How to declare a static const char in your header file

Question

I d like to define a constant char  in my header file for my  cpp file to use   So I ve tried this   private      static const char  SOMETHING    sommething     Which brings me with the following compiler error      error C2864   SomeClass  SOMETHING      only static const integral data   members can be initialized within a   class   I m new to C     What is going on here   Why is this illegal   And how can you do it alternatively

User · Accepted Answer

You need to define static variables in a translation unit, unless they are of integral types.

In your header:

private:
    static const char *SOMETHING;
    static const int MyInt = 8; // would be ok

In the .cpp file:

const char *YourClass::SOMETHING = "something";

C++ standard, 9.4.2/4:

If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression. In that case, the member can appear in integral constant expressions within its scope. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.

User · Answer

The error is that you cannot initialize a static const char  within the class  You can only initialize integer variables there   You need to declare the member variable in the class  and then initialize it outside the class      header file  class Foo       static const char  SOMETHING         rest of class         cpp file  const char  Foo  SOMETHING    sommething     If this seems annoying  think of it as being because the initialization can only appear in one translation unit  If it was in the class definition  that would usually be included by multiple files  Constant integers are a special case  which means the error message perhaps isn t as clear as it might be   and compilers can effectively replace uses of the variable with the integer value   In contrast  a char  variable points to an actual object in memory  which is required to really exist  and it s the definition  including initialization  which makes the object exist  The  one definition rule  means you therefore don t want to put it in a header  because then all translation units including that header would contain the definition  They could not be linked together  even though the string contains the same characters in both  because under current C   rules you ve defined two different objects with the same name  and that s not legal  The fact that they happen to have the same characters in them doesn t make it legal

User · Answer

If you re using Visual C    you can non-portably do this using hints to the linker        In foo h     class Foo   public     static const char  Bar         Still in foo h  doesn t need to be in a  cpp file       declspec selectany  const char  Foo  Bar    Blah       declspec selectany  means that even though Foo  Bar will get declared in multiple object files  the linker will only pick up one   Keep in mind this will only work with the Microsoft toolchain   Don t expect this to be portable

User · Answer

To answer the why question  integral types are special in that they are not a reference to an allocated object but rather values that are duplicated and copied  It s just an implementation decision made when the language was defined  which was to handle values outside the object system and in as efficient and  inline  a fashion as possible   This doesn t exactly explain why they are allowed as initializors in a type  but think of it as essentially a  define and then it will make sense as part of the type and not part of the object

User · Answer

Constant initializer allowed by C   Standard only for integral or enumeration types  See 9 4 2 4 for details      If a static data member is of const integral or const enumeration type  its declaration in the class   definition can specify a constant-initializer which shall be an integral constant expression  5 19    In that   case  the member can appear in integral constant expressions   The member shall still be defined in a name-   space scope if it is used in the program and the namespace scope definition shall not contain an initializer    And 9 4 2 7      Static data members are initialized and destroyed exactly like non-local objects  3 6 2  3 6 3     So you should write somewhere in cpp file   const char  SomeClass  SOMETHING    sommething

User · Answer

To answer the OP s question about why it is only allowed with integral types   When an object is used as an lvalue  i e  as something that has address in storage   it has to satisfy the  one definition rule   ODR   i e it has to be defined in one and only one translation unit  The compiler cannot and will not decide which translation unit to define that object in  This is your responsibility  By defining that object somewhere you are not just defining it  you are actually telling the compiler that you want to define it here  in this specific translation unit   Meanwhile  in C   language integral constants have special status  They can form integral constant expressions  ICEs   In ICEs integral constants are used as ordinary values  not as objects  i e  it is not relevant whether such integral value has address in the storage or not   In fact  ICEs are evaluated at compile time  In order to facilitate such a use of integral constants their values have to be visible globally  And the constant itself don t really need an actual place in the storage  Because of this integral constants received special treatment  it was allowed to include their initializers in the header file  and the requirement to provide a definition was relaxed  first de facto  then de jure    Other constant types has no such properties  Other constant types are virtually always used as lvalues  or at least can t participate in ICEs or anything similar to ICE   meaning that they require a definition  The rest follows

User · Answer

With C  11 you can use the constexpr keyword and write in your header   private      static constexpr const char  SOMETHING    something      Notes    constexpr makes SOMETHING a constant pointer so you cannot write  SOMETHING    something different     later on  Depending on your compiler  you might also need to write an explicit definition in the  cpp file   constexpr const char  MyClass  SOMETHING

User · Answer

class A  public     static const char  SOMETHING     return  something          I do it all the time - especially for expensive const default parameters   class A     static    const expensive to construct amp     default expensive to construct          static const expensive to construct xp2c whatever is needed         return xp2c

User · Answer

There is a trick you can use with templates to provide H file only constants    note  this is an ugly example  but works verbatim in at least in g   4 6 1     values hpp file    include  lt string gt   template lt int dummy gt  class tValues   public     static const char  myValue      template  lt int dummy gt  const char  tValues lt dummy gt   myValue    This is a value    typedef tValues lt 0 gt  Values   std  string otherCompUnit       test from other compilation unit    main cpp    include  lt iostream gt   include  values hpp   int main        std  cout  lt  lt   from main     lt  lt  Values  myValue  lt  lt  std  endl     std  cout  lt  lt   from other     lt  lt  otherCompUnit    lt  lt  std  endl       other cpp    include  values hpp   std  string otherCompUnit         return std  string Values  myValue       Compile  e g  g   -o main main cpp other cpp  amp  amp    main  and see two compilation units referencing the same constant declared in a header   from main  This is a value from other  This is a value     In MSVC  you may instead be able to use   declspec selectany   For example     declspec selectany  const char  data    My data

[c++] How to declare a static const char* in your header file?

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