difference between pointer and reference in c?

Question

what is the difference between pointer , reference and dereference in c?

Answer 1

Here is a memory map; a representation of memory as a sequence of blocks:

    address    01   02   03
             +----+----+----+...
data within  | 23 | 6f | 4a |
             +----+----+----+...

Now suppose we create a character:

char c = 'z';  // 'z' is 7a in hex

Further suppose c is stored at address 01, so our memory looks like so:

    address    01   02   03
             +----+----+----+...
data within  | 7a | 6f | 4a |
             +----+----+----+...

Now, let's create a pointer:

char* p = &c;  // point at c

p may be stored at address 02:

    address    01   02   03
             +----+----+----+...
data within  | 7a | 01 | 4a |
             +----+----+----+...

Here the pointer p is at address 02 and it points at address 01. That's the meaning of p = &c;. When we dereference the pointer p (at address 02) we look at what's in the address pointed at by p. That is, p points at address 01, and so dereferencing p means looking inside address 01.

Finally, lets create a reference:

char& r = c;

Here the memory layout doesn't change. That is, no memory is used to store r. r is a sort of alias for c, so when we refer to r we practically refer to c. r and c are conceptually one. Changing r means changing c, and changing c means changing r.

When you create a reference you must initialize, and once initialized you cannot re-initialize it with another target. That is, above the reference r means and forever will mean c.

Also related are const references. These are the same as a reference, except they are immutable:

const char& r = c;
r = 'y';  // error; you may not change c through r
c = 'y'   // ok. and now r == 'y' as well

We use const references when we are interested in reading the data but frown upon changing it. By using a const reference the compiler will not copy the data, so this gives us ideal performance, but also forbid us from changing the data, for correctness.

In a sense, you can say that references are a compile-time feature, whereas pointers are a runtime feature. So references are faster and cheaper than pointers, but come with certain constraints and implications. Like other compile-time-vs-runtime alternatives, we sometimes pick one over the other for performance, sometimes for static analysis and sometimes for flexibility.