BYR Achieve · 镜像论坛

[This was posted to comp.lang.c by its author, David Anderson, on 1994-05-06.] [本帖层在1994-05-06被它的作者David Anderson发到comp.lang.c邮件组中] 【译：wks】 The ``Clockwise/Spiral Rule'' “顺时针螺旋法则” By David Anderson 作者：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! 这个技巧称为“顺时针螺旋法则”。C程序员们用了它，就可以看懂任何C语言变量类型声明。 There are three simple steps to follow: 一共三步，如下： 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 例1: 简单声明 +-------+ | +-+ | | ^ | | char *str[10]; ^ ^ | | | +---+ | +-----------+ Question we ask ourselves: What is str? 问题：str是什么类型？ ``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 例2：指向函数的指针的声明 +--------------------+ | +---+ | | |+-+| | | |^ || | char *(*fp)( int, float *); ^ ^ ^ || | | | +--+| | | +-----+ | +------------------------+ Question we ask ourselves: What is fp? 问题：fp是什么类型？ ``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'' 例3：终极难题 +-----------------------------+ | +---+ | | +---+ |+-+| | | ^ | |^ || | void (*signal(int, void (*fp)(int)))(int); ^ ^ | ^ ^ || | | +------+ | +--+| | | +--------+ | +----------------------------------+ Question we ask ourselves: What is `signal'? 问题：signal是什么类型？ Notice that signal is inside parenthesis, so we must resolve this first! 首先，必须意识到signal在括号内。所以必须先解决括号内的类型。 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: 同样的规则，const和volatile关键字也有用。比如： 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。” 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. 你可以做一个练习，用上述规则做《K&R的C语言书（第二版）》第122页上的例子。 Copyright © 1993,1994 David Anderson This article may be freely distributed as long as the author's name and this notice are retained. 本文可以自由发布，唯需保留作者名字和这条提示。

顺时针螺旋法则（clockwise/spiral rule）教你认变量类型