correctness errors in them, which have to be eliminated by tangled casts and inappropriate mutables. I wonder if any of you can offer some convincing arguments for the effort involved in const correctness. Or perhaps offer some class of bug caught by it use. Thanks. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Sun Sep 12, 2004 10:09 am    Post subject: Re: What Value Does Const Correctness Offer Jessica B wrote: Quote: I was having an argument recently with a fellow programmer. His take was that const correctness does not add significant value to your software, and, given that it is quite burdensome to maintain, he questioned whether it should be ignored. You can't ignore it, because you are forced to use const anywhere you might need to bind a reference to a temporary. Quote: My primary argument in its favor is that it gives additional information to the compiler about your intentions, which can allow the compiler to detect bugs, however, on challenge I really couldn't remember an example of a bug that it prevented (from my own practical expereience.) snip It should prevent you from putting the source and destination arguments to memcpy or std::copy the wrong way round (note that the correct order differs between the two functions!). -- Ben Hutchings If God had intended Man to program, we'd have been born with serial I/O ports. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Walter Guest Posted: Sun Sep 12, 2004 10:09 am    Post subject: Re: What Value Does Const Correctness Offer "Jessica B" <jessica_boxer (AT) yahoo (DOT) com> wrote Quote: I was having an argument recently with a fellow programmer. His take was that const correctness does not add significant value to your software, and, given that it is quite burdensome to maintain, he questioned whether it should be ignored. My experience is similar to your colleague's. Maintaining const correctness, while generating lots of errors at compile time, has never found an actual bug for me. Peppering declarations with 'const' doesn't look good and tends to visually obfuscate what the types are. Const has numerous arcane and sometimes inconsistent rules with how overloading and partial specialization works (sometimes it's ignored, sometimes it isn't). And lastly, const doesn't help the optimizer, since there are so many legal ways to get around it. Nevertheless, many programmers swear by const and firmly believe it improves program readability and that it does find real bugs. I'm just not one of them <g>. This is why D makes const a storage class, rather than a type modifier. A const storage class means that the data is semantically read-only, and could be placed in ROM or in a read-only data segment. The optimizer can rely on it being constant. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Balog Pal Guest Posted: Sun Sep 12, 2004 3:59 pm    Post subject: Re: What Value Does Const Correctness Offer "Jessica B" <jessica_boxer (AT) yahoo (DOT) com> wrote Quote: I was having an argument recently with a fellow programmer. His take was that const correctness does not add significant value to your software, and, given that it is quite burdensome to maintain, he questioned whether it should be ignored. Rubbish. Quote: My primary argument in its favor is that it gives additional information to the compiler about your intentions, which can allow the compiler to detect bugs, however, on challenge I really couldn't remember an example of a bug that it prevented (from my own practical expereience.) Well. IMHO you can just safely ignore the guy, really. If one thinks a safety belt is just there for some idiots have spare time tomake additional rules -- you will not convince them anyhow. And 'const' in C++ is not just there to detect bugs, but serves a plenty of other purposes -- like communicates intent, documents interface, allows overloads, etc. And built-in rules of the language will make your life a hell if you never write 'const' in your classes and functions. As literals, temporaries will not bind to nonconst refs. And passing by value will be slow for one set of cases and completely inappropriate for others. Quote: I also argued that const correctness was not generally difficult to maintain, It is trivial to maintain. What is cumbersome -- picking up some oldtimer C library and convert it to a const-correct C++ package. But when you write stuff from scratch, it's completely natural to add 'const' wherever your *intetntion* is not to change the object. (And I observe it is the case for the majority of situations.) Quote: but, in all honesty, it is a significant amount of work to maintain it. Just off the top of my head I would say that about 10% of syntax errors are const related errors. You mean mistyping const to cosnt? Anyway, IMHO the syntax-error-pervcentage is not an interesting measure. What counts the overall *time* spent on work. I find the time spent on typing the extra letters, and correct the mistypes, and also misthinks, the recompiles just caused by that -- all that time is still under the measurable limit. Quote: (And the large majority of them only show an error in const correctness, rather than offering any productive value.) Okey, then pick a single case of the minority, and estimate the time it would cause going uncaught. My guess it will be by one magnitude more than all the time consumed by the majority. Quote: Often