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.
- My alltime favorite C++ FQA lite.
- References vs. Pointers.
- An Introduction to References.
- References and const.
- 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
+ 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
- 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.
const&safer for use in argument lists and so forth.