Memory Management – Where Are Null Values Stored?

memorynull

I want to learn about null values or null references.

For example I have a class called Apple and I created an instance of it.

Apple myApple = new Apple("yummy"); // The data is stored in memory

Then I ate that apple and now it needs to be null, so I set it as null.

myApple = null;

After this call, I forgot that I ate it and now want to check.

bool isEaten = (myApple == null);

With this call, where is myApple referencing? Is null a special pointer value? If so, if I have 1000 null objects, do they occupy 1000 object memory space or 1000 int memory space if we think a pointer type as int?

Best Answer

In your example myApple has the special value null (typically all zero bits), and so is referencing nothing. The object that it originally referred to is now lost on the heap. There is no way to retrieve its location. This is known as a memory leak on systems without garbage collection.

If you originally set 1000 references to null, then you have space for just 1000 references, typically 1000 * 4 bytes (on a 32-bit system, twice that on 64). If those 1000 references originally pointed to real objects, then you allocated 1000 times the size of each object, plus space for the 1000 references.

In some languages (like C and C++), pointers always point to something, even when "uninitialized". The issue is whether the address they hold is legal for your program to access. The special address zero (aka null) is deliberately not mapped into your address space, so a segmentation fault is generated by the memory management unit (MMU) when it is accessed and your program crashes. But since address zero is deliberately not mapped in, it becomes an ideal value to use to indicate that a pointer is not pointing to anything, hence its role as null. To complete the story, as you allocate memory with new or malloc(), the operating system configures the MMU to map pages of RAM into your address space and they become usable. There are still typically vast ranges of address space that are not mapped in, and so lead to segmentation faults, too.

Related Solutions

Database – How will you handle NULL values in your application

This is one of the problems with nulls. Whenever the database developer puts them into the database the application developer has to take them out again. Very often the best policy is to avoid or minimise the use of nulls in your base tables to start with. If nulls are used then it's a good idea to record somewhere exactly what the null is supposed to mean and why it is used. You can then put logic in a view or stored procedure to eliminate or replace the nulls as appropriate.

Scala Singleton Design – Null Values vs Empty Singleton for Optional Fields

I also attended the same Scala course and I agree with e-i-s that you should use the Option[T] type for implementing an optional field. Here are some more ideas.

The pattern involving

a trait with method isEmpty
several classes or objects extending the trait
a unique object whose method isEmpty always returns true

is also used to implement sequence-like immutable structures such as lists, streams, and so on.

Compare:

An Option[T] type containing the values Some(t), for each value t in T, and the additional value None.
A List[T] type containing objects built from two subclasses (and their respective constructors) Cons[T](h: T, t: List[T]) and Nil: these represent immutable lists whose elements have type T. Note that here the method Nil.isEmpty returns true.

None indicates the absence of a value, while Nil indicates an empty list.

So an optional value None is not the same as an empty list Nil.

NOTE

If you need to test if an optional value is defined or not you can use the isEmpty method of the Option type, or pattern matching:

class MyClass(f: Option[String])
{
  val field = f
}

and then, to test if the field of an instance obj of MyClass is defined:

if (obj.field.isEmpty)
{
}
else
{
}

obj.field match
{
  case Some(s) => /* do something with s */
  case None    => /* ... */
}

Best Answer

Related Solutions

Database – How will you handle NULL values in your application

Scala Singleton Design – Null Values vs Empty Singleton for Optional Fields

Related Topic