Do you know the Clockwise/Spiral Rule in C/C++?
If your answer is “NO”, then don’t worry in this article you will learn about the Clockwise/Spiral Rule. But, if you already know the Clockwise/Spiral Rule, then this blog post will help you in quick revision.
Let’s Begin.
The Clockwise/Spiral technique is originally introduced by “David Anderson”. This technique enables the C program to parse in their mind any C declaration! It means this technique helps you in understanding a complex declaration and easily you can parse complex C declaration.
Three simple steps of the Clockwise/Spiral Rule in C:
There are the following steps which you need to follow. In beginning, you find these steps complicated but believe me after solving some examples it becomes easy!
➤ Start from the name of the variable, move in a spiral/clockwise direction. When encountering the below-mentioned 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…
➤ Repeat until all tokens have been covered.
➤ Always resolve anything in parenthesis first!
So let’s see some examples, we will move from easy to hard!
Simple declaration:
Example 1:
int *ptr;
Now Question is, What is ptr?
So, here we can use above mentioned techniques, we move in a spiral clockwise direction starting with `ptr’ and the first character seen is a `*’, so… ptr is a pointer to…
Continue in a spiral direction and we see the end of the line (the `;’), so keep going and we get to the type `int’, so… “ptr is a pointer to int”
Example 2:
int * const ptr;
Now Question is, What is ptr?
We need to implement the same rule here, we move in a spiral clockwise direction starting with `ptr’ and the first we see is a const qualifier, so… ptr is a constant…
Continue in a spiral direction and we see a `*’, so… ptr is a constant pointer to…
Continuing in a spiral fashion we see the end of the line (the `;’), so keep going and we get to the type `int’, so… “ptr is a constant pointer to int”.
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Pointers array declaration:
Example:
int*ptr[10];
Question is, What is ptr?
+-------+
| +-+ |
| ^ | |
int *ptr[10];
^ ^ | |
| +---+ |
+-----------+
Start from the ptr and move in a spiral clockwise direction. The first character we see is a `[' so, that means we have an array, so… ptr is an array 10 of…
Continue moving in a spiral clockwise direction, and the next character we encounter is the `*‘ so, that means we have pointers, so… ptr is an array 10 of pointers to…
Keep moving in a spiral direction and we see the end of the line (the `;’), so keep going and we get to the type `int’, so…”ptr is an array 10 of pointers to int”
⚡Note: Mentioned steps are not a universal rule there are some cases where they fail.
Pointer to Function declaration:
Example:
int*(*fp)( int, float *);
Again the same question is, What is fp?
+--------------------+
| +---+ |
| |+-+| |
| |^ || |
int *(*fp)( int, float *);
^ ^ ^ || |
| | +--+| |
| +-----+ |
+------------------------+
Now I believe you have memorized the steps, start from fp and move in a spiral clockwise direction the first thing we see is a `)'; therefore, fp is inside the parenthesis, so we continue the spiral inside the parenthesis and the next character seen is the `*', so… fp is a pointer to…
Now we are outside of the parenthesis and moving in a spiral clockwise direction, we see the `('; therefore, we have a function and the argument is int and float* 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…
Keep continuing in a spiral fashion we see the end of the line (the `;’), so keep going and we get to the type `int’, so… “fp is a pointer to a function passing an int and a pointer to float returning a pointer to an int”.