Unresolved external symbol on static class members

Question

Very simply put   I have a class that consists mostly of static public members  so I can group similar functions together that still have to be called from other classes functions   Anyway  I have defined two static unsigned char variables in my class public scope  when I try to modify these values in the same class  constructor  I am getting an  unresolved external symbol  error at compilation   class test    public      static unsigned char X      static unsigned char Y                test        test  test          X   1      Y   2      I m new to C   so go easy on me  Why can t I do this

User · Accepted Answer

If you are using C   17 you can just use the inline specifier  see https   stackoverflow com a 11711082 55721   If using older versions of the C   standard  you must add the definitions to match your declarations of X and Y unsigned char test  X  unsigned char test  Y   somewhere   You might want to also initialize a static member unsigned char test  X   4   and again  you do that in the definition  usually in a CXX file  not in the declaration  which is often in a  H file

User · Answer

In my case  I was using wrong linking   It was managed c    cli  but with native exporting  I have added to linker -  input -  assembly link resource the dll of the library from which the function is exported  But native c   linking requires  lib file to  see  implementations in cpp correctly  so for me helped to add the  lib file to linker -  input -  additional dependencies    Usually managed code does not use dll export and import  it uses references  but that was unique situation

User · Answer

Since this is the first SO thread that seemed to come up for me when searching for  unresolved externals with static const members  in general  I ll leave another hint to solve one problem with unresolved externals here   For me  the thing that I forgot was to mark my class definition   declspec dllexport   and when called from another class  outside that class s dll s boundaries   I of course got the my unresolved external error  Still  easy to forget when you re changing an internal helper class to a one accessible from elsewhere  so if you re working in a dynamically linked project  you might as well check that  too

User · Answer

in my case  I declared one static variable in  h file  like    myClass h class myClass   static int m nMyVar  static void myFunc        and in myClass cpp  I tried to use this m nMyVar  It got LINK error like   error LNK2001  unresolved external symbol  public  static class    The link error related cpp file looks like     myClass cpp void myClass  myFunc     myClass  m nMyVar   123    I tried to use this m nMyVar here and got link error     So I add below code on the top of myClass cpp    myClass cpp int myClass  m nMyVar    it seems redefine m nMyVar  but it works well void myClass  myFunc     myClass  m nMyVar   123    I tried to use this m nMyVar here and got link error     then LNK2001 is gone

User · Answer

In my case  I was using wrong linking   It was managed c    cli  but with native exporting  I have added to linker -  input -  assembly link resource the dll of the library from which the function is exported  But native c   linking requires  lib file to  see  implementations in cpp correctly  so for me helped to add the  lib file to linker -  input -  additional dependencies    Usually managed code does not use dll export and import  it uses references  but that was unique situation

User · Answer

Static data members declarations in the class declaration are not definition of them  To define them you should do this in the  CPP file to avoid duplicated symbols   The only data you can declare and define is integral static constants   Values of enums can be used as constant values as well   You might want to rewrite your code as   class test   public    const static unsigned char X   1    const static unsigned char Y   2          test        test  test         If you want to have ability to modify you static variables  in other words when it is inappropriate to declare them as const   you can separate you code between  H and  CPP in the following way    H    class test   public     static unsigned char X    static unsigned char Y            test          CPP    unsigned char test  X   1  unsigned char test  Y   2   test  test          constructor is empty       We don t initialize static data member here        because static data initialization will happen on every constructor call

User · Answer

Since this is the first SO thread that seemed to come up for me when searching for  unresolved externals with static const members  in general  I ll leave another hint to solve one problem with unresolved externals here   For me  the thing that I forgot was to mark my class definition   declspec dllexport   and when called from another class  outside that class s dll s boundaries   I of course got the my unresolved external error  Still  easy to forget when you re changing an internal helper class to a one accessible from elsewhere  so if you re working in a dynamically linked project  you might as well check that  too

User · Answer

Static data members declarations in the class declaration are not definition of them  To define them you should do this in the  CPP file to avoid duplicated symbols   The only data you can declare and define is integral static constants   Values of enums can be used as constant values as well   You might want to rewrite your code as   class test   public    const static unsigned char X   1    const static unsigned char Y   2          test        test  test         If you want to have ability to modify you static variables  in other words when it is inappropriate to declare them as const   you can separate you code between  H and  CPP in the following way    H    class test   public     static unsigned char X    static unsigned char Y            test          CPP    unsigned char test  X   1  unsigned char test  Y   2   test  test          constructor is empty       We don t initialize static data member here        because static data initialization will happen on every constructor call

User · Answer

Static data members declarations in the class declaration are not definition of them  To define them you should do this in the  CPP file to avoid duplicated symbols   The only data you can declare and define is integral static constants   Values of enums can be used as constant values as well   You might want to rewrite your code as   class test   public    const static unsigned char X   1    const static unsigned char Y   2          test        test  test         If you want to have ability to modify you static variables  in other words when it is inappropriate to declare them as const   you can separate you code between  H and  CPP in the following way    H    class test   public     static unsigned char X    static unsigned char Y            test          CPP    unsigned char test  X   1  unsigned char test  Y   2   test  test          constructor is empty       We don t initialize static data member here        because static data initialization will happen on every constructor call

User · Answer

in my case  I declared one static variable in  h file  like    myClass h class myClass   static int m nMyVar  static void myFunc        and in myClass cpp  I tried to use this m nMyVar  It got LINK error like   error LNK2001  unresolved external symbol  public  static class    The link error related cpp file looks like     myClass cpp void myClass  myFunc     myClass  m nMyVar   123    I tried to use this m nMyVar here and got link error     So I add below code on the top of myClass cpp    myClass cpp int myClass  m nMyVar    it seems redefine m nMyVar  but it works well void myClass  myFunc     myClass  m nMyVar   123    I tried to use this m nMyVar here and got link error     then LNK2001 is gone

User · Answer

Static data members declarations in the class declaration are not definition of them  To define them you should do this in the  CPP file to avoid duplicated symbols   The only data you can declare and define is integral static constants   Values of enums can be used as constant values as well   You might want to rewrite your code as   class test   public    const static unsigned char X   1    const static unsigned char Y   2          test        test  test         If you want to have ability to modify you static variables  in other words when it is inappropriate to declare them as const   you can separate you code between  H and  CPP in the following way    H    class test   public     static unsigned char X    static unsigned char Y            test          CPP    unsigned char test  X   1  unsigned char test  Y   2   test  test          constructor is empty       We don t initialize static data member here        because static data initialization will happen on every constructor call

[c++] Unresolved external symbol on static class members

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