Define constant variables in C header

Question

A program I am working on has many constants that apply throughout all classes  I want to make one header file  Constants h   and be able to declare all the relevant constants  Then in my other classes  I can just include  include  Constants h    I got it to work fine using  ifndef      define     syntax  However  I would prefer to use the const int    form of constants  I m not quite sure how to though

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You generally shouldn t use e g  const int in a header file  if it s included in several source files  That is because then the variables will be defined once per source file  translation units technically speaking  because global const variables are implicitly static  taking up more memory than required   You should instead have a special source file  Constants cpp that actually defines the variables  and then have the variables declared as extern in the header file   Something like this header file      Protect against multiple inclusions in the same source file  ifndef CONSTANTS H  define CONSTANTS H  extern const int CONSTANT 1    endif   And this in a source file   const int CONSTANT 1   123

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C  17 inline variables  This awesome C  17 feature allow us to    conveniently use just a single memory address for each constant store it as a constexpr  How to declare constexpr extern  do it in a single line from one header   main cpp   include  lt cassert gt    include  notmain hpp   int main            Both files see the same memory address      assert  amp notmain i    notmain func         assert notmain i    42       notmain hpp   ifndef NOTMAIN HPP  define NOTMAIN HPP  inline constexpr int notmain i   42   const int  notmain func      endif   notmain cpp   include  notmain hpp   const int  notmain func         return  amp notmain i      Compile and run   g   -c -o notmain o -std c  17 -Wall -Wextra -pedantic notmain cpp g   -c -o main o -std c  17 -Wall -Wextra -pedantic main cpp g   -o main -std c  17 -Wall -Wextra -pedantic main o notmain o   main   GitHub upstream   See also  How do inline variables work   C   standard on inline variables  The C   standard guarantees that the addresses will be the same  C  17 N4659 standard draft  10 1 6  The inline specifier       6 An inline function or variable with external linkage shall have the same address in all translation units    cppreference https   en cppreference com w cpp language inline explains that if static is not given  then it has external linkage   Inline variable implementation  We can observe how it is implemented with   nm main o notmain o   which contains   main o                   U  GLOBAL OFFSET TABLE                   U  Z12notmain funcv 0000000000000028 r  ZZ4mainE19  PRETTY FUNCTION                    U   assert fail 0000000000000000 T main 0000000000000000 u notmain i  notmain o  0000000000000000 T  Z12notmain funcv 0000000000000000 u notmain i   and man nm says about u       u  The symbol is a unique global symbol   This is a GNU extension to the standard set of ELF symbol bindings   For such a symbol the dynamic linker will make sure that in the entire process                  there is just one symbol with this name and type in use    so we see that there is a dedicated ELF extension for this   C  17 standard draft on  global  const implies static  This is the quote for what was mentioned at  https   stackoverflow com a 12043198 895245  C  17 n4659 standard draft 6 5  Program and linkage       3   A name having namespace scope  6 3 6  has internal linkage if it is the name of          3 1      a variable  function or function template that is explicitly declared static  or     3 2      a non-inline variable of non-volatile const-qualified type that is neither explicitly declared extern nor   previously declared to have external linkage  or    3 3      a data member of an anonymous union        namespace  scope is what we colloquially often refer to as  global    Annex C  informative  Compatibility  C 1 2  Clause 6   basic concepts  gives the rationale why this was changed from C      6 5  also 10 1 7       Change  A name of file scope that is explicitly declared const  and not explicitly declared extern  has   internal linkage  while in C it would have external linkage       Rationale  Because const objects may be used as values during translation in C    this feature urges   programmers to provide an explicit initializer for each const object  This feature allows the user to put   const objects in source files that are included in more than one translation unit       Effect on original feature  Change to semantics of well-defined feature       Difficulty of converting  Semantic transformation       How widely used  Seldom    See also  Why does const imply internal linkage in C    when it doesn  39 t in C   Tested in GCC 7 4 0  Ubuntu 18 04

