C ++联合，Struct，成员类型 [英] C++ Union, Struct, Member type

问题描述

如果我有一个类：

  class Odp 
 {
 int i; 
 int b; 
 union 
 {
 long f; 
 struct 
 {
 WCHAR * pwszFoo; 
 HRESULT hr; 
}; 
}; 
 
} 
  

联盟意味着在列出的所有值中，一次采取这些值之一？在访问这些变量方面如何工作？如何直接访问 hr ？如果我设置 hr ，如果我尝试访问 f 会发生什么？

  union U {
 int a; 
 char c; 
}; 
  

那么：

  U u; 
 u.a = 1; 
 int n = u.a; 
 u.c = 2; 
 char c = u.c; 
  

可以，但是：

  U u; 
 u.a = 1; 
 char c = u.c; 
  

不是。但是，有大量的现有代码，说两个都可以。在任何情况下，都不会为无效访问抛出异常。

基本上，如果你发现自己在C ++代码中使用联合来处理除了C库之外的东西，那就是错误的。

If I have a class:

class Odp
{
    int i;
    int b;
    union
    {
         long f;
         struct
         {
               WCHAR* pwszFoo;
               HRESULT hr;
         };
    };

}

Union means that, of all values listed, it can only take on one of those values at a time? How does that work in terms of accessing these variables? How would I access hr directly? If I set hr, what happens if I try to access f?

解决方案

This is a very fraught area in the C++ standard - basically a union instance, per the standard can only be treated at any one time as if it contained one "active" member - the last one written to it. So:

union U {
   int a;
   char c;
};

then:

U u;
u.a = 1;
int n = u.a;
u.c = 2;
char c = u.c;

is OK, but:

U u;
u.a = 1;
char c = u.c;

is not. However, there are vast volumes of existing code that say that both are OK. and in neither, or any, case will an exception be thrown for an "invalid" access. The C++ language uses exceptions exceptionally (!) sparingly.

Basically, if you find yourself using unions in your C++ code to deal with anything but C libraries, something is wrong.

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