How do I show the value of a define at compile-time

Question

I am trying to figure out what version of Boost my code thinks it s using  I want to do something like this    error BOOST VERSION  but the preprocessor does not expand BOOST VERSION   I know I could print it out at run-time from the program  and I know I could look at the output of the preprocessor to find the answer  I feel like having a way of doing this during compilation could be useful

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If you are using Visual C    you can use  pragma message    include  lt boost preprocessor stringize hpp gt   pragma message  BOOST VERSION   BOOST PP STRINGIZE BOOST VERSION     Edit  Thanks to LB for link  Apparently  the GCC equivalent is  not tested     pragma message  BOOST VERSION   BOOST PP STRINGIZE BOOST VERSION

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define a  lt   BOOST VERSION gt   include a    MSVC2015  fatal error C1083  Cannot open include file     106200   No such file or directory   Works even if preprocess to file is enabled  even if invalid tokens are present    define a  lt         gt   include a    MSVC2015  fatal error C1083  Cannot open include file             No such file or directory GCC4 x  warning  missing terminating   character  -Winvalid-pp-token       define a  lt         gt

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You could write a program that prints out BOOST VERSION and compile and run it as part of your build system  Otherwise  I think you re out of luck

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BOOST VERSION is defined in the boost header file version hpp

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Without boost     define same macro again and compiler HIMSELF will give warning  From warning you can see location of the previous definition  vi file of previous definition     ambarish axiom   cpp  g   shiftOper cpp shiftOper cpp 7 1  warning   LINUX VERSION CODE  redefined shiftOper cpp 6 1  warning  this is the location of the previous definition   define LINUX VERSION CODE 265216  define LINUX VERSION CODE 666  int main

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You could also preprocess the source file and see what the preprocessor value evaluates to

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Looking at the output of the preprocessor is the closest thing to the answer you ask for   I know you ve excluded that  and other ways   but I m not sure why  You have a specific enough problem to solve  but you have not explained why any of the  normal  methods don t work well for you

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Are you looking for   if BOOST VERSION     1 2   error  Bad version   endif   Not great if BOOST VERSION is a string  like I ve assumed  but there may also be individual integers defined for the major  minor and revision numbers

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Instead of  error  try redefining the macro  just before it is being used  Compilation will fail and compiler will provide the current value it thinks applies to the macro   define BOOST VERSION blah

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Take a look at the Boost documentation as well  regarding how you are using the macro   In reference to BOOST VERSION  from http   www boost org doc libs 1 37 0 libs config doc html boost config boost macro reference html boost config boost macro reference boost helper macros      Describes the boost version number in   XXYYZZ format such that     BOOST VERSION   100  is the sub-minor   version    BOOST VERSION   100      1000  is the minor version  and    BOOST VERSION   100000  is the major   version

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In Microsoft C C    you can use the built-in  CRT STRINGIZE   to print constants  Many of my stdafx h files contain some combination of these    pragma message   MSC VER      is    CRT STRINGIZE  MSC VER    pragma message   MFC VER      is    CRT STRINGIZE  MFC VER    pragma message   ATL VER      is    CRT STRINGIZE  ATL VER    pragma message  WINVER        is    CRT STRINGIZE WINVER    pragma message   WIN32 WINNT  is    CRT STRINGIZE  WIN32 WINNT    pragma message   WIN32 IE     is    CRT STRINGIZE  WIN32 IE    pragma message  NTDDI VERSION is    CRT STRINGIZE NTDDI VERSION      and outputs something like this    MSC VER      is 1915  MFC VER      is 0x0E00  ATL VER      is 0x0E00 WINVER        is 0x0600  WIN32 WINNT  is 0x0600  WIN32 IE     is 0x0700 NTDDI VERSION is 0x06000000

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BOOST PP STRINGIZE seems a excellent solution for C    but not for regular C   Here is my solution for GNU CPP      Some test definition here     define DEFINED BUT NO VALUE  define DEFINED INT 3  define DEFINED STR  ABC      definition to expand macro then apply to pragma message     define VALUE TO STRING x   x  define VALUE x  VALUE TO STRING x   define VAR NAME VALUE var   var      VALUE var      Some example here     pragma message VAR NAME VALUE NOT DEFINED    pragma message VAR NAME VALUE DEFINED BUT NO VALUE    pragma message VAR NAME VALUE DEFINED INT    pragma message VAR NAME VALUE DEFINED STR     Above definitions result in   test c 10 9  note   pragma message  NOT DEFINED NOT DEFINED test c 11 9  note   pragma message  DEFINED BUT NO VALUE  test c 12 9  note   pragma message  DEFINED INT 3 test c 13 9  note   pragma message  DEFINED STR  ABC    For  defined as interger    defined as string   and  defined but no value  variables   they work just fine  Only for  not defined  variable  they displayed exactly the same as original variable name  You have to used to it -- or maybe someone can provide a better solution

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As far as I know   error  only will print strings  in fact you don t even need to use quotes   Have you tried writing various purposefully incorrect code using  BOOST VERSION    Perhaps something like  blah BOOST VERSION    foo   will tell you something like  string literal 1 2 1 cannot be used as an array address    It won t be a pretty error message  but at least it ll show you the relevant value   You can play around until you find a compile error that does tell you the value

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I know that this is a long time after the original query  but this may still be useful   This can be done in GCC using the stringify operator      but it requires two stages    define XSTR x  STR x   define STR x   x   The value of a macro can then be displayed with    pragma message  The value of ABC    XSTR ABC    See  3 4 Stringification in the gcc online documentation   How it works   The preprocessor understands quoted strings and handles them differently from normal text  String concatenation is an example of this special treatment  The message pragma requires an argument that is a quoted string  When there is more than one component to the argument then they must all be strings so that string concatenation can be applied  The preprocessor can never assume that an unquoted string should be treated as if it were quoted  If it did then    define ABC 123 int n   ABC    would not compile   Now consider    define ABC abc  pragma message  The value of ABC is    ABC   which is equivalent to   pragma message  The value of ABC is    abc   This causes a preprocessor warning because abc  unquoted  cannot be concatenated with the preceding string   Now consider the preprocessor stringize  Which was once called stringification  the links in the documentation have been changed to reflect the revised terminology   Both terms  incidentally  are equally detestable  The correct term is  of course  stringifaction  Be ready to update your links    operator  This acts only on the arguments of a macro and replaces the unexpanded argument with the argument enclosed in double quotes  Thus    define STR x   x char  s1    abc   char  s2   STR abc     will assign identical values to s1 and s2  If you run gcc -E you can see this in the output  Perhaps STR would be better named something like ENQUOTE   This solves the problem of putting quotes around an unquoted item  the problem now is that  if the argument is a macro  the macro will not be expanded  This is why the second macro is needed  XSTR expands its argument  then calls STR to put the expanded value into quotes

[macros] How do I show the value of a #define at compile-time?

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