联盟在C中的应用 [英] Application of Union in C
问题描述
我想在C中使用union的应用。请解释
I want what is the application of union in C.Please explain
推荐答案
2006-04-19，balasam< ; BK ***** @ gmail.com>写道：
On 2006-04-19, balasam <bk*****@gmail.com> wrote:
我想在C中使用union的什么应用。请解释
I want what is the application of union in C.Please explain
他们经常被用于某种多态性。
例如，如果您正在为动态类型的
语言编写解释器，则可以使用类似的联合来表示语言中的值
这个：
enum value_type
{
INT，
REAL，
STRING，
OBJECT
};
struct value
{
enum value_type type;
union
{
int val_int;
double val_real;
char * val_string;
struct object * val_object;
}
值;
};
所有值都是struct value的实例，但可以代表
int，double，char或object。
They''re quite often used for a sort of "polymorphism".
For example, if you were writing an interpreter for a dynamically typed
language, you might represent values in the language using unions like
this:
enum value_type
{
INT,
REAL,
STRING,
OBJECT
};
struct value
{
enum value_type type;
union
{
int val_int;
double val_real;
char *val_string;
struct object *val_object;
}
value;
};
All values are instances of "struct value", but that can represent an
int, a double, a char or an object.
union的一个常见用途是简化分解输入的STREAM，在下面的
示例中将int值读入两个短值。
/ * tunion.c * /
#include< stdio.h>
#include< stdlib.h>
#include< string.h>
/ *用法：tunion< binfile * /
int
main（int argc ，char * argv []）
{
union {
unsigned ii;
struct {
短高;
短低;
}字;
}解码;
printf（" sizeof（decode）=％d \ n"，sizeof（decode））;
printf（" sizeof（decode.ii）=％d \ n" ，sizeof（decode.ii））;
printf（" sizeof（decode.word）=％d \ n"，sizeof（decode.word））;
printf（" sizeof（decode.word.high）=
％d \ n"，sizeof（decode.word.high））;
printf（" ; sizeof（decode.word.low）=
％d \ n& quot;，sizeof（decode.word.low））;
/ *从标准输入读取0x01020304 * /
读取（0，& decode.ii ，sizeof（decode.ii））;
printf（" high word =％04hx \ n"，decode.word.high）;
printf（" low word =％04hx \ n"，decode.word.low）;
/ *从标准输入读取0x04030201 * /
读取（0，& decode.ii，sizeof（decode.ii））;
printf（" high word =％04hx \ n"，decode.word.high）;
printf（" low word =％04hx \ n"，decode.word.low）;
}
重定向一个包含8个字节的文件，你可以使用union
变量解码来直接访问int
值读取的两个16位字。 / b
binfile包含：^ A ^ B ^ C ^ D ^ D ^ C ^ B ^ A （报价不在档案中）
CJ
A common use for union is to ease breaking up a STREAM of input, in the
example below breaking an int value read into two short values.
/* tunion.c */
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
/* Usage: tunion <binfile */
int
main(int argc,char *argv[])
{
union {
unsigned ii;
struct {
short high;
short low;
} word;
} decode;
printf("sizeof(decode) = %d\n",sizeof(decode));
printf("sizeof(decode.ii) = %d\n",sizeof(decode.ii));
printf("sizeof(decode.word) = %d\n",sizeof(decode.word));
printf("sizeof(decode.word.high) =
%d\n",sizeof(decode.word.high));
printf("sizeof(decode.word.low) =
%d\n",sizeof(decode.word.low));
/* Reads 0x01020304 from stdin */
read(0,&decode.ii,sizeof(decode.ii));
printf("high word = %04hx\n",decode.word.high);
printf("low word = %04hx\n",decode.word.low);
/* Reads 0x04030201 from stdin */
read(0,&decode.ii,sizeof(decode.ii));
printf("high word = %04hx\n",decode.word.high);
printf("low word = %04hx\n",decode.word.low);
}
Redirecting a file containing eight bytes, below, you can use the union
variable decode to get direct access to the two 16 bit words of the int
value read.
binfile contains: "^A^B^C^D^D^C^B^A" (quotes not in file)
CJ
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