Static constant string class member

Question

I d like to have a private static constant for a class  in this case a shape-factory   I d like to have something of the sort  class A      private        static const string RECTANGLE    quot rectangle quot      Unfortunately I get all sorts of error from the C    g    compiler  such as   ISO C   forbids initialization of member    RECTANGLE    invalid in-class initialization of static data member of non-integral type    std  string    error  making    RECTANGLE    static  This tells me that this sort of member design is not compliant with the standard  How do you have a private literal constant  or perhaps public  without having to use a  define directive  I want to avoid the uglyness of data globality   Any help is appreciated

User · Answer

The current standard only allows such initialization for static constant integral types  So you need to do as AndreyT explained  However  that will be available in the next standard through the new member initialization syntax

User · Answer

This is just extra information  but if you really want the string in a header file  try something like   class foo   public      static const std  string amp  RECTANGLE void                static const std  string str    rectangle            return str             Though I doubt that s recommended

User · Answer

You can either go for the const char  solution mentioned above  but then if you need string all the time  you re going to have a lot of overhead  On the other hand  static string needs dynamic initialization  thus if you want to use its value during another global static variable s initialization  you might hit the problem of initialization order  To avoid that  the cheapest thing is accessing the static string object through a getter  which checks if your object is initialized or not       in a header   class A      static string s     public       static string getS             in implementation   string A  s    namespace      bool init A s          A  s   string  foo           return true            bool A s initialized   init A s          string A  getS            if   A s initialized        A s initialized   init A s        return s          Remember to only use A  getS    Because any threading can only started by main    and A s initialized is initialized before main    you don t need locks even in a multithreaded environment  A s initialized is 0 by default  before the dynamic initialization   so if you use getS   before s is initialized  you call the init function safely   Btw  in the answer above   static const std  string RECTANGLE   const    static functions cannot be const because they cannot change the state if any object anyway  there is no this pointer

User · Answer

To use that in-class initialization   syntax  the constant must be a static   const of integral or enumeration type   initialized by a constant expression    This is the restriction  Hence  in this case you need to define variable outside the class  refer answwer from  AndreyT

User · Answer

In C   17 you can use inline variables   class A    private    static inline const std  string my string    some useful string constant        Note that this is different from abyss 7 s answer  This one defines an actual std  string object  not a const char

User · Answer

The class static variables can be declared in the header but must be defined in a  cpp file  This is because there can be only one instance of a static variable and the compiler can t decide in which generated object file to put it so you have to make the decision  instead   To keep the definition of a static value with the declaration in C  11  a nested static structure can be used  In this case the static member  is a structure and has to be defined in a  cpp file  but the values  are in the header   class A   private    static struct  Shapes        const std  string RECTANGLE   rectangle         const std  string CIRCLE   circle        shape       Instead of initializing individual members the whole static structure is initialized in  cpp   A   Shapes A  shape    The values are accessed with  A  shape RECTANGLE    or -- since the members are private and are meant to be used only from A -- with   shape RECTANGLE    Note that this solution still suffers from the problem of the order of  initialization of the static variables  When a static value is used to  initialize another static variable  the first may not be initialized   yet      file h class File   public    static struct  Extensions       const std  string h    h         const std  string hpp    hpp         const std  string c    c         const std  string cpp    cpp         extension         file cpp File   Extensions File  extension      module cpp static std  set lt std  string gt  headers  File  extension h  File  extension hpp      In this case the static variable headers will contain either         or     h     hpp     depending on the order of initialization created by the linker   As mentioned by  abyss 7 you could also use constexpr if the value of the variable can be computed at compile time  But if you declare your strings with static constexpr const char  and your program uses std  string otherwise there will be an overhead because a new std  string object will be created every time you use such a constant   class A   public     static constexpr const char  STRING    some value      void foo const std  string amp  bar   int main        foo A  STRING      a new std  string is constructed and destroyed

User · Answer

possible just do   static const std  string RECTANGLE   const       return  rectangle        or    define RECTANGLE  rectangle

User · Answer

You have to define your static member outside the class definition and provide the initializer there  First    In a header file  if it is in a header file in your case  class A      private          static const string RECTANGLE      and then    In one of the implementation files const string A  RECTANGLE    quot rectangle quot    The syntax you were originally trying to use  initializer inside class definition  is only allowed with integral and enum types   Starting from C  17 you have another option  which is quite similar to your original declaration  inline variables    In a header file  if it is in a header file in your case  class A      private          inline static const string RECTANGLE    quot rectangle quot       No additional definition is needed  Starting from C  20 instead of const you can declare it constexpr in this variant  Explicit inline would no longer be necessary  since constexpr implies inline

User · Answer

Fast forward to 2018 and C  17    do not use std  string  use std  string view literals please do notice the  constexpr  bellow  This is also an  compile time  mechanism  no inline does not mean repetition no cpp files are not necessary for this static assert  works  at compile time only  using namespace std  literals   namespace STANDARD   constexpr  inline  auto  compiletime static string view constant        make and return string view literal    will stay the same for the whole application lifetime    will exhibit standard and expected interface    will be usable at both    runtime and compile time    by value semantics implemented for you     auto when needed      compile time sv      return when needed              Above is a proper and legal standard C   citizen  It can get readily involved in any and all std   algorithms  containers  utilities and a such  For example      test the resilience auto return by val              auto return by val                  auto return by val                      auto return by val          return STANDARD  compiletime static string view constant                               return return by val                       return return by val               return return by val           actually a run time   ASSERTE return by val       compile time        compile time  static assert     STANDARD  compiletime static string view constant           compile time         Enjoy the standard C

User · Answer

Inside class definitions you can only declare static members  They have to be defined outside of the class  For compile-time integral constants the standard makes the exception that you can  initialize  members  It s still not a definition  though  Taking the address would not work without definition  for example   I d like to mention that I don t see the benefit of using std  string over const char   for constants  std  string is nice and all but it requires dynamic initialization  So  if you write something like  const std  string foo    hello     at namespace scope the constructor of foo will be run right before execution of main starts and this constructor will create a copy of the constant  hello  in the heap memory  Unless you really need RECTANGLE to be a std  string you could just as well write     class definition with incomplete static member could be in a header file class A       static const char RECTANGLE           this needs to be placed in a single translation unit only const char A  RECTANGLE      rectangle     There  No heap allocation  no copying  no dynamic initialization   Cheers  s

User · Answer

In C  11 you can do now   class A    private    static constexpr const char  STRING    some useful string constant

[c++] Static constant string (class member)

Examples related to c++

Examples related to string

Examples related to class

Examples related to static

Examples related to constants