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You could simply define a series of const ints in a header file     Constants h  if  defined MYLIB CONSTANTS H   define MYLIB CONSTANTS H 1  const int a   100  const int b   0x7f    endif  This works because in C   a name at namespace scope  including the global namespace  that is explicitly declared const and not explicitly declared extern has internal linkage  so these variables would not cause duplicate symbols when you link together translation units  Alternatively you could explicitly declare the constants as static  static const int a   100  static const int b   0x7f   This is more compatible with C and more readable for people that may not be familiar with C   linkage rules  If all the constants are ints then another method you could use is to declare the identifiers as enums  enum mylib constants       a   100      b   0x7f      All of these methods use only a header and allow the declared names to be used as compile time constants  Using extern const int and a separate implementation file prevents the names from being used as compile time constants   Note that the rule that makes certain constants implicitly internal linkage does apply to pointers  exactly like constants of other types  The tricky thing though is that marking a pointer as const requires syntax a little different that most people use to make variables of other types const  You need to do  int   const ptr   to make a constant pointer  so that the rule will apply to it  Also note that this is one reason I prefer to consistently put const after the type  int const instead of const int  I also put the   next to the variable  i e  int  ptr  instead of int  ptr   compare also this discussion   I like to do these sorts of things because they reflect the general case of how C   really works  The alternatives  const int  int  p  are just special cased to make some simple things more readable  The problem is that when you step out of those simple cases  the special cased alternatives become actively misleading  So although the earlier examples show the common usage of const  I would actually recommend people write them like this  int const a   100  int const b   0x7f   and static int const a   100  static int const b   0x7f

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Rather than making a bunch of global variables  you might consider creating a class that has a bunch of public static constants  It s still global  but this way it s wrapped in a class so you know where the constant is coming from and that it s supposed to be a constant   Constants h   ifndef CONSTANTS H  define CONSTANTS H  class GlobalConstants     public      static const int myConstant      static const int myOtherConstant       endif   Constants cpp   include  Constants h   const int GlobalConstants  myConstant   1  const int GlobalConstants  myOtherConstant   3    Then you can use this like so    include  Constants h   void foo       int foo   GlobalConstants  myConstant

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I like the namespace better for this kind of purpose   Option 1     ifndef MYLIB CONSTANTS H  define MYLIB CONSTANTS H      File Name   LibConstants hpp    Purpose   Global Constants for Lib Utils namespace LibConstants     const int CurlTimeOut   0xFF         Just some example          endif     source cpp  include  lt LibConstants hpp gt  int value   LibConstants  CurlTimeOut    Option 2     ifndef MYLIB CONSTANTS H  define MYLIB CONSTANTS H     File Name   LibConstants hpp    Purpose   Global Constants for Lib Utils namespace CurlConstants     const int CurlTimeOut   0xFF         Just some example          namespace MySQLConstants     const int DBPoolSize   0xFF          Just some example          endif       source cpp  include  lt LibConstants hpp gt  int value   CurlConstants  CurlTimeOut  int val2    MySQLConstants  DBPoolSize    And I would never use a Class to hold this type of HardCoded Const variables

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It seems that bames53 s answer can be extended to defining integer and non-integer constant values in namespace and class declarations even if they get included in multiple source files   It is not necessary to put the declarations in a header file but the definitions in a source file   The following example works for Microsoft Visual Studio 2015  for z OS V2 2 XL C C   on OS 390  and for g    GCC  8 1 1 20180502 on GNU Linux 4 16 14  Fedora 28    Note that the constants are declared defined in only a single header file that gets included in multiple source files   In foo cc    include  lt cstdio gt                   for puts   include  messages hh   include  bar hh   include  zoo hh   int main int argc  const char  argv        puts  Hello       bar      zoo      puts Message  third     return 0      In messages hh    ifndef MESSAGES HH  define MESSAGES HH  namespace Message     char const   const first    Yes  this is the first message      char const   const second    This is the second message      char const   const third    Message  3         endif   In bar cc    include  messages hh   include  lt cstdio gt   void bar void      puts  Wow       printf  bar   s n   Message  first       In zoo cc    include  lt cstdio gt   include  messages hh   void zoo void      printf  zoo   s n   Message  second       In bar hh    ifndef BAR HH  define BAR HH   include  messages hh   void bar void     endif   In zoo hh    ifndef ZOO HH  define ZOO HH   include  messages hh   void zoo void     endif   This yields the following output   Hello  Wow  bar  Yes  this is the first message  zoo  This is the second message  Message  3    The data type char const   const means a constant pointer to an array of constant characters   The first const is needed because  according to g     ISO C   forbids converting a string constant to  char      The second const is needed to avoid link errors due to multiple definitions of the  then insufficiently constant  constants   Your compiler might not complain if you omit one or both of the consts  but then the source code is less portable

[c++] Define constant variables in C++ header

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