some of the strangest syntax errors arise from const correctness errors, and they sometimes take some detangling. Example? Quote: Further, many external libraries one uses often have const correctness errors in them, That is indeed a problem -- but the fact you face defective libraries is hardly an excuse to write more defective software of your own. The general solution if you can't drop that library or manage to get it fixed by supplier is to create some wrappers, that are const-correct. [btw most shops use external libraries through domestic wrappers anyway, so it is rarely a new and extra work.] Quote: which have to be eliminated by tangled casts and inappropriate mutables. The important rule is to have those casts (just as any such stuff) AWAY from the user code. :) Quote: I wonder if any of you can offer some convincing arguments for the effort involved in const correctness. Or perhaps offer some class of bug caught by it use. Hm, it's hard to show the bugs for a simple reason -- they are caught so early in the process and so easy to correct wihout cost, likely no one bothers to document or even remember them. See the estimates for a cost of bug caught at design/coding/autotests/review/other tests/production, there are steep jumps. OTOH you can't hack that problem in isolation. The issue of 'code shall reflect the intent' and 'the design shall be clear about what changes what' is essential to write correct stuff. If you have people who can't think that way, sure tools that help will really just cause headache, and yield no real benefits. Paul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sun Sep 12, 2004 3:59 pm    Post subject: Re: What Value Does Const Correctness Offer In article <bPJ0d.74330$3l3.5033@attbi_s03>, Walter <walter (AT) digitalmars (DOT) nospamm.com> writes Quote: My experience is similar to your colleague's. Maintaining const correctness, while generating lots of errors at compile time, has never found an actual bug for me. Peppering declarations with 'const' doesn't look good and tends to visually obfuscate what the types are. Const has numerous arcane and sometimes inconsistent rules with how overloading and partial specialization works (sometimes it's ignored, sometimes it isn't). And lastly, const doesn't help the optimizer, since there are so many legal ways to get around it. You can't be serious:-) double const pi(3.14159265); is a very different beast to double value(2.1); The compiler 'knows' that unless the programmer does something silly like take the address of pi that it need not provide storage for it, however in the case of value, it better provide storage unless it can categorically prove that value never ever changes. Equally, it is fully entitled to ROM objects that are declared as const and if the programmer tries to change the object the programmer is wrong, not the compiler. We even introduced an extra keyword, mutable, in order to deal with the case of logically const objects that would still have a mutable element. The only people I know that have a problem with const are those that come from a C background. Novices quickly learn to write const correct code if correctly introduced to the concept. Now the problem is that const is an overloaded keyword. At top level it effectively means immutable (and so ROMable) at other levels it means 'read only'. BTW that is yet another reason for placing const at the extreme right of the type being qualified; if there is then any other type element (* or &) to its right it the overall type is not immutable, only write protected. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Sun Sep 12, 2004 4:06 pm    Post subject: Re: What Value Does Const Correctness Offer Jessica B wrote: Quote: I was having an argument recently with a fellow programmer. His take was that const correctness does not add significant value to your software, and, given that it is quite burdensome to maintain, he questioned whether it should be ignored. My primary argument in its favor is that it gives additional information to the compiler about your intentions, which can allow the compiler to detect bugs, however, on challenge I really couldn't remember an example of a bug that it prevented (from my own practical expereience.) [snip] const correctness enables you to express the concept of immutable data, and have the compiler support it; for example, if your data is in ROM. If you use memory mapped I/O, writing to certain memory locations can have unintended side effects. With the proper const specifications you can avoid such situations at compile time. The promise of constness increases the probability of better optimization. And, if you're dealing with big objects, it saves you a lot of copying to pass a const&. There is little question that constness and its requirements have been introduced to solve some real issues, and many love it. It's a lot easier to deal with problems at compile time than after releasing to the customer. -- Antoun Kanawati [email]antounk.at (AT) comcast (DOT) dot.net[/email] [remove .dot and .at before use] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sun Sep 12, 2004 4:08 pm    Post subject: Re: What Value Does Const Correctness Offer In article <642e01d8.0409101150.63271032 (AT) posting (DOT) google.com>, Jessica B <jessica_boxer (AT) yahoo (DOT) com> writes Quote: I was having an argument recently