References in D

[Simen Kjaeraas]

On Sat, 15 Sep 2012 16:06:38 +0200, Maxim Fomin <maxim at maxim-fomin.ru>  
wrote:

> On Saturday, 15 September 2012 at 13:49:02 UTC, Simen Kjaeraas wrote:
>>> void foo(ref S s); // cannot pass null pointer S* or null - "always  
>>> there"
>>
>> void bar( ref int n ) {
>>     n = 3;
>> }
>>
>> void main( ) {
>>     int* p = null;
>>     bar( *p );
>> }
>>
>> Good luck.
>>
>> Of course, it's not that obvious in production code, but I've had this
>> happen to me in C++, and - obviously - it can happen in D, too.
>
> The problem happens because you deliberately passed hidden null pointer.

In this very specific example, yes. In my case it happened because I
called a function with a vector of pointers to some class, some of which
were null, and that function did not expect nulls in the vector, passing
them on as references to null.


> And in real code it could crash after dereferencing and before passing  
> to bar, if accessed (meaning that problem is in erroneous pointer usage,  
> not accepting ints through reference in bar). Certainly, it is possible  
> to break even in such cases, as well as in other parts of the language  
> which doesn't necessarily mean that such parts of the language are  
> broken. What I was talking about is:
>
> void bar( ref int n ) {
>      n = 3;
> }
>
> void main( ) {
>      int* p = null;
>      bar(p); // error
>      bar(null); //error
> }

But that's like saying bar(int n) doesn't accept null as a parameter - of
course it doesn't - int and int* are different types.

What I'm saying is references may be a bit safer than pointers, but the
'guarantee' that they're not null pointers in disguise is a lie.

-- 
Simen