[geos-devel] Why empty LineString should return a null pointer from getCoordiante()

[Mateusz Łoskot]

Hi,

Please, forgive me that I'm annoying with all those recent questions but
I really would like to understand GEOS internals.


Here is getCoordinate:

const Coordinate* LineString::getCoordinate() const
{
   // should use auto_ptr here or return NULL or throw an exception !
   // 	--strk;
   if (isEmpty())
      return(new Coordinate());

   return &(points->getAt(0));
}

as you see, there is Sandro's comment about what could be best.
I think it's time to answer this question :-)

For comparison and reference, here is JTS' version of this function:

public Coordinate getCoordinate()
{
   if (isEmpty()) return null;
   return points.getCoordinate(0);
}

Here are my comments to current solution:
1. Returning newly allocated Coordinate is confusing and will cause
problems that user won't free the memory

2. This brokes the logic because how empty linestring can return any
Coordinate? This does not make any sense.


Here are my comments to possible solution:

1. auto_ptr will not solve the problem because it won't fix broken logic
will

2. My suggestion is to return *null pointer*

As you can see above, JTS version returns null reference.

What is Java null reference (a pointer to nothing)?
It is a reference that does not refer to any instance of any type.

What we can do with Java null reference?
null reference can be safely compared with other references (null or not
null), null reference can be safely passed to the function, returned
from function, null reference can be safely assigned to the existing
reference etc.
What we can not do with null reference is dereferencing, e.g.:

Type x = null;
x.Fire(); // BUM!

Now, how to transit this Javism to C++. It's very simple - just use null
pointer.

What is null pointer in C++?
Null pointer is a *valid* pointer value that doesn’t point to anything.
Just as null reference in Java.

Similarly, with null pointers in C++ we can do (almost) the same as what
we can do with Java null reference.

The only thin that can not be done with C++ null pointer is
dereferencing. Dereferencing null pointer causes undefined behavior
This rule applies to every pointer that does not point to point to anything.

What I'd like to repeat is that "null pointer is a *valid* pointer",
but not invalid. Invalid pointer is another category and *any* use of
invalid pointer causes Undefined Behaviour, even comparing, etc.

So, I vote for returning null pointer.

3. Throwing an exception is a bad idea

Exceptions should be used for unexpected situations but when user is
asking object for coordinate he is aware that the object may or may not
include any coordinates. So, there is no unexpected behaviour here.


BTW, I'm really sorry if my explanation seems to be like in books of
seriece "XXX for dummies". This is completely not my intetion!
Simply, I usualy try to explain my point as clear as possible.


Cheers
-- 
Mateusz Łoskot
http://mateusz.loskot.net