Why are the constructor Zlib_init kept private in class Zlib_init in the book by Bjarne Stroustrup

Question

In the book The C++ Programming Language, by Bjarne Stroustrup, the author says:

Sometimes when you design a library, it is necessary, or simply convenient, to invent a type with a constructor and a destructor with the sole purpose of initialization and cleanup. Such a type would be used once only: to allocate a static object so that the constructor and the destructor are called. For example:

 class  Zlib_init
{
    Zlib_init() ; //get Zlib ready for use
   ~Zlib_init()  ; //clean up after Zlib
};
Class Zlib
{
   static  Zlib_init   x;
   /  /...
};

Unfortunately, it is not guaranteed that such an object is initialized before its first use and destroyed after its last use in a program consisting of separately compiled units.

Why does the author keep the constructor and destructor as private members? And why won't this method work if we use it in a program consisting of seperately compiled units? Won't it require definition of the member x for calling the constructor Zlib_init() and destructor ~Zlib_init()? Then what is the use of this method? It's in section 10.4.9 of the book.

Answer 1

Why does the author keep the constructor and destructor as private members?

The constructor & destructor being private seems to be a typo.
Class static members needs to be defined in order that you can use them. In order that the static member x is defined the constructor needs to be accessible. if not the linker will complain about undefined reference.

Online Sample:

class  Zlib_init 
{    
   Zlib_init() ; //get Zlib ready for use  
  ~Zlib_init()  ; //clean up after Zlib 
   public:
     int j;
};
class Zlib 
{  
   public:  
   static  Zlib_init   x;    
}; 

Zlib_init Zlib::x;

int main()
{
    Zlib::x.j = 10;

    return 0;
}

Output:

prog.cpp:3: error: ‘Zlib_init::Zlib_init()’ is private     
prog.cpp:14: error: within this context      
prog.cpp: In static member function ‘static void Zlib::__static_initialization_and_destruction_0(int, int)’:     
prog.cpp:4: error: ‘Zlib_init::~Zlib_init()’ is private     
prog.cpp:14: error: within this context     

And why won't this method work if we use it in a program consisting of seperately compiled units?

If you fix the typo mentioned above by making the constructor and destructor public or by making Zlib a friend of class Zlib_init the code still faces another problem.
The problem is popularly known as Static Initialization Fiasco in C++.

Good Read:

[10.14] What's the "static initialization order fiasco"?
[10.17] How do I prevent the "static initialization order fiasco" for my static data members?