How to initialize private static members in C

Question

What is the best way to initialize a private  static data member in C     I tried this in my header file  but it gives me weird linker errors   class foo       private          static int i      int foo  i   0    I m guessing this is because I can t initialize a private member from outside the class   So what s the best way to do this

User · Answer

I follow the idea from Karl  I like it and now I use it as well   I ve changed a little bit the notation and add some functionality   include  lt stdio h gt   class Foo      public        int   GetMyStaticValue    const    return MyStatic            int  amp  GetMyStaticVar               return MyStatic            static bool isMyStatic  int  amp  num     return  amp  num     amp  MyStatic          private         static int  amp  MyStatic                     static int mStatic   7           return mStatic              int main  int  char          Foo obj      printf   mystatic value  d n   obj GetMyStaticValue        obj GetMyStaticVar      3     printf   mystatic value  d n   obj GetMyStaticValue         int valMyS   obj GetMyStaticVar        int  amp  iPtr1   obj GetMyStaticVar        int  amp  iPtr2   valMyS      printf   is my static  d  d n   Foo  isMyStatic iPtr1   Foo  isMyStatic iPtr2        this outputs   mystatic value 7 mystatic value 3 is my static 1 0

User · Answer

int foo  i   0     Is the correct syntax for initializing the variable  but it must go in the source file   cpp  rather than in the header    Because it is a static variable the compiler needs to create only one copy of it  You have to have a line  int foo i  some where in your code to tell the compiler where to put it otherwise you get a link error  If that is in a header you will get a copy in every file that includes the header  so get multiply defined symbol errors from the linker

User · Answer

One  old-school  way to define constants is to replace them by a enum   class foo       private          enum  i   0      default type   int         enum  int64 t  HUGE   1000000000000      may specify another type      This way doesn t require providing a definition  and avoids making the constant lvalue  which can save you some headaches  e g  when you accidentally ODR-use it

User · Answer

Does this serves your purpose     header file  struct MyStruct   public      const std  unordered map lt std  string  uint32 t gt  str to int             a   1               b   2                           z   26              const std  unordered map lt int   std  string gt  int to str            1   a               2   b                           26   z               std  string some string    justanotherstring         uint32 t some int   42       static MyStruct  amp  Singleton             static MyStruct instance          return instance        private      MyStruct             Usage in cpp file int main        std  cout lt  lt MyStruct  Singleton   some string lt  lt std  endl      std  cout lt  lt MyStruct  Singleton   some int lt  lt std  endl      return 0

User · Answer

You can also include the assignment in the header file if you use header guards  I have used this technique for a C   library I have created  Another way to achieve the same result is to use static methods  For example     class Foo         public       int GetMyStatic   const               return  MyStatic               private       static int  MyStatic                 static int mStatic   0         return  amp mStatic                The above code has the  bonus  of not requiring a CPP source file  Again  a method I use for my C   libraries

User · Answer

I just wanted to mention something a little strange to me when I first encountered this   I needed to initialize a private static data member in a template class   in the  h or  hpp  it looks something like this to initialize a static data member of a template class   template lt typename T gt  Type ClassName lt T gt   dataMemberName   initialValue

User · Answer

For future viewers of this question  I want to point out that you should avoid what monkey0506 is suggesting   Header files are for declarations   Header files get compiled once for every  cpp file that directly or indirectly  includes them  and code outside of any function is run at program initialization  before main     By putting  foo  i   VALUE  into the header  foo i will be assigned the value VALUE  whatever that is  for every  cpp file  and these assignments will happen in an indeterminate order  determined by the linker  before main   is run   What if we  define VALUE to be a different number in one of our  cpp files  It will compile fine and we will have no way of knowing which one wins until we run the program   Never put executed code into a header for the same reason that you never  include a  cpp file   include guards  which I agree you should always use  protect you from something different  the same header being indirectly  included multiple times while compiling a single  cpp file

User · Answer

For a variable   foo h   class foo   private      static int i       foo cpp   int foo  i   0    This is because there can only be one instance of foo  i in your program  It s sort of the equivalent of extern int i in a header file and int i in a source file   For a constant you can put the value straight in the class declaration   class foo   private      static int i      const static int a   42

