C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  const bool as debugging switch          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message lotusny78@yahoo.com Guest Posted: Thu Apr 21, 2005 4:50 pm    Post subject: const bool as debugging switch In "C++ Gotchas," Stephen Dewhurst says that using: const bool debug = ...; is preferable to #ifdefs because: 1) all modern compilers should be able to easily optimize it away 2) all the code in your project will be parsed and checked, not just that of the current configuration He also suggests using a "makefile or similar facility" to choose between debug and production versions, instead of: const bool debug = #ifdef NDEBUG false #else true #endif ; 1) How would you apply the "makefile or similar facility" part ( I use VC++, I don't think I've ever even seen a makefile )? 2) Does anyone have any positive/negative experiences with this technique? Thanks everybody. - Sean [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top mlimber Guest Posted: Fri Apr 22, 2005 2:26 pm    Post subject: Re: const bool as debugging switch In VC++ 6.0 you can generate a makefile for your project from the project menu (of course, you have to use a compatible make utility to use that file). Alternately, you can specify a preprocessor definition in the project settings (Project | Settings... | C/C++ | Preprocessor). If you look there, you might see something like: WIN32,_DEBUG to which you can add your variable and its value: WIN32,_DEBUG,MY_DEBUG_ENABLE=true which can be used in the code thusly: const bool debug = MY_DEBUG_ENABLE; // Defined in the project settings Cheers! M [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (ne Spange Guest Posted: Sat Apr 23, 2005 1:59 am    Post subject: Re: const bool as debugging switch Hello Sean, [email]lotusny78 (AT) yahoo (DOT) com[/email] schrieb: Quote: In "C++ Gotchas," Stephen Dewhurst says that using: const bool debug = ...; is preferable to #ifdefs because: 1) all modern compilers should be able to easily optimize it away 2) all the code in your project will be parsed and checked, not just that of the current configuration He also suggests using a "makefile or similar facility" to choose between debug and production versions, instead of: const bool debug = #ifdef NDEBUG false #else true #endif ; 1) How would you apply the "makefile or similar facility" part ( I use VC++, I don't think I've ever even seen a makefile )? Poor child . I don't know any IDE which does not allow providing preprocessor definitions. Note that by default the VC++ environment will define NDEBUG for you in release builds and not so in debug builds (compliant to the above test). Quote: 2) Does anyone have any positive/negative experiences with this technique? Positive, definitly, although I also use the ol' way via std::assert. The big advantage is, that you can use template techniques delegating to configuration-dependent code. The simplest way is something like: template <bool Debug> struct Checker { static void test(Args args) { // Perform some debug tests using arguments } }; template <> struct Checker<false> { static void test(Args) {} // Do nothing }; // Usage: void foo(Args args) { Checker<debug>::test(args); } More advanced techniques can use the template pattern or similar things to perform prefix/suffix code tests. Greetings from Bremen, Daniel Krügler [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top lotusny78@yahoo.com Guest Posted: Sat Apr 23, 2005 5:20 pm    Post subject: Re: const bool as debugging switch Of course you can #define through the IDE, but the point from the book is that there is a way to use the makefile so that you don't have any conditional compilations in your code. But how? [ 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 Apr 24, 2005 9:25 am    Post subject: Re: const bool as debugging switch [email]lotusny78 (AT) yahoo (DOT) com[/email] wrote: Quote: Of course you can #define through the IDE, but the point from the book is that there is a way to use the makefile so that you don't have any conditional compilations in your code. But how? Put conditional code in separate source files, and then select different sets of source files in the makefile depending on make variables. For example, if you want to build Linux and Windows versions of an app with some shared code and some platform-dependent, you could use: sources := foo-common.cpp bar.cpp ifeq ($(TARGET),Linux) sources += foo-linux.cpp baz-linux.cpp endif ifeq ($(TARGET),Windows) sources += foo-win32.cpp baz-win32.cpp endif objects := $(sources:.cpp=.o) app : $(objects) $(CXX) $(CXXFLAGS) $(LDFLAGS) $< -o $@ Then it's just a case of "make TARGET=Linux" or "make TARGET=Windows". (Some of the above might require GNU make. I don't bother trying to use other versions.) If you use an IDE you may be able to exclude certain source files from certain build configurations. Visual Studio certainly provides this option. -- Ben Hutchings Having problems with C++ templates? Your questions may be answered by [ 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 Apr 24, 2005 8:59 pm    Post subject: Re: const bool as debugging switch <lotusny78 (AT) yahoo (DOT) com> wrote Quote: In "C++ Gotchas," Stephen Dewhurst says that using: const bool debug = ...; is preferable to #ifdefs because: 1) all modern compilers should be able to easily optimize it away 2) all the code in your project will be parsed and checked, not just that of the current configuration The debug constant can deal with expressions and statements, but not declarations. How would one