## 顺时针螺旋法则（clockwise/spiral rule）教你认变量类型

2013年02月22日 ⁄ 综合 ⁄ 共 7335字 ⁄ 字号 评论关闭

The ``Clockwise/Spiral Rule''
“顺时针螺旋法则”

By David Anderson

There is a technique known as the ``Clockwise/Spiral Rule'' which enables any C programmer to parse in their head any C declaration!

There are three simple steps to follow:

Starting with the unknown element, move in a spiral/clockwise direction; when ecountering the following elements replace them with the corresponding english statements:
从变量名开始，沿着顺时针螺旋移动。每次遇到以下这几种符号，把它换成相对应的汉语句子：

[X] or []
=> Array X size of... or Array undefined size of...
-> ...类型的长度为X的数组， 或者 ...类型的长度未知的数组
(type1, type2)
=> function passing type1 and type2 returning...
-> 传入type1和type2类型，返回...类型的函数。
*
=> pointer(s) to...
-> 指向...类型的指针

【译注：英语怎么都是前缀表达式？郁闷。谁帮我把汉语也改成前缀或者后缀表达式吧。

Keep doing this in a spiral/clockwise direction until all tokens have been covered.
不断地沿着顺时针螺旋方向做上述动作，直到所有的符号都遍历到。

Always resolve anything in parenthesis first!
遇到括号，一定要先确定括号内的类型。

Example #1: Simple declaration

+-------+
| +-+   |
| ^ |   |
char *str[10];
^   ^   |   |
|   +---+   |
+-----------+

Question we ask ourselves: What is str?

``str is an...
“str的类型是...

We move in a spiral clockwise direction starting with `str' and the first character we see is a `[' so, that means we have an array, so...
我们沿着顺时针螺旋移动。从 “str”开始，遇到的第一个符号是 “[”。所以，它是个数组。 所以……

``str is an array 10 of...
“str是一个容纳……类型的长度为10的数组

Continue in a spiral clockwise direction, and the next thing we encounter is the `*' so, that means we have pointers, so...
继续沿着顺时针螺旋移动。下一个遇到的符号是`*'。这是指针的意思。所以……

``str is an array 10 of pointers to...
“str是一个容纳指向……的指针的长度为10的数组

Continue in a spiral direction and we see the end of the line (the `;'), so keep going and we get to the type `char', so...
继续沿着顺治真螺旋移动。遇到了行末（其实就是分号“;”），不管它，继续移动，我们遇到了“char”。所以……

``str is an array 10 of pointers to char''
“str是一个容纳指向char类型的指针的长度为10的数组。”

We have now ``visited'' every token; therefore we are done!
我们已经“遍历”了每个符号。所以，完成了！

Example #2: Pointer to Function declaration

+--------------------+
| +---+              |
| |+-+|              |
| |^ ||              |
char *(*fp)( int, float *);
^   ^ ^  ||              |
|   | +--+|              |
|   +-----+              |
+------------------------+

Question we ask ourselves: What is fp?

``fp is a...
“fp是一个……

Moving in a spiral clockwise direction, the first thing we see is a `)'; therefore, fp is inside parenthesis, so we continue the spiral inside the parenthesis and the next character seen is the `*', so...
沿着顺时针螺旋移动，第一个遇到的是右括号“)”。我们发现，fp被放在一个括号内。我们必须先留在括号内，继续沿着螺旋移动。下一个遇到的符号是“*”。所以……

``fp is a pointer to...
“fp是一个指向……的指针

We are now out of the parenthesis and continuing in a spiral clockwise direction, we see the `('; therefore, we have a function, so...
我们现在离开了括号。继续沿着顺时针螺旋移动。我们遇到“(”。这是函数的意思。所以……

``fp is a pointer to a function passing an int and a pointer to float returning...
“fp是一个指向一个参数为一个int和一个指向float型的指针，返回值是……型的函数的指针。

Continuing in a spiral fashion, we then see the `*' character, so...
继续绕圈。遇到了“*”，所以……

