Initialize static variables in C class

Question

I have noticed that some of my functions in a class are actually not accessing the object  so I made them static  Then the compiler told me that all variables they access must also be static     well  quite understandable so far  I have a bunch of string variables such as  string RE ANY         n      string RE ANY RELUCTANT         n         and so on in the class  I have then made them all static const because they never change  However  my program only compiles if I move them out of the class  Otherwise  MSVC  2010 complains  Only static constant integral variables may be initialized within a class    Well that s unfortunate  Is there a workaround  I would like to leave them inside the class they belong to

User · Answer

Optionally  move all your constants to  cpp file without declaration in  h file  Use anonymous namespace to make them invisible beyond the cpp module      MyClass cpp   include  MyClass h      anonymous namespace namespace       string RE ANY         n          string RE ANY RELUCTANT         n             member function  static or not  bool MyClass  foo            logic that uses constants     return RE ANY RELUCTANT size    gt  0

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If your goal is to initialize the static variable in your header file  instead of a   cpp file  which you may want if you are sticking to a  header only  idiom   then you can work around the initialization problem by using a template  Templated static variables can be initialized in a header  without causing multiple symbols to be defined   See here for an example   Static member initialization in a class template

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Just to add on top of the other answers  In order to initialize a complex static member  you can do it as follows   Declare your static member as usual      myClass h class myClass   static complexClass s complex             Make a small function to initialize your class if it s not trivial to do so  This will be called just the one time the static member is initialized   Note that the copy constructor of complexClass will be used  so it should be well defined      class cpp      include myClass h complexClass initFunction         complexClass c      c add           c compute           c sort              Etc      return c     complexClass myClass  s complex   initFunction

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Some answers seem to be a little misleading   You don t have to       Assign a value to some static object when initializing  because assigning a value is Optional  Create another  cpp file for initializing since it can be done in the same Header file    Also  you can even initialize a static object in the same class scope just like a normal variable using the inline keyword     Initialize with no values in the same file   include  lt string gt  class A       static std  string str      static int x     std  string A  str  int A  x    Initialize with values in the same file   include  lt string gt  class A       static std  string str      static int x     std  string A  str    SO    int A  x   900    Initialize in the same class scope using the inline keyword   include  lt string gt  class A       static inline std  string str    SO        static inline int x   900

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Mike Seymour has given you the right answer  but to add    C   lets you declare and define in your class body only static const integral types  as the compiler tells  So you can actually do   class Foo       static const int someInt   1      static const short someShort   2         etc       And you can t do that with any other type  in that cases you should define them in your  cpp file

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I feel it is worth adding that a static variable is not the same as a constant variable   using a constant variable in a class  struct Foo      const int a      Foo int b    a b        and we would declare it like like so  fooA   new Foo 5   fooB   new Foo 10      fooA a   5     fooB a   10    For a static variable  struct Bar      static int a      Foo int b           a   b          Bar  a   0     set value for a   which is used like so  barA   new Bar 5   barB   new Bar 10      barA a   10     barB a   10     Bar  a   10    You see what happens here  The constant variable  which is instanced along with each instance of Foo  as Foo is instanced has a separate value for each instance of Foo  and it can t be changed by Foo at all   Where as with Bar  their is only one value for Bar  a no matter how many instances of Bar are made  They all share this value  you can also access it with their being any instances of Bar  The static variable also abides rules for public private  so you could make it that only instances of Bar can read the value of Bar  a

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Since C  11 it can be done inside a class with constexpr   class stat       public             init inside class         static constexpr double inlineStaticVar   22       The variable can now be accessed with   stat  inlineStaticVar

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They can t be initialised inside the class  but they can be initialised outside the class  in a source file      inside the class class Thing       static string RE ANY      static string RE ANY RELUCTANT         in the source file string Thing  RE ANY         n      string Thing  RE ANY RELUCTANT         n         Update  I ve just noticed the first line of your question - you don t want to make those functions static  you want to make them const  Making them static means that they are no longer associated with an object  so they can t access any non-static members   and making the data static means it will be shared with all objects of this type  This may well not be what you want  Making them const simply means that they can t modify any members  but can still access them

User · Answer

Static member variables must be declared in the class and then defined outside of it   There s no workaround  just put their actual definition in a source file     From your description it smells like you re not using static variables the right way  If they never change you should use constant variable instead  but your description is too generic to say something more   Static member variables always hold the same value for any instance of your class  if you change a static variable of one object  it will change also for all the other objects  and in fact you can also access them without an instance of the class - ie  an object

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