Static variables in C

Question

I would like to know what is the difference between static variables in a header file vs declared in a class  When static variable is declared in a header file is its scope limited to  h file or across all units  Also generally static variable is initialized in  cpp file when declared in a class right  So that does mean static variable scope is limited to 2 compilation units

User · Answer

A static variable declared in a header file outside of the class would be file-scoped in every .c file which includes the header. That means separate copy of a variable with same name is accessible in each of the .c files where you include the header file.

A static class variable on the other hand is class-scoped and the same static variable is available to every compilation unit that includes the header containing the class with static variable.

User · Answer

Excuse me when I answer your questions out-of-order  it makes it easier to understand this way      When static variable is declared in a header file is its scope limited to  h file or across all units    There is no such thing as a  header file scope   The header file gets included into source files  The translation unit is the source file including the text from the header files  Whatever you write in a header file gets copied into each including source file   As such  a static variable declared in a header file is like a static variable in each individual source file   Since declaring a variable static this way means internal linkage  every translation unit  includeing your header file gets its own  individual variable  which is not visible outside your translation unit   This is usually not what you want      I would like to know what is the difference between static variables in a header file vs declared in a class    In a class declaration  static means that all instances of the class share this member variable  i e   you might have hundreds of objects of this type  but whenever one of these objects refers to the static  or  class   variable  it s the same value for all objects  You could think of it as a  class global       Also generally static variable is initialized in  cpp file when declared in a class right     Yes  one  and only one  translation unit must initialize the class variable      So that does mean static variable scope is limited to 2 compilation units     As I said    A header is not a compilation unit  static means completely different things depending on context    Global static limits scope to the translation unit  Class static means global to all instances   I hope this helps   PS  Check the last paragraph of Chubsdad s answer  about how you shouldn t use static in C   for indicating internal linkage  but anonymous namespaces   Because he s right   -

User · Answer

Static variable in a header file    say  common h  has  static int zzz    This variable  zzz  has internal linkage  This same variable can not be accessed in other translation units   Each translation unit which includes  common h  has it s own unique object of name  zzz    Static variable in a class   Static variable in a class is not a part of the subobject of the class  There is only one copy of a static data member shared by all the objects of the class       9 4 2 6 -  Static data members of a   class in namespace scope have external   linkage  3 5  A local class shall not   have static data members     So let s say  myclass h  has  struct myclass     static int zzz            this is only a declaration      and myclass cpp has    include  myclass h   int myclass  zzz   0              this is a definition                                     should be done once and only once   and  hisclass cpp  has   include  myclass h   void f   myclass  zzz   2         myclass  zzz is always the same in any                                    translation unit   and  ourclass cpp  has   include  myclass h  void g   myclass  zzz   2         myclass  zzz is always the same in any                                    translation unit   So  class static members are not limited to only 2 translation units  They need to be defined only once in any one of the translation units      Note  usage of  static  to declare   file scope variable is deprecated and   unnamed namespace is a superior   alternate

[c++] Static variables in C++

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