c-faq The Clockwise/Spiral Rule

Clockwise 顺时针 Spiral 螺旋

This was posted to comp.lang.c by its author, David Anderson, on 1994-05-06.

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!

There are three simple steps to follow:

Step 1:

Starting with the unknown element, move in a spiral/clockwise direction（顺时针、旋转方向）; when ecountering the following elements replace them with the corresponding english statements:

1、[ ] or []

=> Array X size of... or Array undefined size of...

NOTE:

1、array和它的length是一起的

2、(type1, type2)

=> function passing type1 and type2 returning...

NOTE:

1、function parameter 和 return type是一起的

3、*

=> pointer(s) to...

Step 2:

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

Step 3:

Always resolve anything in parenthesis first!

NOTE: 上述...表示需要根据declaration替换的

Example #1: Simple declaration

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

Question we ask ourselves: What is str?

str is an...

1、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...

2、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...

3、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''

4、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...

1、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...

2、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...

3、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...

4、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''

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

Question we ask ourselves: What is signal?

Notice that signal is inside parenthesis, so we must resolve this first!

1、Moving in a clockwise direction we see ( so we have...

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

2、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...

3、Continue in a spiral clockwise direction and we get to (, so...

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

4、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)''

5、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...

6、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...

7、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...

8、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)

CV

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.

Practice this rule with the examples found in K&R II on page 122.

Excercise

int (*(*foo)(double))[3] = nullptr;

foo is a pointer to a function passing double returning pointer to an array size of 3 type of int

Website

https://cdecl.org/