What are the differences between a pointer variable and a reference variable?

  • 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.

  • 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.

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