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

C_plus_plus_at_woodlands_school
 

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:

  1. A pointer can be re-assigned any number of times while a reference can not be re-seated after binding.
  2. Pointers can point nowhere (NULL), whereas reference always refer to an object.
  3. You can’t take the address of a reference like you can with pointers.
  4. 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:

prakash

Answer:

  1. 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;
  2. 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);
  3. 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);
  4. 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
  5. 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.
  6. 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 ..
  7. 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.
  8. References cannot be stuffed into an array, whereas pointers can be (Mentioned by user @litb)
  9. 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.

Brian R. Bondy

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