User · Answer

For a variable   foo h   class foo   private      static int i       foo cpp   int foo  i   0    This is because there can only be one instance of foo  i in your program  It s sort of the equivalent of extern int i in a header file and int i in a source file   For a constant you can put the value straight in the class declaration   class foo   private      static int i      const static int a   42

User · Answer

Since C  17  static members may be defined in the header with the inline keyword   http   en cppreference com w cpp language static   A static data member may be declared inline  An inline static data member can be defined in the class definition and may specify a default member initializer  It does not need an out-of-class definition    struct X       inline static int n   1

User · Answer

I follow the idea from Karl  I like it and now I use it as well   I ve changed a little bit the notation and add some functionality   include  lt stdio h gt   class Foo      public        int   GetMyStaticValue    const    return MyStatic            int  amp  GetMyStaticVar               return MyStatic            static bool isMyStatic  int  amp  num     return  amp  num     amp  MyStatic          private         static int  amp  MyStatic                     static int mStatic   7           return mStatic              int main  int  char          Foo obj      printf   mystatic value  d n   obj GetMyStaticValue        obj GetMyStaticVar      3     printf   mystatic value  d n   obj GetMyStaticValue         int valMyS   obj GetMyStaticVar        int  amp  iPtr1   obj GetMyStaticVar        int  amp  iPtr2   valMyS      printf   is my static  d  d n   Foo  isMyStatic iPtr1   Foo  isMyStatic iPtr2        this outputs   mystatic value 7 mystatic value 3 is my static 1 0

User · Answer

Since C  17  static members may be defined in the header with the inline keyword   http   en cppreference com w cpp language static   A static data member may be declared inline  An inline static data member can be defined in the class definition and may specify a default member initializer  It does not need an out-of-class definition    struct X       inline static int n   1

User · Answer

What about a set default   method   class foo       public          static void set default int       private          static int i      void foo  set default int x        i   x      We would only have to use the set default int x  method and our static variable would be initialized   This would not be in disagreement with the rest of the comments  actually it follows the same principle of initializing the variable in a global scope  but by using this method we make it explicit  and easy to see-understand  instead of having the definition of the variable hanging there

User · Answer

int foo  i   0     Is the correct syntax for initializing the variable  but it must go in the source file   cpp  rather than in the header    Because it is a static variable the compiler needs to create only one copy of it  You have to have a line  int foo i  some where in your code to tell the compiler where to put it otherwise you get a link error  If that is in a header you will get a copy in every file that includes the header  so get multiply defined symbol errors from the linker

User · Answer

Also working in privateStatic cpp file     include  lt iostream gt   using namespace std   class A   private    static int v      int A  v   10     possible initializing  int main     A a    cout  lt  lt  A  v  lt  lt  endl     no access because of private scope return 0        g   privateStatic cpp -o privateStatic  amp  amp    privateStatic

User · Answer

The class declaration should be in the header file  Or in the source file if not shared   File  foo h  class foo       private          static int i       But the initialization should be in source file  File  foo cpp  int foo  i   0    If the initialization is in the header file then each file that includes the header file will have a definition of the static member  Thus during the link phase you will get linker errors as the code to initialize the variable will be defined in multiple source files  The initialisation of the static int i must be done outside of any function   Note  Matt Curtis  points out that C   allows the simplification of the above if the static member variable is of const int type  e g  int  bool  char   You can then declare and initialize the member variable directly inside the class declaration in the header file   class foo       private          static int const i   42

User · Answer

The linker problem you encountered is probably caused by    Providing both class and static member definition in header file  Including this header in two or more source files    This is a common problem for those who starts with C    Static class member must be initialized in single translation unit i e  in single source file   Unfortunately  the static class member must be initialized outside of the class body  This complicates writing header-only code  and  therefore  I am using quite different approach  You can provide your static object through static or non-static class function for example   class Foo          int amp  getObjectInstance   const       static int amp  getObjectInstance             static int object          return object             void func             int  amp object   getValueInstance            object    5