with a fellow programmer. His take was that const correctness does not add significant value to your software, and, given that it is quite burdensome to maintain, he questioned whether it should be ignored. Let me guess that he came from a C background where they (with good reason) talk of const poisoning. Without function overloading const is costly because it is often possible for an object to acquire const protection by 'accident' Such possibility means that it then has to be cast away, and the programmer now has the responsibility to ensure that the cast is valid (i.e. do not try to cast away const qualification of an object that was declared as const, while safely casting away an acquired const attribute) Quote: My primary argument in its favor is that it gives additional information to the compiler about your intentions, which can allow the compiler to detect bugs, however, on challenge I really couldn't remember an example of a bug that it prevented (from my own practical expereience.) You mean you have never had to deal with accidental attempts to modify a string literal? The concept of const in C++ is also essential to allowing the passing of object by reference when the object must not be changed by the function being called. Again this is an issue for C++ but not for C. Quote: I also argued that const correctness was not generally difficult to maintain, but, in all honesty, it is a significant amount of work to maintain it. Just off the top of my head I would say that about 10% of syntax errors are const related errors. (And the large majority of them only show an error in const correctness, rather than offering any productive value.) Just because 10% of your syntax errors are caused by breaches of const correctness does not make const correctness hard to maintain but it does mean that the compiler is identifying places in your code where you are doing something suspect. Quote: Often some of the strangest syntax errors arise from const correctness errors, and they sometimes take some detangling. Further, many external libraries one uses often have const correctness errors in them, which have to be eliminated by tangled casts and inappropriate mutables. So? Incompetence in developers may be a reason not to buy their products but it certainly isn't a reason to allow their incompetence by changing the language. Quote: I wonder if any of you can offer some convincing arguments for the effort involved in const correctness. Or perhaps offer some class of bug caught by it use. But like exception safety, it isn't a problem once one changes ones coding habits. Those who write C++ as if it were C will have problems, but trying to write C++ without const correctness would result in a mess. const wasn't introduced to C++ on a whim, though perhaps C would have been better off had it not copied the keyword and then watered down its semantics. C might have found it better to have made const a keyword and then specified that it did nothing in C but was only there for C++ compatibility. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Walter Guest Posted: Mon Sep 13, 2004 10:28 am    Post subject: Re: What Value Does Const Correctness Offer "Francis Glassborow" <francis (AT) robinton (DOT) demon.co.uk> wrote Quote: You can't be serious:-) double const pi(3.14159265); is a very different beast to double value(2.1); The compiler 'knows' that unless the programmer does something silly like take the address of pi that it need not provide storage for it, however in the case of value, it better provide storage unless it can categorically prove that value never ever changes. Equally, it is fully entitled to ROM objects that are declared as const and if the programmer tries to change the object the programmer is wrong, not the compiler. In this case, using const as the 'top level', const is essentially being used as a storage class, and it can be put in ROM. I was referring, however, to things like pointer to const. The optimizer cannot rely on whatever the pointer is pointing to being the same value after, say, a function call or an assignment through another pointer. Quote: We even introduced an extra keyword, mutable, in order to deal with the case of logically const objects that would still have a mutable element. I've always been bemused by mutable - it seems to be driven by the notion that const things are better, so we'll enable things to be declared as const even though they're not <g>. Isn't mutable is an admission of the semantic failure of const? Maybe there's a compelling argument for it I haven't seen. Quote: The only people I know that have a problem with const are those that come from a C background. Novices quickly learn to write const correct code if correctly introduced to the concept. I come from a C background <g> (and BASIC, FORTRAN, Pascal, and asm), and I know how to write const correct code. I just haven't found a practical benefit to it - it doesn't find real bugs, it doesn't look good, and it doesn't help code generation. Quote: Now the problem is that const is an overloaded keyword. At top level it effectively means immutable (and so ROMable) D keeps this meaning, and so making const a storage class makes sense. Quote: at other levels it means 'read only'. BTW that is yet another reason for placing const at the extreme right of the type being qualified; if there is then any other type element (* or &) to its right it the overall type is not immutable, only write protected. