阅读复杂的C类型声明，通常采用右左法则，也就是Clockwise/Spiral Rule (顺时针/螺旋法则)。 本文将首先介绍工具(cdecl)（个人比较偏好使用工具提高学习和工作效率），然后中英文对照翻译一下著名的文章The ``Clockwise/Spiral Rule''。
1. 工具(cdecl)
1.1 命令行(cdecl)
在Ubuntu上，使用如下命令安装cdecl。
$ sudo apt-get install cdecl
如何使用cdecl，举几个例子：
$ cdecl -i Type `help' or `?' for help cdecl> explain int *foo[3] declare foo as array 3 of pointer to int cdecl> cdecl> explain int (*foo)[3] declare foo as pointer to array 3 of int cdecl> cdecl> explain void (*signal(int, void (*)(int)))(int); declare signal as function (int, pointer to function (int) returning void) returning pointer to function (int) returning void cdecl> cdecl> exit $
注意： 阅读C++类型声明，请使用工具c++decl。
1.2 在线服务 （https://cdecl.org/）
这个在线服务实在是太贴心了，体验截图如下：
2. 文章（The ``Clockwise/Spiral Rule"）
[This was posted to comp.lang.c by its author, David Anderson, on 1994-05-06.]
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!
C程序员们如果使用"顺时针/螺旋法则"，就可以看懂任何C语言类型声明。
There are three simple steps to follow: 一共分为三步，描述如下：
1. Starting with the unknown element, move in a spiral/clockwise direction; when ecountering the following elements replace them with the corresponding english statements: 从未知元素(变量名?)开始，沿着顺时针螺旋移动。每次遇到以下几种符号，把它换成相应的句子：
[X] or [] => Array X size of... or Array undefined size of... (type1, type2) => function passing type1 and type2 returning... * => pointer(s) to... 2. Keep doing this in a spiral/clockwise direction until all tokens have been covered. 反复地沿着顺时针/螺旋方向做上述动作，直到所有的符号都被遍历到;
3. Always resolve anything in parenthesis first!
如果遇到括号，一定要先确定括号内的类型。
Example #1: Simple declaration | 简单声明 +-------+ | +-+ | | ^ | | char *str[10]; ^ ^ | | | +---+ | +-----------+ Question we ask ourselves: What is str?
问题： str是什么？ ``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... 从"str"开始，沿顺时针做螺旋移动，遇到的第一个符号是"["。所以，它是一个数组。所以...
``str is an array 10 of... ``str是一个元素类型为...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... 继续沿顺时针做螺旋移动，下一个遇到的符号是'*'。这意味着有指针。所以...
``str is an array 10 of pointers to...
``str是一个元素类型为指向...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...
继续沿顺时针做螺旋移动，这一行最后的符号是';', 继续沿顺时针做螺旋移动，遇到类型'char'，所以 ... ``str is an array 10 of pointers to char''
``str是一个元素类型为指向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"了每一个符号。收工！
Example #2: Pointer to Function declaration | 函数指针声明 +--------------------+ | +---+ | | |+-+| | | |^ || | char *(*fp)( int, float *); ^ ^ ^ || | | | +--+| | | +-----+ | +------------------------+ Question we ask ourselves: What is fp?
问题： fp是什么类型？ ``fp is a... 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...
沿顺时针做螺旋移动，第一个遇到的是")"。我们发现，fp被放在一个()内。我们必须先留在()内，
继续沿着顺时针做螺旋移动。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...
离开了括号，继续沿顺时针做螺旋移动。我们遇到"("。因此，这是一个函数。所以... ``fp is a pointer to a function passing an int and a pointer to float returning...
``fp是一个指向函数的指针，函数参数为一个int型和一个float型指针，返回值为... Continuing in a spiral fashion, we then see the `*' character, so...
继续沿顺时针做螺旋移动，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是一个指向函数的指针，函数参数为一个int型和一个float型指针，返回值为
一个指向...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...
继续沿顺时针做螺旋移动，遇到了";"，但是我们还没有遍历完所有符号，于是继续绕圈圈，
最后遇到类型"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是一个指向函数的指针，函数参数为一个int型和一个float型指针，返回值为
一个指向char类型的指针"
Example #3: The ``Ultimate'' | 终极难题 +-----------------------------+ | +---+ | | +---+ |+-+| | | ^ | |^ || | void (*signal(int, void (*fp)(int)))(int); ^ ^ | ^ ^ || | | +------+ | +--+| | | +--------+ | +----------------------------------+ Question we ask ourselves: What is `signal'? 问题： signal是什么？
Notice that signal is inside parenthesis, so we must resolve this first!
注意signal在括号内，于是我们必须先搞清楚括号内的类型！ 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 是一个函数，该函数的参数为一个整数和... Hmmm, we can use this same rule on `fp', so... What is fp? fp is also inside parenthesis so continuing we see an `*', so...
嗯嗯，我们可以用同样的规则来确定fp的类型，那么...fp是什么类型？ fp也在括号内。所以，继续绕圈圈，
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...
继续沿顺时针方向绕圈圈，我们遇到'('，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之后，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...
解析fp部分结束了。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:
继续绕圈圈，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:
同样的规则可用于const和volatile。例如： const char *chptr; Now, what is chptr??
那么现在，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。" How about this one:
下面这个呢？ 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: 最后一道题：
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.
请使用此规则对K&R II第122页上的例子做练习。
Copyright © 1993,1994 David Anderson
This article may be freely distributed as long as the author's name and this notice are retained.
本文可以自由地发布，唯一需要保留的就是作者的名字和这条提示。
参考文献
- Reading C type declarations
- How to interpret complex C/C++ declarations
- The ``Clockwise/Spiral Rule''
结束语： 此时无声胜有声
cdecl> explain void (*signal(int, void (*)(int)))(int); declare signal as function (int, pointer to function (int) returning void) returning pointer to function (int) returning void