Clockwise/Spiral Rule

时间:2024-01-17 18:31:14

Clockwise/Spiral Rule

  There is a technique known as the ``Clockwise/Spiral Rule''. (顺时针螺旋法则).

  There are three simple steps to follow:

  1. Starting with the unknown element, move in a spiral/clockwise direction; when ecountering the following elements replace them with the corresponding english statements:
    [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.
  3. Always resolve anything in parenthesis first!

Example #1: Simple declaration

  Clockwise/Spiral Rule

Question we ask ourselves: What is str?

``str is an...

  • 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...

    ``str is an array 10 of...

  • Continue in a spiral clockwise direction, and the next thing we encounter is the `*' so, that means we have pointers, so...

    ``str is an array 10 of pointers to...

  • 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...

    ``str is an array 10 of pointers to char''

  • We have now ``visited'' every token; therefore we are done!

Example #2: Pointer to Function declaration

  Clockwise/Spiral Rule

  Question we ask ourselves: What is fp?

``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...

    ``fp is a pointer to...

  • We are now out of the parenthesis and continuing in a spiral clockwise direction, we see the `('; therefore, we have a function, so...

    ``fp is a pointer to a function passing an int and a pointer to float returning...

  • Continuing in a spiral fashion, we then see the `*' character, so...

    ``fp is a pointer to a function passing an int and a pointer to float returning a pointer to...

  • 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...

    ``fp is a pointer to a function passing an int and a pointer to float returning a pointer to a char''

Example #3: The ``Ultimate''

  Clockwise/Spiral Rule  

  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...

    ``signal is a function passing an int and a...

  • Hmmm, we can use this same rule on `fp', so... What is fp? fp is also inside parenthesis so continuing we see an `*', so...

    fp is a pointer to...

  • Continue in a spiral clockwise direction and we get to `(', so...

    ``fp is a pointer to a function passing int returning...''

  • Now we continue out of the function parenthesis and we see void, so...

    ``fp is a pointer to a function passing int returning nothing (void)''

  • We have finished with fp so let's catch up with `signal', we now have...

    ``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning...

  • We are still inside parenthesis so the next character seen is a `*', so...

    ``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer to...

  • We have now resolved the items within parenthesis, so continuing clockwise, we then see another `(', so...

    ``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...

  • Finally we continue and the only thing left is the word `void', so the final complete definition for signal is:

    ``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)''

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

	const char *chptr;
  • Now, what is chptr??

    ``chptr is a pointer to a char constant''

How about this one:

	char * const chptr;
  • Now, what is chptr??

    ``chptr is a constant pointer to char''

Finally:

	volatile char * const chptr;
  • Now, what is chptr??

    ``chptr is a constant pointer to a char volatile.''

参考:http://c-faq.com/decl/spiral.anderson.html