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pavel Vozenilek Guest Posted: Mon Sep 13, 2004 10:29 am    Post subject: Re: What Value Does Const Correctness Offer "Francis Glassborow" wrote: Walter <walter (AT) digitalmars (DOT) nospamm.com> writes Quote: My experience is similar to your colleague's. Maintaining const correctness, while generating lots of errors at compile time, has never found an actual bug for me. You can't be serious:-) double const pi(3.14159265); is a very different beast to double value(2.1); I guess the experience is more about const qualified member functions than variables. Const qualifier for member function helps to catch errors (though not as often I would like) but single change somewhere deep in code can propagate across whole project with dozens of places needing update. That's how I understand the complaint. /Pavel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Harris Guest Posted: Mon Sep 13, 2004 10:33 am    Post subject: Re: What Value Does Const Correctness Offer [email]jessica_boxer (AT) yahoo (DOT) com[/email] (Jessica B) wrote (abridged): Quote: I wonder if any of you can offer some convincing arguments for the effort involved in const correctness. Or perhaps offer some class of bug caught by it use. For me the main use is when you have a reference and aren't sure what the effects of changing it would be. For example: void move( Rect *pRect ) { pRect->topLeft().x += 10; } Does this move the given rectangle 10 units to the left? It depends on whether what topLeft() returns is a copy of the rectangle's point, or the actual defining point itself. If you have garbage collection or are using smart pointers, then a result can be a (smart) pointer and still be a copy, and the examples are not so trivial. Anyway, const cuts this Gordian Knot by disallowing it; by not allowing you to change through the reference. It makes it much easier to use references and shared data without unwanted side effects. When I first started using Smalltalk (a language which has garbage collection, but no static type checks) I found const type errors to be the ones which took most of my debugging time. (Admittedly when I returned to Smalltalk a few years later my experienced changed, so this may be a matter of programming style or competence). The importance of "const", in the broader sense of state-free programming, is such that the entire school of Functional Programming is largely based upon the idea. I find it quite useful when organising my thoughts and code to distinguish between operations which may change the program's state, and operations which will not. It helps when debugging live code, for example - if a routine is const, I know I can set the program counter back and re-execute a call to it without changing anything. It helps when reasoning about the code. Whether all this is compelling is another matter. I agree the cost is high. I am especially bothered by the frequent need to replicate code, to have const and non-const versions of the same function. -- Dave Harris, Nottingham, UK [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Mon Sep 13, 2004 10:38 am    Post subject: Re: What Value Does Const Correctness Offer "Walter" <walter (AT) digitalmars (DOT) nospamm.com> writes: Quote: "Jessica B" <jessica_boxer (AT) yahoo (DOT) com> wrote in message news:642e01d8.0409101150.63271032 (AT) posting (DOT) google.com... I was having an argument recently with a fellow programmer. His take was that const correctness does not add significant value to your software, and, given that it is quite burdensome to maintain, he questioned whether it should be ignored. My experience is similar to your colleague's. Maintaining const correctness, while generating lots of errors at compile time, has never found an actual bug for me. Peppering declarations with 'const' doesn't look good and tends to visually obfuscate what the types are. Const has numerous arcane and sometimes inconsistent rules with how overloading and partial specialization works (sometimes it's ignored, sometimes it isn't). And lastly, const doesn't help the optimizer, since there are so many legal ways to get around it. I can understand your point of view; I find it particularly agravating that I sometimes have to provide two exactly identical functions (e.g. like operator[]), one of which can be called on a const object, and the other which allows modification through the returned reference. On the other hand, I find that when joined with the refusal to bind a const reference to a temporary, it does catch some errors -- cases when an unintentional implicit conversion reared its head. IMHO, the better solution would have been to get rid of implicit conversions, but somehow, I don't think that that would have flown. At any rate, given the current rules, you really don't have the choice in general. The choice you might have is whether to use const, and enforce const correctness, for entity objects, i.e. objects you pass by reference or pointer, and never copy. Even their, all things considered, I prefer the consistency of const correctness. -- James Kanze 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 David B. Held Guest Posted: Mon Sep 13, 2004 10:39 am    Post subject: Re: What Value Does Const Correctness