c++ - union的用法和乱码

Question

我有一些代码是关于 union 的用法，如下所示:

int main(){

  typedef union{int a;char b[10];float c;}Union;

  Union x,y = {100};
  printf("Union x :%d|   |%s|   |%f \n",x.a,x.b,x.c );
  printf("Union y :%d|   |%s|   |%f \n\n",y.a,y.b,y.c);

  x.a = 50;
  printf("Union x :%d|   |%s|   |%f \n",x.a,x.b,x.c );
  printf("Union y :%d|   |%s|   |%f \n\n",y.a,y.b,y.c);

  strcpy(x.b,"hello");
  printf("Union x :%d|   |%s|   |%f \n",x.a,x.b,x.c );
  printf("Union y :%d|   |%s|   |%f \n\n",y.a,y.b,y.c);

  x.c = 21.50;
  printf("Union x :%d|   |%s|   |%f \n",x.a,x.b,x.c );
  printf("Union y :%d|   |%s|   |%f \n\n",y.a,y.b,y.c);

  return 0;
}

在我编译并执行上面的代码后，我得到了这样的结果:

  Union x :0|     ||    |0.000000 
  Union y :100|   |d|   |0.000000 

  Union x :50|    |2|   |0.000000 
  Union y :100|   |d|   |0.000000 

  Union x :1819043176|     |hello|      |1143141483620823940762435584.000000 
  Union y :100|   |d|   |0.000000 

  Union x :1101791232|   ||    |21.500000 
  Union y :100|   |d|   |0.000000 

不知为何y.b初始化为“d”？以及为什么 x.a 和 x.c 的值在之后发生了变化？为什么 strcpy(x.b,"hello") 不起作用？

Answer 1

严格按照类型双关标准，在狭窄的情况下期望是undefined behavior但实际上许多编译器都支持它，例如 gcc manual points here for type-punning在 -fstrict-aliasing 部分说:

The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type.

我会推荐阅读 Understanding Strict Aliasing如果您打算大量使用双关语。

y.b 的值为 b 因为 union 的所有元素都共享内存并且您用 100 初始化了 y > 在 ASCII 中是 b。这就是为什么 x 的其他字段在您修改一个字段时发生变化的原因，包括 strcpy 的情况，并且根据您的编译器，这可能是未定义的或明确定义的(在 gcc 的情况下，它是定义的)。

为了完整起见，C++ 草案标准部分 9.5 Union 段落 1 说(强调我的):

In a union, at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time. [ Note: One special guarantee is made in order to simplify the use of unions: If a standard-layout union contains several standard-layout structs that share a common initial sequence (9.2), and if an object of this standard-layout union type contains one of the standard-layout structs, it is permitted to inspect the common initial sequence of any of standard-layout struct members; see 9.2. —end note ] The size of a union is sufficient to contain the largest of its non-static data members. Each non-static data member is allocated as if it were the sole member of a struct.