What are the differences between a pointer variable and a reference variable in C++?

Question：

I know references are syntactic sugar, so code is easier to read and write.

But what are the differences?

Summary from answers and links below:

A pointer can be re-assigned any number of times while a reference can not be re-seated after binding.

Pointers can point nowhere (

NULL

),
whereas reference always refer to an object.

You can't take the address of a reference like you can with pointers.

There's no "reference arithmetics" (but you can take the address of an object pointed by a reference and do pointer arithmetics on it as in 

&obj
+ 5

).

To clarify a misconception:

The C++ standard is very careful to avoid dictating how a compiler must implement references, but every C++ compiler implements references as pointers. That is, a declaration such as:

int &ri = i;

if it's not optimized away entirely, allocates
the same amount of storage as a pointer, and places the address of i into that storage.

So, a pointer and a reference both occupy the same amount of memory.

As a general rule,

Use references in function parameters and return types to define useful and self-documenting interfaces.

Use pointers to implement algorithms and data structures.

Interesting read:

My alltime favorite C++ FQA lite.

References vs. Pointers.

An Introduction to References.

References and const.

Answer：

A pointer can be re-assigned:

int x = 5;
int y = 6;
int *p;
p =  &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);

A reference cannot, and must be assigned at initialization:

int x = 5;
int y = 6;
int &r = x;

A pointer has its own memory address and size on the stack (4 bytes on x86), whereas a reference shares the same memory address (with the original variable) but also takes up some space on the stack. Since a reference has the same address as the original variable
itself, it is safe to think of a reference as another name for the same variable. Note: What a pointer points to can be on the stack or heap. Ditto a reference. My claim in this statement is not that a pointer must point to the stack. A pointer is just a variable
that holds a memory address. This variable is on the stack. Since a reference has its own space on the stack, and since the address is the same as the variable it references. More on stack
vs heap. This implies that there is a real address of a reference that the compiler will not tell you.

int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;
assert(p == p2);

You can have pointers to pointers to pointers offering extra levels of indirection. Whereas references only offer one level of indirection.

int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;
pp = &q;//*pp = q
**pp = 4;
assert(y == 4);
assert(x == 0);

Pointer can be assigned NULL directly, whereas reference cannot. If you try hard enough, and you know how, you can make the address of a reference NULL. Likewise, if you try hard enough you can have a reference to a pointer, and then that reference can contain
NULL.

int *p = NULL;
int &r = NULL; <--- compiling error

Pointers can iterate over an array, you can use 

++

 to
go to the next item that a pointer is pointing to, and 

+
4

 to go to the 5th element. This is no matter what size the object is that the pointer points to.

A pointer needs to be dereferenced with 

*

 to
access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses 

->

 to
access it's members whereas a reference uses a 

.

.

A pointer is a variable that holds a memory address. Regardless of how a reference is implemented, a reference has the same memory address as the item it references.

References cannot be stuffed into an array, whereas pointers can be (Mentioned by user @litb)

Const references can be bound to temporaries. Pointers cannot (not without some indirection):

const int &x = int(12); //legal C++
int *y = &int(12); //illegal to dereference a temporary.

This makes 

const&

 safer
for use in argument lists and so forth.