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

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.

Last edited Dec 10, 2015 at 12:39 PM by vineetchoudhary, version 1