C++ - Pointers vs References

References can’t be NULL, they are safer to use.
A pointer can be re-assigned while reference cannot, and must be assigned at initialization only.
A pointer is a variable that holds a memory address. A reference has the same memory address as the item it references.
For function parameter: use mutable references for non-optional mutable parameters and remove the == nullptr checks; otherwise use pointer.

Pointers

Pointer variables store memory locations. When dealing with pointers, you use the * symbol in two distinct but related ways:

As part of a type specifier (such as in int* p), meaning p is a pointer to an int.
As the dereference operator (such as in *p), meaning use the value a pointer points to.

For example:

int x = 42;
int* p = &x;
std::cout << "p points to memory location " << p << " which contains " << *p;

References

Reference variables are defined using the & symbol, but don't confuse that usage with the address-of operator. You use the & symbol in two completely unrelated ways:

As part of a type specifier (such as int& r), meaning r is another name for some int object.
As the address-of operator (such as &r), meaning get the address of a variable. Here's an example of both kinds of &:

int x = 42;
int& r = x;        // r is another name for x
assert(r == x);    // they have the same value
assert(&r == &x);  // they have the same address!

Notice that you can treat r just like x, including taking its address with the & operator. Once it's defined, its behavior and notation are indistinguishable from the original variable.

Unlike pointers, references can never be moved to refer to a different object. In the following code, pointer p starts out pointing at x, then switches to point to y:

int x = 42;
int* p = &x;  // p points at x
int y = 1138;
p = &y;       // now p points at y

In contrast to pointers, the following code uses references. Reference r always refers to x. The assignment r = y assigns the value contained in y to the variable called r, another name for x:

int x = 42;    // x == 42
int& r = x;    // x == 42    &r == &x
int y = 1138;  // x == 42    &r == &x  y == 1138
r = y;         // x == 1138  &r == &x  y == 1138