User · Answer

If you want to initialize some compound type  f e  string  you can do something like that   class SomeClass     static std  list lt string gt   list     public      static const std  list lt string gt  amp  getList           struct Initializer            Initializer                   Here you may want to put mutex             list push back  FIRST                list push back  SECOND                                             static Initializer ListInitializationGuard        return  list             As the ListInitializationGuard is a static variable inside SomeClass  getList   method it will be constructed only once  which means that constructor is called once  This will initialize  list variable to value you need  Any subsequent call to getList will simply return already initialized  list object   Of course you have to access  list object always by calling getList   method

User · Answer

For a variable   foo h   class foo   private      static int i       foo cpp   int foo  i   0    This is because there can only be one instance of foo  i in your program  It s sort of the equivalent of extern int i in a header file and int i in a source file   For a constant you can put the value straight in the class declaration   class foo   private      static int i      const static int a   42

User · Answer

What about a set default   method   class foo       public          static void set default int       private          static int i      void foo  set default int x        i   x      We would only have to use the set default int x  method and our static variable would be initialized   This would not be in disagreement with the rest of the comments  actually it follows the same principle of initializing the variable in a global scope  but by using this method we make it explicit  and easy to see-understand  instead of having the definition of the variable hanging there

User · Answer

I just wanted to mention something a little strange to me when I first encountered this   I needed to initialize a private static data member in a template class   in the  h or  hpp  it looks something like this to initialize a static data member of a template class   template lt typename T gt  Type ClassName lt T gt   dataMemberName   initialValue

User · Answer

The class declaration should be in the header file  Or in the source file if not shared   File  foo h  class foo       private          static int i       But the initialization should be in source file  File  foo cpp  int foo  i   0    If the initialization is in the header file then each file that includes the header file will have a definition of the static member  Thus during the link phase you will get linker errors as the code to initialize the variable will be defined in multiple source files  The initialisation of the static int i must be done outside of any function   Note  Matt Curtis  points out that C   allows the simplification of the above if the static member variable is of const int type  e g  int  bool  char   You can then declare and initialize the member variable directly inside the class declaration in the header file   class foo       private          static int const i   42

User · Answer

For future viewers of this question  I want to point out that you should avoid what monkey0506 is suggesting   Header files are for declarations   Header files get compiled once for every  cpp file that directly or indirectly  includes them  and code outside of any function is run at program initialization  before main     By putting  foo  i   VALUE  into the header  foo i will be assigned the value VALUE  whatever that is  for every  cpp file  and these assignments will happen in an indeterminate order  determined by the linker  before main   is run   What if we  define VALUE to be a different number in one of our  cpp files  It will compile fine and we will have no way of knowing which one wins until we run the program   Never put executed code into a header for the same reason that you never  include a  cpp file   include guards  which I agree you should always use  protect you from something different  the same header being indirectly  included multiple times while compiling a single  cpp file

User · Answer

With a Microsoft compiler 1   static variables that are not int-like can also be defined in a header file  but outside of the class declaration  using the Microsoft specific   declspec selectany    class A       static B b       declspec selectany  A  b    Note that I m not saying this is good  I just say it can be done    1  These days  more compilers than MSC support   declspec selectany  - at least gcc and clang  Maybe even more

User · Answer

C  11 static constructor pattern that works for multiple objects  One idiom was proposed at  https   stackoverflow com a 27088552 895245 but here goes a cleaner version that does not require creating a new method per member   main cpp   include  lt cassert gt   include  lt vector gt      Normally on the  hpp file  class MyClass   public      static std  vector lt int gt  v  v2      static struct StaticConstructor           StaticConstructor                 v push back 1               v push back 2               v2 push back 3               v2 push back 4                    staticConstructor         Normally on the  cpp file  std  vector lt int gt  MyClass  v  std  vector lt int gt  MyClass  v2     Must come after every static member  MyClass  StaticConstructor MyClass   staticConstructor   int main         assert MyClass  v 0     1       assert MyClass  v 1     2       assert MyClass  v2 0     3       assert MyClass  v2 1     4       GitHub upstream   Compile and run   g   -ggdb3 -O0 -std c  11 -Wall -Wextra -pedantic -o main out main cpp   main out   See also  static constructors in C    I need to initialize private static objects  Tested on Ubuntu 19 04   C  17 inline variable  Mentioned at  https   stackoverflow com a 45062055 895245 but here is a multifile runnable example to make it even clearer  How do inline variables work

