1.1.7.2 static

• In C++, global and static variables initialized to default values. But auto variable is random value, because of efficiency consideration.
• You can’t initialize a static member variable inside the class declaration, you need to put it in a .cpp file. But you can if the static data member is const of integer or enumeration.
  class{  //in .h file 
    static int obj_num;  // you can’t initialize 
    const static int months = 12; // you can  initialize 
  }; 
   
  //In .cpp file 
  Int class::obj_num = 0; // no static keyword anymore. 
  // obj_num will be default value(0) even you don’t init it.
• static member function has two usages:
  1. It can be invoked just by class name, not object instance, so you can define math class and define a lot static math function inside it. Just like name space.
  2. It can’t access class data member, only can access class static member data.
• static can be used restrain the scope.
  Int global = 0;  //All files 
  static int one_file = 50; //just in this file 
  main(){ 
      Static int inside_main = 100;  //just in this block 
  }