C++: Difference between setting a pointer to nullptr and initializing it as a new variable type

Question

I am learning C++ and I know the 'new' key word is used to allocate an address in memory to a pointer. And I think when using 'nullptr' initializes a pointer which points to nothing. Is that correct? Example for reference:

//using nullptr
int *x = nullptr; //this is just a pointer that points to nothing and
                  //will need to be initialized with an address before it  
                  //it can be used. Correct?

//using new
int *x = new int; //this is basically giving x an address in memory. Will the 
                  //address have some residual value stored in it or will
                  //it contain zero?

When would you use one over the other? Is new only used for dynamic memory allocation or are there other applications for it? Why would you initialize a pointer to nullptr if you could just declare it and then initialize it later?

Thanks for the help!

Answer 1

Your understanding of new int and int *x = new int; is not correct (or at least, the way you worded it is not correct).

new int allocates some memory. It doesn't "allocate memory to a pointer", it just allocates memory. It supplies a pointer to that memory, which didn't exist previously.

int *x; also allocates memory for a variable called x. x is a variable whose value is the address of another object. (as opposed to other types of variable, whose value may be a number or a string for example).

int *x = nullptr; means x holds a special value called "null pointer" that does not indicate any other object.

int *x = new int means that x holds the address of the piece of memory that was allocated by new int. So there are two allocations involved in this line: x, and the unnamed memory allocated by new.

In the latter case you could output the address of each of these allocations separately, e.g.:

cout << &x << ',' << x << '\n';