Interpret the clockwise/spiral rule (clockwise spiral rules) __c++ of C + + declarations

The ' clockwise/spiral rule ' by David Anderson

There is a technique known as the "clockwise/spiral rule" which enables any C programmer to parse into their head any C de claration!

There are three simple steps to follow:starting and the unknown element, move in a spiral/clockwise direction; When ecountering the "following elements replace them with the corresponding 中文版 statements:

[x] or [] => Array X size of ... or array undefined size of ... (Type1, type2) => function passing type1 and type2 returning ... * => pointer (s) to ...

Keep doing a spiral/clockwise direction until all tokens have been.

Always resolve anything in parenthesis first! Example #1: Simple declaration

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

Question we ask Ourselves:what is STR? ' Str is ' ... We move in a spiral clockwise direction starting with ' str ' and the ' the ' of the ' the ' and ' the ' we are a ' [' So, that character we means An array, so ... ' str ' is ' an array ' ... Continue in a spiral clockwise direction, and the next thing we encounter are the ' * ' so, which means we have pointers, so. . ' Str is ' a array of pointers to ... Continue in a spiral direction and we are the end of the "the", so keep going and we have to the type ' char '. . ' Str is ' a array of pointers to Char ' We have now ' visited ' every token; Therefore we are done! Example #2: Pointer to Function declaration

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

Question we ask Ourselves:what is FP? ' FP is a ... Moving in a spiral clockwise direction, the ' a ' thing we are ' a ') '; Therefore, FP is inside parenthesis, so we continue the spiral inside the parenthesis and the next character is the ' * ', so ... ' FP is a pointer to ... We are now out of the "parenthesis" and continuing in a spiral clockwise direction, we have the ' ('; therefore, we have a fun Ction, so ... ' FP is a pointer to a function passing a int and a pointer to float returning ... Continuing in a spiral fashion, we then you the ' * ' character, so ... ' FP is a pointer to a function passing a int and a Pointer to float returning a pointer ... Continuing in a spiral fashion we are the '; ', but we haven ' t visited all tokens, so we continue and finally get to the Ty PE ' 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 '

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

Question we ask ourselves:what is ' signal '?

Notice that signal is inside parenthesis and so we must resolve this first! Moving in A clockwise direction we are (' so we have ... ' signal is a function passing an int and a ... Hmmm, we can use this same the rule in ' FP ', so ... What is FP? FP is also inside parenthesis so continuing we are ' * ', so ... FP is a pointer to ... Continue in a spiral clockwise direction and we have to ' (', so ... ' FP is a pointer to a function passing int returning.. .'' Now we are continue out of the function parenthesis and we are void, so ... ' FP is a pointer to a function passing int returni ng nothing (void) "We have finished with FP so let's catch up with ' signal '", We now have ... ' signal is a function passin G 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 be a ' * ', so ... ' signal is a function passing a int and a POI Nter to a function passing a int returning nothing (void) returning a pointEr to ... We have now resolved the items within parenthesis, so continuing clockwise, we then to the another ' (', so ... ' signal is a f Unction passing an int and a pointer to a function passing an int returning nothing (void) Returning a pointer to a FUNCTI On passing an int returning ... Finally we continue and the only thing left is the word ' void ', so the final complete definition for signal is: ' SIG NAL is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer t o a function passing an int returning nothing (void) '

The same 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. '

Practice this and the examples found in K&r II on page 122.

copyright©1993,1994 David Anderson

This article may is freely distributed as long as the author ' name and this notice are.