User · Answer

The class declaration should be in the header file  Or in the source file if not shared   File  foo h  class foo       private          static int i       But the initialization should be in source file  File  foo cpp  int foo  i   0    If the initialization is in the header file then each file that includes the header file will have a definition of the static member  Thus during the link phase you will get linker errors as the code to initialize the variable will be defined in multiple source files  The initialisation of the static int i must be done outside of any function   Note  Matt Curtis  points out that C   allows the simplification of the above if the static member variable is of const int type  e g  int  bool  char   You can then declare and initialize the member variable directly inside the class declaration in the header file   class foo       private          static int const i   42

User · Answer

One  old-school  way to define constants is to replace them by a enum   class foo       private          enum  i   0      default type   int         enum  int64 t  HUGE   1000000000000      may specify another type      This way doesn t require providing a definition  and avoids making the constant lvalue  which can save you some headaches  e g  when you accidentally ODR-use it

User · Answer

For a variable   foo h   class foo   private      static int i       foo cpp   int foo  i   0    This is because there can only be one instance of foo  i in your program  It s sort of the equivalent of extern int i in a header file and int i in a source file   For a constant you can put the value straight in the class declaration   class foo   private      static int i      const static int a   42

User · Answer

With a Microsoft compiler 1   static variables that are not int-like can also be defined in a header file  but outside of the class declaration  using the Microsoft specific   declspec selectany    class A       static B b       declspec selectany  A  b    Note that I m not saying this is good  I just say it can be done    1  These days  more compilers than MSC support   declspec selectany  - at least gcc and clang  Maybe even more

User · Answer

I don t have enough rep here to add this as a comment  but IMO it s good style to write your headers with  include guards anyway  which as noted by Paranaix a few hours ago would prevent a multiple-definition error  Unless you re already using a separate CPP file  it s not necessary to use one just to initialize static non-integral members    ifndef FOO H  define FOO H  include  bar h   class foo   private      static bar i      bar foo  i   VALUE   endif   I see no need to use a separate CPP file for this  Sure  you can  but there s no technical reason why you should have to

User · Answer

I don t have enough rep here to add this as a comment  but IMO it s good style to write your headers with  include guards anyway  which as noted by Paranaix a few hours ago would prevent a multiple-definition error  Unless you re already using a separate CPP file  it s not necessary to use one just to initialize static non-integral members    ifndef FOO H  define FOO H  include  bar h   class foo   private      static bar i      bar foo  i   VALUE   endif   I see no need to use a separate CPP file for this  Sure  you can  but there s no technical reason why you should have to

User · Answer

int foo  i   0     Is the correct syntax for initializing the variable  but it must go in the source file   cpp  rather than in the header    Because it is a static variable the compiler needs to create only one copy of it  You have to have a line  int foo i  some where in your code to tell the compiler where to put it otherwise you get a link error  If that is in a header you will get a copy in every file that includes the header  so get multiply defined symbol errors from the linker

User · Answer

With a Microsoft compiler 1   static variables that are not int-like can also be defined in a header file  but outside of the class declaration  using the Microsoft specific   declspec selectany    class A       static B b       declspec selectany  A  b    Note that I m not saying this is good  I just say it can be done    1  These days  more compilers than MSC support   declspec selectany  - at least gcc and clang  Maybe even more

User · Answer

You can also include the assignment in the header file if you use header guards  I have used this technique for a C   library I have created  Another way to achieve the same result is to use static methods  For example     class Foo         public       int GetMyStatic   const               return  MyStatic               private       static int  MyStatic                 static int mStatic   0         return  amp mStatic                The above code has the  bonus  of not requiring a CPP source file  Again  a method I use for my C   libraries

User · Answer

Does this serves your purpose     header file  struct MyStruct   public      const std  unordered map lt std  string  uint32 t gt  str to int             a   1               b   2                           z   26              const std  unordered map lt int   std  string gt  int to str            1   a               2   b                           26   z               std  string some string    justanotherstring         uint32 t some int   42       static MyStruct  amp  Singleton             static MyStruct instance          return instance        private      MyStruct             Usage in cpp file int main        std  cout lt  lt MyStruct  Singleton   some string lt  lt std  endl      std  cout lt  lt MyStruct  Singleton   some int lt  lt std  endl      return 0

