It is a matter of convention and you should clearly have one in your head and document it (at least in comments).

Sometimes a pointer really should always point to a valid address (see this intSwap example, both arguments should be valid pointers). At other times, it should either be such a valid address, or be NULL. Conceptually the pointer type is then by convention a sum type (between genuine pointer addresses and the special NULL value).

Notice that the C language does not have a type (or a notation) which enforces that some given pointer is always valid and non-null. BTW, with GCC specifically, you can annotate a function with __attribute__ using nonnull to express that a given argument is never null.

A typical example is FILE* pointers in <stdio.h>. The fopen function is documented to be able to return NULL (on failure), or some valid pointer. But the fprintf function is expecting a valid pointer (and passing NULL to it as the first argument is some undefined behavior, often a segmentation fault; and UB is really bad).

Some non-portable programs even use several "special" pointer values (which should not be dereferenced), e.g. (on Linux/x86-64) #define SPECIAL_SLOT (void*)((intptr_t)-1) (which we know that on Linux it is never a valid address). Then we could have the convention that a pointer is a valid pointer to a valid memory zone, or NULL or SPECIAL_SLOT (hence, if seen as an abstract data type, it is a sum type of two distinct invalid pointers NULL and SPECIAL_SLOT and the set of valid addresses). Another example is MAP_FAILURE as result of mmap(2) on Linux.

BTW, when using pointers in C to heap allocated data (indirectly obtained with malloc), you also need conventions about who is in charge of releasing the data (by using free, often thru a supplied function to free a data and all its internal stuff).

Good C programming requires many explicit conventions regarding pointers, and it is essential to understand them precisely and document them well. Look for example[s] into GTK. Read also about restrict.

This function

const string& foo(); 

does not return a pointer. It returns a constant reference to an object of type std::string. So its return value may not be assigned to a pointer.

According to the C++ Standard

4.10 Pointer conversions [conv.ptr]

1 A null pointer constant is an integer literal (2.14.2) with value zero or a prvalue of type std::nullptr_t. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of object pointer or function pointer type. Such a conversion is called a null pointer conversion. Two null pointer values of the same type shall compare equal. The conversion of a null pointer constant to a pointer to cv-qualified type is a single conversion, and not the sequence of a pointer conversion followed by a qualification conversion (4.4). A null pointer constant of integral type can be converted to a prvalue of type std::nullptr_t. [ Note: The resulting prvalue is not a null pointer value. —end note ]

So when you use null pointer constant defined with macro NULL it is assigned to the return pointer of the function that will contain null pointer value of type node *

Q: "Isn't obj also pointing to null, since both object were pointing to the same object?"

No, both obj and d_heater contain addresses in memory. Changing one does not change the other. This is easy to see if you think about non-pointer variables:

int foo = 13;
int bar = foo;
foo = 42;

Obviously we know that bar still holds 13, and in the same way obj still holds the original address.

If you want obj to keep the same value as d_heater you can make it a reference, then obj and d_heater are the same variable they don't simply share the same value. We can see this again looking at non-pointer variables, but this time let's make bar a reference:

int foo = 13;
int& bar = foo;
foo = 42;

Now both foo and bar will equal 42. If you want to accomplish the same thing with obj make it a pointer reference:

WaterHeater*& obj = d_heater;

You can see a more detailed example here: http://ideone.com/I8CTba

Q. "Is nullptr in this case would be the same as delete?"

No, again d_heater is only an address. If you assign a new value to d_heater it is simply referencing a different point in memory. If by reassigning d_heater you lose the address of dynamically allocated memory that's very bad, it's known as a memory leak. To prevent leaking you must always release dynamically allocated memory before losing the last address to your dynamically allocated memory (call delete for memory allocated with new.)

That said, use of dynamic memory allocation is best left to the standard libraries, unless you really know what you're doing. So I'd strongly recommend you look at using auto-pointers. In C++11 those are unique_ptr and shared_ptr.