一个简单而有用的例子是....

想象一下:

你有一个uint32_t数组[2]，想要访问字节链的第3个和第4个字节。 你可以做*((uint16_t*) &数组[1])。 但遗憾的是，这打破了严格的混叠规则!

但是已知的编译器允许你做以下事情:

union un
{
    uint16_t array16[4];
    uint32_t array32[2];
}

严格来说，这仍然是违反规则的。但是所有已知的标准都支持这种用法。

这里有一个来自我自己代码库的联合的例子(来自记忆和转述，所以可能不准确)。它被用来在我构建的解释器中存储语言元素。例如，以下代码:

set a to b times 7.

由以下语言元素组成:

[设置]符号 可变[a] 符号[到] 可变[b] 符号[时报] 康斯坦[7] 符号[。]

语言元素被定义为“#define”值，如下:

#define ELEM_SYM_SET        0
#define ELEM_SYM_TO         1
#define ELEM_SYM_TIMES      2
#define ELEM_SYM_FULLSTOP   3
#define ELEM_VARIABLE     100
#define ELEM_CONSTANT     101

下面的结构被用来存储每个元素:

typedef struct {
    int typ;
    union {
        char *str;
        int   val;
    }
} tElem;

然后，每个元素的大小是最大联合的大小(typ为4字节，联合为4字节，尽管这些是典型值，但实际大小取决于实现)。

为了创建一个“set”元素，你可以使用:

tElem e;
e.typ = ELEM_SYM_SET;

为了创建一个“variable[b]”元素，你可以使用:

tElem e;
e.typ = ELEM_VARIABLE;
e.str = strdup ("b");   // make sure you free this later

为了创建一个常量[7]元素，你可以使用:

tElem e;
e.typ = ELEM_CONSTANT;
e.val = 7;

你可以很容易地将其扩展为包含浮点数(float flt)或有理数(struct ratnl {int num;Int denom;})和其他类型。

基本前提是str和val在内存中不是连续的，它们实际上是重叠的，所以这是一种在同一块内存上获得不同视图的方法，如图所示，其中结构基于内存位置0x1010，整数和指针都是4字节:

       +-----------+
0x1010 |           |
0x1011 |    typ    |
0x1012 |           |
0x1013 |           |
       +-----+-----+
0x1014 |     |     |
0x1015 | str | val |
0x1016 |     |     |
0x1017 |     |     |
       +-----+-----+

如果只是在一个结构中，它看起来会是这样的:

       +-------+
0x1010 |       |
0x1011 |  typ  |
0x1012 |       |
0x1013 |       |
       +-------+
0x1014 |       |
0x1015 |  str  |
0x1016 |       |
0x1017 |       |
       +-------+
0x1018 |       |
0x1019 |  val  |
0x101A |       |
0x101B |       |
       +-------+

在C的早期版本中，所有结构声明都共享一组公共字段。考虑到:

struct x {int x_mode; int q; float x_f};
struct y {int y_mode; int q; int y_l};
struct z {int z_mode; char name[20];};

a compiler would essentially produce a table of structures' sizes (and possibly alignments), and a separate table of structures' members' names, types, and offsets. The compiler didn't keep track of which members belonged to which structures, and would allow two structures to have a member with the same name only if the type and offset matched (as with member q of struct x and struct y). If p was a pointer to any structure type, p->q would add the offset of "q" to pointer p and fetch an "int" from the resulting address.

Given the above semantics, it was possible to write a function that could perform some useful operations on multiple kinds of structure interchangeably, provided that all the fields used by the function lined up with useful fields within the structures in question. This was a useful feature, and changing C to validate members used for structure access against the types of the structures in question would have meant losing it in the absence of a means of having a structure that can contain multiple named fields at the same address. Adding "union" types to C helped fill that gap somewhat (though not, IMHO, as well as it should have been).

An essential part of unions' ability to fill that gap was the fact that a pointer to a union member could be converted into a pointer to any union containing that member, and a pointer to any union could be converted to a pointer to any member. While the C89 Standard didn't expressly say that casting a T* directly to a U* was equivalent to casting it to a pointer to any union type containing both T and U, and then casting that to U*, no defined behavior of the latter cast sequence would be affected by the union type used, and the Standard didn't specify any contrary semantics for a direct cast from T to U. Further, in cases where a function received a pointer of unknown origin, the behavior of writing an object via T*, converting the T* to a U*, and then reading the object via U* would be equivalent to writing a union via member of type T and reading as type U, which would be standard-defined in a few cases (e.g. when accessing Common Initial Sequence members) and Implementation-Defined (rather than Undefined) for the rest. While it was rare for programs to exploit the CIS guarantees with actual objects of union type, it was far more common to exploit the fact that pointers to objects of unknown origin had to behave like pointers to union members and have the behavioral guarantees associated therewith.

低级系统编程就是一个合理的例子。

IIRC中，我使用联合将硬件寄存器分解为组件位。因此，您可以访问一个8位寄存器(在我这样做的那天;-)到组件位。

(我忘记了确切的语法，但是……)这种结构将允许控制寄存器作为control_byte或通过单个位来访问。对于给定的字节顺序，确保位映射到正确的寄存器位是很重要的。

typedef union {
    unsigned char control_byte;
    struct {
        unsigned int nibble  : 4;
        unsigned int nmi     : 1;
        unsigned int enabled : 1;
        unsigned int fired   : 1;
        unsigned int control : 1;
    };
} ControlRegister;

联合通常用于整数和浮点数的二进制表示之间的转换:

union
{
  int i;
  float f;
} u;

// Convert floating-point bits to integer:
u.f = 3.14159f;
printf("As integer: %08x\n", u.i);

尽管根据C标准，这在技术上是未定义的行为(您只应该阅读最近编写的字段)，但它将在几乎任何编译器中以定义良好的方式起作用。

联合有时也被用来实现C语言中的伪多态性，通过给一个结构一些标记来指示它包含什么类型的对象，然后将可能的类型联合在一起:

enum Type { INTS, FLOATS, DOUBLE };
struct S
{
  Type s_type;
  union
  {
    int s_ints[2];
    float s_floats[2];
    double s_double;
  };
};

void do_something(struct S *s)
{
  switch(s->s_type)
  {
    case INTS:  // do something with s->s_ints
      break;

    case FLOATS:  // do something with s->s_floats
      break;

    case DOUBLE:  // do something with s->s_double
      break;
  }
}

这使得struct S的大小只有12字节，而不是28字节。

当你有一个函数，你返回的值可以不同，这取决于函数做了什么，使用联合。