Offer Jessica B wrote: Quote: [...] My primary argument in its favor is that it gives additional information to the compiler about your intentions, which can allow the compiler to detect bugs, however, on challenge I really couldn't remember an example of a bug that it prevented (from my own practical expereience.) How about any kind of ownership transfer scenario? For instance, ownership probably should not leave a const object, since that would imply that the object should lose its reference to the object. Thus, a perfectly obvious and ubiquitous example would be std::auto_ptr<>. You should never be able to move from a const auto_ptr<>. Quote: I also argued that const correctness was not generally difficult to maintain, but, in all honesty, it is a significant amount of work to maintain it. Just off the top of my head I would say that about 10% of syntax errors are const related errors. (And the large majority of them only show an error in const correctness, rather than offering any productive value.) Do you say that because you've written the code in a const-free form and obtained correct programs, or because the programs that forced you to use const-correctness turned out to be correct? Make sure you don't infer causality in the wrong direction. Quote: Often some of the strangest syntax errors arise from const correctness errors, and they sometimes take some detangling. Further, many external libraries one uses often have const correctness errors in them, which have to be eliminated by tangled casts and inappropriate mutables. [...] Is that the fault of const or people who don't use it correctly? Does that fact that people abuse pointers imply that pointers are a flawed language concept that should be removed? (Well, I guess Java has answered that question). Dave [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Clem Dickey Guest Posted: Mon Sep 13, 2004 7:59 pm    Post subject: Re: What Value Does Const Correctness Offer "Balog Pal" <pasa (AT) lib (DOT) hu> wrote Quote: it's completely natural to add 'const' wherever your *intention* is not to change the object. (And I observe it is the case for the majority of situations.) Agreed. And therefore const correctness would be simpler and programs would be shorter if "const" were the default, and "var" modifier to designated modifiable items. If not for backwards compatibility ... [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jessica B Guest Posted: Mon Sep 13, 2004 8:00 pm    Post subject: Re: What Value Does Const Correctness Offer Francis Glassborow <francis (AT) robinton (DOT) demon.co.uk> wrote Quote: In article <642e01d8.0409101150.63271032 (AT) posting (DOT) google.com>, Jessica B [email]jessica_boxer (AT) yahoo (DOT) com[/email]> writes You mean you have never had to deal with accidental attempts to modify a string literal? Perhaps once or twice. I rarely use string literals anyway, since I produce shrink-wrap software that must be localized, meaning text strings usually come from an external source into a string variable. Let me start by clarifying the question. If is not whether const is useful or necessary, clearly it is. It is whether const correctness throughout the system is necessary or, more specifically, a net benefit. Quote: The concept of const in C++ is also essential to allowing the passing of object by reference when the object must not be changed by the function being called. Again this is an issue for C++ but not for C. But that begs the question. You say const is essential for that, which is undoubtedly true, but the goal you set out (making in immutable) is the very matter under question. Quote: Just because 10% of your syntax errors are caused by breaches of const correctness does not make const correctness hard to maintain but it does mean that the compiler is identifying places in your code where you are doing something suspect. But if, as the premise of the thread suggests, const correctness adds little value, then the extra work involved in the aforementioned syntax problems is still significant enough to offset any minimal benefit conveyed. Of course that is a judgment call. Const obviously is going to catch some bugs, with a cost of n hours work saved, and the challenge of maintaining cost is m hours. The question is whether in practice n > m. If n -> 0, and m>0 then we have an answer. Quote: Often some of the strangest syntax errors arise from const correctness errors, and they sometimes take some detangling. Further, many external libraries one uses often have const correctness errors in them, which have to be eliminated by tangled casts and inappropriate mutables. So? Incompetence in developers may be a reason not to buy their products but it certainly isn't a reason to allow their incompetence by changing the language. I find that a curious statement. What if you have no choice but to use a particular library (as is often the case in many, many situations)? What does one do then? Toss the project out of some sense of linguistic purity? This is not some meaningless debate either. There are many such real world problems. For example, one of the most widely used libraries in the world, the Microsoft Foundational Class library has at least a half a dozen const correctness issues I know of. Should I not use it because of that reason? Should I really write a mega wrapper around it? Further I can think of