c++ - 为什么构造函数 Zlib_init 在 Bjarne Stroustrup 的书中的类 Zlib_init 中保持私有(private)

Question

在 Bjarne Stroustrup 的《The C++ Programming Language》一书中，作者说:

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.

为什么作者将构造函数和析构函数保留为私有(private)成员？如果我们在由单独编译的单元组成的程序中使用它，为什么这种方法不起作用？调用构造函数Zlib_init()和析构函数~Zlib_init()不需要定义成员x吗？那么这个方法有什么用呢？它在本书的第 10.4.9 节中。

Answer 1

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

private 的构造函数和析构函数似乎是一个错字。
类 static 成员需要被定义，以便您可以使用它们。为了定义静态成员 x ，构造函数需要可访问。如果不是，链接器将提示 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;
}

输出:

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?

如果你通过 making the constructor and destructor public 修正了上面提到的错字或者通过制作 Zlib a friend of class Zlib_init代码仍然面临另一个问题。
该问题在 C++ 中通常称为 Static Initialization Fiasco。

好读:

[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?