c++ - Eckel Vol1，pg283，类型规范不完整，编译器在做什么？

Question

struct X;

struct Y {
  void f(X*);
};

struct X { //definition
private:
  int i;
public:
  friend void Y::f(X*);
  <SNIP'd>
};

"struct Y has a member function f() that will modify an object of type X. This is a bit of a conundrum because the C++ compiler requires you to declare everything before you can refer to it, so struct Y must be declared before its member Y::f(X*) can be declared as a friend in struct X. But for Y::f(X*) to be declared struct X must be declared first!

Here's the solution. Notice that Y::f(X*) takes the address of an X object. This is critical because the compiler always knows how to pass an address, which is of a fixed size regardless of the object being passed, even if it doesn't have full information about the size of the type."

所以这里我们有:

struct X; struct Y { ... }; struct X { ... };

我的问题是:

1. 为什么编译器坚持不完整地声明X:struct X;??

   毕竟，正如作者所说:“编译器总是知道如何传递一个地址这是固定大小的。”这只是一个声明，它告诉编译器 X 是一个结构并且很快就会跟进，那么为什么编译器如此坚持让您输入这些字母:struct X; 毕竟我不是在使用 X 本身吗？此时没有生成汇编语言代码。当它读取 X* 当然它可以告诉它它是一个地址(指针).. 为什么:struct X; 需要？

   声明函数的全部意义在于在实际执行之前进行类型检查用法。所以如果我这样做:

    int foo(void); foo(30); 
   

   然后编译器会咆哮。但是在上面，如果我不说:struct X; 有什么不同呢？他只是在检查我对 X(函数名称)的拼写吗？

2. 为什么颠倒结构 Y 和 X 的位置不起作用？像这样:
   结构 Y；结构 X { ... };结构 Y { ... };
   我得到:
   

   error: invalid use of incomplete type 'struct main()::Y'

   显然，编译器对我不完整的类型规范不满意Y(struct Y;)。但是缺少什么宝贵的小信息？后所有结构 Y;应该告诉编译器 Y 很快就会跟进(与做我们在 Q1 中做了什么: struct X;结构 Y {};结构 X{}; )

3. 为什么作者将 struct X 称为定义？如果我做:struct X { ... }; 这肯定是声明？当我实例化结构如下:X foo;那么它是一个定义？正确吗？

4. 我该怎么做:

   struct X {
     friend void Y::f(X*);
     void f(Y*);
   };
   
   struct Y {
     friend void X::f(Y*);
     void f(X*);
   };
   

Answer 1

Why does the compiler insist that X be declared incompletely as: struct X?? After all, as the author notes: "compiler always knows how to pass an address which is of a fixed size.."

你不需要:这也行:

struct Y { void f(struct X*); };

class X {
    int i;
    friend void Y::f(X*);
};

您还有什么建议？

此外，当按值传递时，您甚至不必拥有完整的类型除非您实际上定义了方法:

struct Y { void f(struct X); };

// too early:    
void Y::f(struct X) { /* whoops compile error */ }

class X {
    int i;
    friend void Y::f(X);
};

// ok here:
void Y::f(X) { /* compile success */ }