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<html><!-- Mirrored from c-faq.com/decl/spiral.anderson.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:17 GMT --><head><title>Clockwise/Spiral Rule</title></head><body>[This was posted to comp.lang.c by its author,David Anderson, on 1994-05-06.]<!-- Message-ID: <17188@anderson> --><H1>The ``Clockwise/Spiral Rule''</H1><H2>By David Anderson</H2><p>There is a technique known as the ``Clockwise/Spiral Rule'' which enablesany C programmer to parse in their head any C declaration!<p>There are three simple steps to follow:<ol><li>Starting with the unknown element, move in a spiral/clockwise direction;when ecountering the following elements replace them with the correspondingenglish statements:<p><dl><dt>[X] or []</dt><dd>=> Array X size of... or Array undefined size of...</dd><dt>(type1, type2)</dt><dd>=> function passing type1 and type2 returning...</dd><dt>*</dt><dd>=> pointer(s) to...</dl><p><li>Keep doing this in a spiral/clockwise direction until all tokens have beencovered.<p><li>Always resolve anything in parenthesis first!</ol><h3>Example #1: Simple declaration</h3><pre> +-------+ | +-+ | | ^ | | char *str[10]; ^ ^ | | | +---+ | +-----------+</pre>Question we ask ourselves: What is str?<blockquote>``str is an...</blockquote><ul><li>We move in a spiral clockwise direction starting with `str' and the firstcharacter we see is a `[' so, that means we have an array, so...<blockquote>``str is an array 10 of...</blockquote><li>Continue in a spiral clockwise direction, and the next thing we encounter isthe `*' so, that means we have pointers, so...<blockquote>``str is an array 10 of pointers to...</blockquote><li>Continue in a spiral direction and we see the end of the line (the `;'), sokeep going and we get to the type `char', so...<blockquote>``str is an array 10 of pointers to char''</blockquote><li>We have now ``visited'' every token; therefore we are done!</ul><h3>Example #2: Pointer to Function declaration</h3><pre> +--------------------+ | +---+ | | |+-+| | | |^ || | char *(*fp)( int, float *); ^ ^ ^ || | | | +--+| | | +-----+ | +------------------------+</pre>Question we ask ourselves: What is fp?<blockquote>``fp is a...</blockquote><ul><li>Moving in a spiral clockwise direction, the first thing we see is a `)';therefore, fp is inside parenthesis, so we continue the spiral inside theparenthesis and the next character seen is the `*', so...<blockquote>``fp is a pointer to...</blockquote><li>We are now out of the parenthesis and continuing in a spiral clockwisedirection, we see the `('; therefore, we have a function, so...<blockquote>``fp is a pointer to a function passing an int and a pointer to floatreturning...</blockquote><li>Continuing in a spiral fashion, we then see the `*' character, so...<blockquote>``fp is a pointer to a function passing an int and a pointer to floatreturning a pointer to...</blockquote><li>Continuing in a spiral fashion we see the `;', but we haven't visited alltokens, so we continue and finally get to the type `char', so...<blockquote>``fp is a pointer to a function passing an int and a pointer to floatreturning a pointer to a char''</blockquote></ul><h3>Example #3: The ``Ultimate''</h3><pre> +-----------------------------+ | +---+ | | +---+ |+-+| | | ^ | |^ || | void (*signal(int, void (*fp)(int)))(int); ^ ^ | ^ ^ || | | +------+ | +--+| | | +--------+ | +----------------------------------+</pre><p>Question we ask ourselves: What is `signal'?<p>Notice that signal is <em>inside</em> parenthesis, so we must resolve this first!<ul><li>Moving in a <em>clockwise</em> direction we see `(' so we have...<blockquote>``signal is a function passing an int and a...</blockquote><li>Hmmm, we can use this same rule on `fp', so... What is fp? fp is also insideparenthesis so continuing we see an `*', so...<blockquote>fp is a pointer to...</blockquote><li>Continue in a spiral clockwise direction and we get to `(', so...<blockquote>``fp is a pointer to a function passing int returning...''</blockquote><li>Now we continue out of the function parenthesis and we see void, so...<blockquote>``fp is a pointer to a function passing int returning nothing (void)''</blockquote><li>We have finished with fp so let's catch up with `signal', we now have...<blockquote>``signal is a function passing an int and a pointer to a function passing anint returning nothing (void) returning...</blockquote><li>We are still inside parenthesis so the next character seen is a `*', so...<blockquote>``signal is a function passing an int and a pointer to a function passing anint returning nothing (void) returning a pointer to...</blockquote><li>We have now resolved the items within parenthesis, so continuing clockwise,we then see another `(', so...<blockquote>``signal is a function passing an int and a pointer to a function passing anint returning nothing (void) returning a pointer to a function passing anint returning...</blockquote><li><em>Finally</em> we continue and the only thing left is the word `void', so thefinal complete definition for signal is:<blockquote>``signal is a function passing an int and a pointer to a function passing anint returning nothing (void) returning a pointer to a function passing anint returning nothing (void)''</blockquote></ul><p>The same rule is applied for const and volatile. For Example:<pre> const char *chptr;</pre><ul><li>Now, what is chptr??<blockquote>``chptr is a pointer to a char constant''</blockquote></ul><p>How about this one:<pre> char * const chptr;</pre><ul><li>Now, what is chptr??<blockquote>``chptr is a constant pointer to char''</blockquote></ul><p>Finally:<pre> volatile char * const chptr;</pre><ul><li>Now, what is chptr??<blockquote>``chptr is a constant pointer to a char volatile.''</blockquote></ul><p>Practice this rule with the examples found in K&R II on page 122.<p><hr>Copyright © 1993,1994 David Anderson<p>This article may be freely distributed as long as the author's name andthis notice are retained.<hr></body><!-- Mirrored from c-faq.com/decl/spiral.anderson.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:17 GMT --></html>⌨️ 快捷键说明
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