CODEWITHSUNDEEP
cpointers
webpersistence
webpersistence

Reputation: 904

How does pointer incrementation work?

We have an array: int p[100].
Why p[i] is equivalent to *(p+i) and not *(p+i*sizeof(int)) ?

Upvotes: 1

Views: 152

Answers (5)

wefwefa3
wefwefa3

Reputation: 4036

Why p[i] is equivalent to *(p+i) and not *(p+i*sizeof(int)) ?

Because some processor architectures cannot dereference a pointer that does not point to an address that is aligned by the size of its type. That basicly means that a pointer to a 4 byte integer should always point to an adress that is the multiple of 4.

When a program tries to dereference a misaligned pointer it might cause a "Bus error". You can read more about it here on Wikipedia.

What you are asking for is that p + 1 should increment the pointer by one byte instead of one element. If the language was designed that way writing p++ would no longer be valid for pointers of other types than char. It would also cause big problems with pointer alignment when a programmer forgets to write * sizeof(*p) to make the addition.

It might be confusing but there are very valid reasons for why the language was designed this way.

Upvotes: 3

glglgl
glglgl

Reputation: 91139

Why p[i] is equivalent to *(p+i) and not *(p+i*sizeof(int))?

This is because of the way pointer arithmetics work: adding an integer n to a pointer yields a pointer to the nth element (not byte) from the first on (0-based).

Upvotes: 0

haccks
haccks

Reputation: 106102

This is because before adding i to p compiler calculates internally the size of data type p points to and then add it i times to p.

Upvotes: 1

Thiyagu
Thiyagu

Reputation: 17920

Array elements are stored contiguously.

*(p+i)

Here p has the base address of array p and i ranges from 0 to 99.

So you can iterate over the elements of p by incrementing i.

Upvotes: 0

ouah
ouah

Reputation: 145899

Why p[i] is equivalent to *(p+i) and not *(p+i*sizeof(int)) ?

Because *(p+i) is also the same as *((int *) ((char *) p + i * sizeof (int))). When you add an integer i to a pointer, the pointer is moved i times the size of the pointed object.

Upvotes: 7

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