你可以有一个“指向成员的指针”到工会的成员？ [英] Can you have a "pointer to member" to a member of a union?

问题描述

在类的上下文中，我可以找到的所有描述都涉及指向成员的指针。工会与结构非常相似，特别是有成员。

  
 union x {
 int a; 
 float b; 
}; 
 int x :: * p =& x :: a; 
  

我不是指向联合的指针，指向联合的成员，等等。 p 在上面的例子中真的是一个偏移量，显然是大小为0.我需要这个构造来回答此问题。

解决方案

§3.9.2/ 1：复合类型可以通过以下方式构造：...指向非静态50类成员的指针，它标识给定类的对象内给定类型的成员， p>

§8.3.3/ 1：在声明TD中，D的格式为...，而且 nested-name-specifier / p>

§5.3.1/ 3：一元& 运算符的结果是指向其操作数的指针。操作数应为左值或 qualified-id 。如果操作数是一个 C m 的 qualified-id c>类型 T ，结果具有类型指向类型T的类C的成员的指针，并且是指定 C :: m 。

当然，§9.5/ 5：是 c $ c> union ...

（§3.9.2/ 1也提到：联盟 ...）

没有提到类不能是联合，所以是的，你可以形成这样的PTM类型和值。

All descriptions that I can find talk about "pointer to member" in the context of a class. Unions are very similar to structures, and in particular have members too. Can you have a pointer to those members too?

E.g.

union x {
    int a;
    float b;
};
int x::*p = &x::a;

I'm not talking about pointer to the union as a whole, pointers as members of a union, etc. p in the example above would really be an offset, obviously of size 0. I'd need this construct to answer this question.

解决方案

§3.9.2/1: Compound types can be constructed in the following ways: … pointers to non-static 50 class members, which identify members of a given type within objects of a given class,

§8.3.3/1: In a declaration T D where D has the form … and the nested-name-specifier denotes a class, …

§5.3.1/3: The result of the unary & operator is a pointer to its operand. The operand shall be an lvalue or a qualified-id. If the operand is a qualified-id naming a non-static member m of some class C with type T, the result has type "pointer to member of class C of type T" and is a prvalue designating C::m.

And of course §9.5/5: A union is a class defined with the class-key union…

(§3.9.2/1 also mentions: unions, which are classes…)

No mention that the class cannot be a union, so yes, you can form such a PTM type and value.

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