On Friday, 5 October 2012 at 04:50:18 UTC, Jonathan M Davis wrote:
On Friday, October 05, 2012 05:42:03 Alex Burton wrote:
I realise what is currently the case I am making an argument as
to why I this should be changed (at least for class references
in
D).
This was talking about C++ references, not D, giving an example
of how they
can be null even though most people think that they can't be.
int& isn't even
legal syntax in D.
I was talking about both. Regardless of whether the int&
reference or the int * reference was used to assign to memory
address 0 in Walters example the result is still a crash and will
be in D with equivalent code using explicit pointers or instances
of classes (which are always pointers in D).
The crash is a result of two mistakes:
One is the language designer allowing null to be an acceptable
value for a pointer to an int.
As should be blatently obvious that null is not a pointer to an
int, but for historical reasons inherited from C (when people
were just happy to get out of assembly language) it has been
allowed.
The second mistake is that someone chose to use the language
feature which clearly makes no sense.
This is bad programming for two reasons:
1) It is logically incorrect to state that 0 is a pointer to
something.
2) It is a case of using magic numbers in code - an antipattern.
It is trying to create some universal consensus that the magic
number 0 means something special. What I am supposed to do with a
null pointer is not so universal.
Do I construct it ? Do I throw an exception ? Why on earth has
someone sent me this 0 when my type system specifies I want a
pointer to an int ?
Regardless, references in D will _never_ be non-nullable. It
would break too
much code to change it now regardless of whether nullable or
non-nullable is
better.
I don't think this argument is any more powerful than any of the
other 'lets remain compatible with C to avoid breakage' ones.
If it were changed there could be compiler errors for
uninitialised references, and tests for null. These sorts of
compile time errors are much more preferable than undefined
behaviour in released code that crashes IMHO.
Alex