User · Answer

With a Microsoft compiler 1   static variables that are not int-like can also be defined in a header file  but outside of the class declaration  using the Microsoft specific   declspec selectany    class A       static B b       declspec selectany  A  b    Note that I m not saying this is good  I just say it can be done    1  These days  more compilers than MSC support   declspec selectany  - at least gcc and clang  Maybe even more

User · Answer

Also working in privateStatic cpp file     include  lt iostream gt   using namespace std   class A   private    static int v      int A  v   10     possible initializing  int main     A a    cout  lt  lt  A  v  lt  lt  endl     no access because of private scope return 0        g   privateStatic cpp -o privateStatic  amp  amp    privateStatic

User · Answer

The class declaration should be in the header file  Or in the source file if not shared   File  foo h  class foo       private          static int i       But the initialization should be in source file  File  foo cpp  int foo  i   0    If the initialization is in the header file then each file that includes the header file will have a definition of the static member  Thus during the link phase you will get linker errors as the code to initialize the variable will be defined in multiple source files  The initialisation of the static int i must be done outside of any function   Note  Matt Curtis  points out that C   allows the simplification of the above if the static member variable is of const int type  e g  int  bool  char   You can then declare and initialize the member variable directly inside the class declaration in the header file   class foo       private          static int const i   42

User · Answer

C  11 static constructor pattern that works for multiple objects  One idiom was proposed at  https   stackoverflow com a 27088552 895245 but here goes a cleaner version that does not require creating a new method per member   main cpp   include  lt cassert gt   include  lt vector gt      Normally on the  hpp file  class MyClass   public      static std  vector lt int gt  v  v2      static struct StaticConstructor           StaticConstructor                 v push back 1               v push back 2               v2 push back 3               v2 push back 4                    staticConstructor         Normally on the  cpp file  std  vector lt int gt  MyClass  v  std  vector lt int gt  MyClass  v2     Must come after every static member  MyClass  StaticConstructor MyClass   staticConstructor   int main         assert MyClass  v 0     1       assert MyClass  v 1     2       assert MyClass  v2 0     3       assert MyClass  v2 1     4       GitHub upstream   Compile and run   g   -ggdb3 -O0 -std c  11 -Wall -Wextra -pedantic -o main out main cpp   main out   See also  static constructors in C    I need to initialize private static objects  Tested on Ubuntu 19 04   C  17 inline variable  Mentioned at  https   stackoverflow com a 45062055 895245 but here is a multifile runnable example to make it even clearer  How do inline variables work

User · Answer

If you want to initialize some compound type  f e  string  you can do something like that   class SomeClass     static std  list lt string gt   list     public      static const std  list lt string gt  amp  getList           struct Initializer            Initializer                   Here you may want to put mutex             list push back  FIRST                list push back  SECOND                                             static Initializer ListInitializationGuard        return  list             As the ListInitializationGuard is a static variable inside SomeClass  getList   method it will be constructed only once  which means that constructor is called once  This will initialize  list variable to value you need  Any subsequent call to getList will simply return already initialized  list object   Of course you have to access  list object always by calling getList   method

User · Answer

The linker problem you encountered is probably caused by    Providing both class and static member definition in header file  Including this header in two or more source files    This is a common problem for those who starts with C    Static class member must be initialized in single translation unit i e  in single source file   Unfortunately  the static class member must be initialized outside of the class body  This complicates writing header-only code  and  therefore  I am using quite different approach  You can provide your static object through static or non-static class function for example   class Foo          int amp  getObjectInstance   const       static int amp  getObjectInstance             static int object          return object             void func             int  amp object   getValueInstance            object    5

User · Answer

int foo  i   0     Is the correct syntax for initializing the variable  but it must go in the source file   cpp  rather than in the header    Because it is a static variable the compiler needs to create only one copy of it  You have to have a line  int foo i  some where in your code to tell the compiler where to put it otherwise you get a link error  If that is in a header you will get a copy in every file that includes the header  so get multiply defined symbol errors from the linker

[c++] How to initialize private static members in C++?

Examples related to c++

Examples related to initialization

Examples related to static-members