I have seen many times people complaining of interviewer giving them shit looking declaration and ask us to tell them what it is..
Ex. :
int * p :
pointer to an integer :
[:D]..
okay.. how about this..
void (*signal(int, void (*fp)(int)))(int);
To some even this may look easy but to me I put down my pencil and got back to facebook to relieve from tension.
Following is the illustration of one simple method I stumbled across and felt it worked for any kind of declaration.
If there is/are any exception(s) to the following, please feel free to discuss.
[This was posted to comp.lang.c by its author, David Anderson, on 1994-05-06.].. and I read on c-facts ;)
The ``Clockwise/Spiral Rule''
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:
- 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...
- Keep doing this in a spiral/clockwise direction until all tokens have been covered.
- Always resolve anything in parenthesis first!
Example #1: Simple declaration
+-------+
| +-+ |
| ^ | |
char *str[10];
^ ^ | |
| +---+ |
+-----------+
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
+--------------------+
| +---+ |
| |+-+| |
| |^ || |
char *(*fp)( int, float *);
^ ^ ^ || |
| | +--+| |
| +-----+ |
+------------------------+
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''
+-----------------------------+
| +---+ |
| +---+ |+-+| |
| ^ | |^ || |
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!
- 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.''
Practice this rule with the examples found in K&R II on page 122.
Copyright © 1993,1994 David AndersonThis article may be freely distributed as long as the author's name and this notice are retained.