``fp is a pointer to a function passing an int and a pointer to float returning a pointer to...
“fp是一个指向一个参数为一个int和一个指向float型的指针，返回值是一个指向……的指针的函数的指针。

Continuing in a spiral fashion we see the `;', but we haven't visited all tokens, so we continue and finally get to the type `char', so...
继续绕圈，遇到“;”，但是符号还没遍历完。继续绕圈，最后遇到char。所以……

``fp is a pointer to a function passing an int and a pointer to float returning a pointer to a char''
“fp是一个指向一餐参数为一个int和一个指向float型的指针，返回值是一个指向char的指针的函数的指针。”

Example #3: The ``Ultimate''

+-----------------------------+
|                  +---+      |
|  +---+           |+-+|      |
|  ^   |           |^ ||      |
void (*signal(int, void (*fp)(int)))(int);
^    ^      |      ^    ^  ||      |
|    +------+      |    +--+|      |
|                  +--------+      |
+----------------------------------+

Question we ask ourselves: What is `signal'?

Notice that signal is inside parenthesis, so we must resolve this first!

Moving in a clockwise direction we see `(' so we have...
沿着顺时针移动，遇到“(”，所以……

``signal is a function passing an int and a...
“signal是一个参数为一个int和一个……型变量，返回值是……的函数。

Hmmm, we can use this same rule on `fp', so... What is fp? fp is also inside parenthesis so continuing we see an `*', so...
吼吼，我们可以用同样的规则确定fp的类型。所以……fp是什么类型呢？fp也在括号内。所以，继续绕圈，我们遇见“*”。所以……

fp is a pointer to...
fp是一个指向……的指针

Continue in a spiral clockwise direction and we get to `(', so...
继续绕圈，遇到“(”。所以……

``fp is a pointer to a function passing int returning...''
“fp是一个指向一个参数为int，返回值是……的函数的指针。”

Now we continue out of the function parenthesis and we see void, so...
离开函数参数的括号，我们遇见void。所以……

``fp is a pointer to a function passing int returning nothing (void)''
“fp是一个指向参数为int，什么也不返回（返回void）的函数的指针。”

We have finished with fp so let's catch up with `signal', we now have...
fp部分结束了。我们回到signal的类型。目前是……

``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning...
“signal是一个参数为一个int和一个指向一个参数为int，什么也不返回的函数的指针，返回值为……的函数。”

We are still inside parenthesis so the next character seen is a `*', so...
我们还在括号内。下一个符号是“*”。所以

``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer to...
“signal是一个参数位一个int和一个指向一个参数为int，什么也不返回的函数的指针，返回值为指向……的指针的函数。”

We have now resolved the items within parenthesis, so continuing clockwise, we then see another `(', so...
现在，括号内的类型已经解决了。继续绕圈，遇到另一个“(”。所以……

``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer to a function passing an int returning...
“signal是一个参数位一个int和一个指向一个参数为int，什么也不返回的函数的指针，返回值为指向一个参数为int，返回值为……的函数的指针的函数。”

Finally we continue and the only thing left is the word `void', so the final complete definition for signal is:
最后，继续绕圈。最后一个符号就是最左边的“void”。所以，signal的最终定义就是：

``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer to a function passing an int returning nothing (void)''
“signal是一个参数位一个int和一个指向一个参数为int，什么也不返回的函数的指针，返回值为指向一个参数为int，什么也不返回的函数的指针的函数。”

The same rule is applied for const and volatile. For Example:

const char *chptr;

Now, what is chptr??
那么，chptr是什么类型？？

``chptr is a pointer to a char constant''
“chptr是一个指向char型常量的指针变量。”

char * const chptr;

Now, what is chptr??
现在，chptr又是什么？？

``chptr is a constant pointer to char''
“chptr是一个指向char型变量的指针常量。”

Finally:

volatile char * const chptr;

Now, what is chptr??
现在，chptr又是什么类型？？

``chptr is a constant pointer to a char volatile.''
“chptr是一个指向char volatile型变量的指针常量。”

Practice this rule with the examples found in K&R II on page 122.