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);
A pointer can be assigned
nullptrdirectly, whereas reference cannot. If you try hard enough, and you know how, you can make the address of a reference
nullptr. Likewise, if you try hard enough, you can have a reference to a pointer, and then that reference can contain
int *p = nullptr;
int &r = nullptr; <--- compiling error int &r = *p; <--- likely no compiling error, especially if the nullptr is hidden behind a function call, yet it refers to a non-existent int at address 0
Pointers can iterate over an array; you can use
++to go to the next item that a pointer is pointing to, and
+ 4to 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
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.
const& safer for use in argument lists and so forth.