如何初始化属于类的成员并且与它们所在的类具有相同类型的静态对象？

Question

Bjarne Strousrup in The C++ Programming Language illustrates the use of static members of a class with code similar to this:

class Date{
  int d, m, y;
  static Date default_date;
  public:
      Date(int dd=0, int mm=0, int yy=0);
      static void set_default(int, int, int);
      int year()const{return y;}
      int month() const{return m;}
      int day() const {return d;}

};
Date::Date(int dd, int mm, int yy){
d=dd ? dd : default_date.d;
m=mm ? mm : default_date.m;
y=yy ? yy : default_date.y;
}

void f()
{
    Date::set_default(4,5,1945);
}

Date Date::default_date(16,12,1770);


void Date::set_default(int d,int m, int y)
{
    Date::default_date=Date(d,m,y);
}


int main(){


Date ob(5,5);
cout<<ob.day()<<endl<<ob.month()<<endl<<ob.year();


return 0;
}

The output of the code was 5 5 1770

He states that the default_date must be defined somewhere, as it is done between f() and set default. While we are at it can somebody tell me why is it with two Date before :: and not one?

OK default_date can be constructed with the constructor provided since all arguments are supplied.

Now i tried removing the year from the list of arguments for default_date:

Date Date::default_date(16,12);

and the program compiled fine with output 5,5,0. This means that when in the constructor, since the last argument is not provided yy is 0 and default_date.y should get default_date.y and it turns out that it is 0. To me this only makes sense if the members of default_date are set to 0 when default_date is declared, and changed when default_date is defined. If that is true why then must we define default_date, why cant we leave it be 0,0,0?

Answer 1

default_date is a Date object which is a static member of the Date class. That's why there are two Date s in front of its definition:

        qualified id
     vvvvvvvvvvvvvvvvvv
Date Date::default_date(16,12,1770);
^^^^ ^^^^
 |     default_date is a member of Date
 |
The type of default_date

By removing the year from the definition, the Date constructor will end up assigning default_date.y to itself. Why does this have value 0? Shouldn't it be uninitialized? Actually, static objects will be zero-initialized before any other initialization takes place. So the members of default_date will all be set to 0 before the constructor runs.

Answer 2

// This is simply how static variables are initialized
// type classname::attributename followed by either = or constructor invocation
Date Date::default_date(16,12,1770);

// two ways of initializing an int in c++
int i = 0;
int i(0);

What you see in the example is similar to the second form above, except that it is within a class so you need the scope to be included.

If that is true why then must we define default_date, why cant we leave it be 0,0,0?

If you are in a large scale app with multiple compilation units, you will get linker errors if you don't create a static initializer in the cpp file for the corresponding class declaration. In your example everything is within one file, and it appears that you don't need to do it. I deleted that line, and it still compiled fine with VS2010.