Static members and the default constructor C++

Question

I came across a statement in my book that said:

You don't have to initialize a static member when you declare it; C++ will invoke the default constructor if you don't.

This really has me confused as to what it means. Are they talking about object members only? If so, at what point would it call the default constructor? Also, how would you initialize a static member object without the default constructor?

Answer 1

Let's break it down. Suppose there's some class Foo; somewhere. Now we make this a static member of our class,

class Star
{
  static Foo z;
  // ...
};

Now in essence that declares a global object Foo Star::z -- so how does this get instantiated? The standard tells you: it gets default-constructed. But remember that you have to provide the actual object instance in one of your translation units:

// in, say, star.cpp
Foo Star::z;  // OK, object lives here now

Now suppose that Foo doesn't actually have a default constructor:

class Foo
{
public:
  Foo(char, double); // the only constructor
  // ...
};

Now there's a problem: How do we construct Star::z? The answer is "just like above", but now we have to call a specific constructor:

// again in star.cpp
Foo Star::z('a', 1.5);

_{The standard actually has two distinct notions of "initialization" (a grammatical concept) and "construction" (a function call), but I don't think we need to go into this just now.}