several features added to the language to facilitate the use of errant or old libraries such as extern "C" or the use of the "using namespace std" idiom, both of which fall under this category, and that's what I can think of off the top of my head. (One facilitates the linking of old C libraries, and the other facilitates old code that does not use namespace qualification.) Anyway, the question is not one of changing the language rather it is a question of whether a particular feature of the language should be used. And that is an entirely different thing. Quote: But like exception safety, it isn't a problem once one changes ones coding habits. But that isn't true. Regardless of ones coding habits it still takes extra cognitive effort to maintain const correctness. And I might add that to suggest that providing exception safety is easy once one has good habits is not a viewpoint I agree with at all. If I remember correctly Stroustrup wrote a whole appendix in "The C++ Programming Language" on the subtleties of exception safety in the standard library. I find it curious indeed that despite the fact that there is almost universal acceptance of the dogma that const correctness is key to great software, (and I am an priest of that church also), that there is no good quality list of reasons why. For many of the other good software practices I use I can clearly articulate a list of reasons for their use, that are clear, irrefutable, and legion. For example, can you give me a list of reasons to prefer exceptions over error return codes? I can give you at least a dozen excellent reasons. What about encapsulation? Why shouldn't I use protected member variables? Why should I avoid multiple inheritance? Why should I prefer the standard library over roll your own code? I can give lists of reasons for these that are simply beyond question. However, const correctness seems different. Here are the reasons I have heard here: * "it is needed for embedded programming", sure, you need to put stuff in ROM, but that hardly applies to programming in general, and seems more like the sort of thing that the toolset should deal with, in a pragma for example; * "bind refs to temps need it" which seems to me to be something that can be dealt with locally rather than globally (i.e., const is useful, const correctness not so much); * "const pi = 3.14" again is important, but local; * "const provides opportunities for optimization", which it does in theory, however, I doubt it really translates into anything significant in practice. For example: class C { int f() const; }; int test(const C c) { return c.f() + c.f(); } Would any compiler actually optimize out the second call to f? I doubt there is a complier in existence that would. I may be wrong, however, perhaps there are some aggressive compilers that do. However, make c non const, and there is no realistic way to expect a compiler to eliminate the second call. For optimization to succeed requires a lot of stars to line up just right. As I say, I doubt it happens much in the real world. (Of course except for constant folding const pi = 3.14; but that example doesn't count since it doesn't require systematic const correctness to use.) Even Myers in his Tips books says const correctness is crucial, but doesn't really give a good explanation of why. I feel unsatisfied. Perhaps my problem is that after reading the article linked below I wonder if the whole concept of static type checking is more effort than it is worth (written by Java maven Bruce Eckel -- not some inexperienced kid from college.) <http://mindview.net/WebLog/log-0025> This article basically argues that static type checking is a test applied by the compiler to test a small subset of the correctness of a program. However, the cost of maintaining and implementing it is large enough that it slows the programmer down reducing the amount of unit testing that can be done, and bang for buck, unit testing is much more valuable than static type checking for validating your software. To use pseudo math: For some project P, n is the number of hours saved by static type checking m is the number of hours cost maintaining static checks and f(x) is the number of hours saved by spending x hours writing and running unit tests (which presumably declines exponentially as x -> infinity.) Static type checking is inefficient if n < f(m) and it is Eckel's contention in the above article that n is much smaller than f(m). If he is right, that is a pretty scary result. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Mon Sep 13, 2004 8:01 pm    Post subject: Re: What Value Does Const Correctness Offer In article <2qjhk1F102644U1 (AT) uni-berlin (DOT) de>, Pavel Vozenilek <pavel_vozenilek (AT) yahoo (DOT) co.uk> writes Quote: I guess the experience is more about const qualified member functions than variables. But the two necessarily go hand-in-glove. The concept of immutable objects only works if we have a way to guarantee that they are not going to be violated. Quote: Const qualifier for member function helps to catch errors (though not as often I would like) but single change somewhere deep in code can propagate across whole project with dozens of places needing update. Indeed, correcting a design fault often has that property. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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