conditionally add a member to a class with this? It won't work. Equally bad, the debug expressions must still undergo semantic analysis, and be semantically correct. This precludes any referencing of conditionally compiled debug declarations. -Walter www.digitalmars.com C, C++, D compilers "code of the nerds" [ 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: Tue Apr 26, 2005 9:19 am    Post subject: Re: const bool as debugging switch [email]lotusny78 (AT) yahoo (DOT) com[/email] wrote: Quote: Of course you can #define through the IDE, but the point from the book is that there is a way to use the makefile so that you don't have any conditional compilations in your code. But how? bool const debugOn = DEBUGON ; Compiled with either -DDEBUGON=true or -DDEBUGON=false. To be frank, however, I don't quite see the interest in the technique. It's only formally that you don't have condition compilation; practically, the effects are the same. Maybe even worse -- if I see #ifdef, I know that what follows may not be a part of the code; if I see if ( debugOn ), it's less obvious. On the whole, however, I'd avoid either. -- 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 Allan W Guest Posted: Wed Apr 27, 2005 8:38 am    Post subject: Re: const bool as debugging switch k... (AT) gabi-soft (DOT) fr wrote: Quote: bool const debugOn = DEBUGON ; Compiled with either -DDEBUGON=true or -DDEBUGON=false. To be frank, however, I don't quite see the interest in the technique. It's only formally that you don't have condition compilation; practically, the effects are the same. Maybe even worse -- if I see #ifdef, I know that what follows may not be a part of the code; if I see if ( debugOn ), it's less obvious. If I see if(debugOn), I assume that debugOn is something that can be set by the user during a debugging session. Maybe the user has detected a problem on iteration number 4000 through a loop, so they enable some log messages starting on iteration 3998 to see what's different on the 4000th time through. My point is that I wouldn't expect debugOn to be immutable. Most debuggers can alter const objects. I'd declare it volatile though, just to make sure that the compiler doesn't optimize out the part after if(debugOn)... (not an issue if optimizations have been disabled). [ 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: Wed May 04, 2005 8:01 am    Post subject: Re: const bool as debugging switch Allan W wrote: Quote: k... (AT) gabi-soft (DOT) fr wrote: bool const debugOn = DEBUGON ; Compiled with either -DDEBUGON=true or -DDEBUGON=false. To be frank, however, I don't quite see the interest in the technique. It's only formally that you don't have condition compilation; practically, the effects are the same. Maybe even worse -- if I see #ifdef, I know that what follows may not be a part of the code; if I see if ( debugOn ), it's less obvious. If I see if(debugOn), I assume that debugOn is something that can be set by the user during a debugging session. Maybe the user has detected a problem on iteration number 4000 through a loop, so they enable some log messages starting on iteration 3998 to see what's different on the 4000th time through. I totally agree there (although for filtering log messages, grep and akw are pretty powerful tools). But in the example, debugOn is declared const. Quote: My point is that I wouldn't expect debugOn to be immutable. Most debuggers can alter const objects. Which probably won't help. Typically, if you compile with debugOn false, the compiler won't even generate the code for the unexecuted branches. And of course, if you've instrumented the code, why on earth would you bother with a debugger:-)? Quote: I'd declare it volatile though, just to make sure that the compiler doesn't optimize out the part after if(debugOn)... (not an issue if optimizations have been disabled). If I saw a bool const volatile, I'd immediately suppose that it must be memory mapped IO. -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre 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 billvanvalk@no_spam_gmail Guest Posted: Wed May 04, 2005 11:57 pm    Post subject: Re: const bool as debugging switch const is advisatory in C++. Can't one still do the following in some module where this variable is visible: *(bool*)&debug = false; So can the compiler really optimize this code away? It is a global variable, how does the compiler know that debug's value has not been changed in some other module other than that which it is currently compiling? BillV [ 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: Tue May 10, 2005 2:27 pm    Post subject: Re: const bool as debugging switch [email]billvanvalk (AT) no_spam_gmail_no_spam (DOT) com[/email] wrote: Quote: const is advisatory in C++. No. A const-qualified object must never be modified (with the exception of any mutable members it may have). Quote: Can't one still do the following in some module where this variable is visible: *(bool*)&debug = false; One can try that, but it has undefined behaviour. Quote: So can the compiler really optimize this code away? Yes. Quote: It is a global variable, how does the compiler know that debug's value has not been changed in some other module other than that which it is currently compiling? There is no legal way to do that, so it is allowed to assume that it doesn't. -- Ben Hutchings Having problems with C++ templates? Your questions may be answered by <http://womble.decadentplace.org.uk/c++/template-faq.html>. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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