static member in a class?

想起来Scott那天还问道static member in a class和non-static member有什么不同。前几天一直在看《Accelerated C++》，对这个有印象，就回答static member is not associated with any instance/object of the class（Section 13.4, Page 244~245）。

但是Scott又继续追问：“这个性质有什么作用？”我立刻就有些傻眼，当时书上好象是谈到这样做的话，外面的程序call这个static member func的时候，前面要加一个classi qualifier，比如这样：sort(students.begin(), students.end(), Student_info::compare)。好处是For our purposes, static member functions have one significant advantage: Their names are within the scope of their class. So, when we say that compare is a static member we are defining a function named Student_info::compare. Because the function has a qualified name, it does not overload the compare that we used to compare Core objects. Thus, our users will be able to call sort, passing Student_info::compare, and the compiler can know which function they want.



更详细的描述可以见于Section 15.2.3, Page 282~283：
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15.2.3 Padding the output

We can now think about our padding function. Because we want to access this function from each of the derived classes, we'll define the pad operation as a function member of Pic_base that is both static and protected:

class Pic_base {
    // as before
protected:
    static void pad(std::ostream& os, wd_sz beg, wd_sz end) {
        while (beg != end) {
            os << " ";
            ++beg;
        }
    }
};

This function takes an ostream on which to write blanks, and two values that control how many blanks to write. When a display function needs to call pad, it will pass the current column number and one past the last column number that needs to be filled in by the current display operation. The pad function will fill this range with blanks.

Note the use of the static keyword on the declaration of pad. As we saw in §13.4/244, this use of static indicates that pad is a static member function. Such functions differ from an ordinary member function in that they are not associated with an object of the class type.

It may also be surprising that we can define a member function for an abstract base class. After all, if there can be no objects of the base class, why should there be member functions? However, remember that each derived object contains a base-class part. Each derived class also inherits any member functions defined in the base. Thus, a base-class function will execute on the base-class portion of a derived object. In this particular case, the function that we are defining is a static member, so the question of access to members of the base is moot. But it is important to realize that abstract classes may define data members, and (ordinary) member functions, as well as static ones. These functions will access the base-class objects that are part of derived objects.

Static members (both functions and static data members, which we can also define) are useful in that they let us minimize the names that are defined globally. Our pad function is a good example. We can imagine many abstractions that have the notion of padding. In this book we talked about padding in the context of writing a formatted report of student grades, as well as in the context of writing Pictures. If the Picture class were to define pad as a global function, then we would not also be able to define a pad function for Student_info, or vice versa. By making pad a static member, we allow for the fact that other abstractions in our program might have the notion of padding. As long as each class defines what pad means only in the context of the class, these mutually independent notions of padding can coexist within our program.
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简言之，static member的好处就是可以minimize the names that are defined globally。但是当时没能明确地说出这一点，还是概念模糊所导致的。