dereferencing null

[Chad J]

On 03/07/2012 04:41 AM, Timon Gehr wrote:
> On 03/07/2012 02:40 AM, Chad J wrote:
>> But to initialize non-null fields, I suspect we would need to be able to
>> do stuff like this:
>>
>> class Foo
>> {
>> int dummy;
>> }
>>
>> class Bar
>> {
>> Foo foo = new Foo();
>>
>> this() { foo.dummy = 5; }
>> }
>>
>> Which would be lowered by the compiler into this:
>>
>> class Bar
>> {
>> // Assume we've already checked for bogus assignments.
>> // It is now safe to make this nullable.
>> Nullable!(Foo) foo;
>>
>> this()
>> {
>> // Member initialization is done first.
>> foo = new Foo();
>>
>> // Then programmer-supplied ctor code runs after.
>> foo.dummy = 5;
>> }
>> }
>>
>> I remember C# being able to do this. I never understood why D doesn't
>> allow this. Without it, I have to repeat myself a lot, and that is just
>> wrong ;).]
>
> It is not sufficient.
>
> class Bar{
> Foo foo = new Foo(this);
> void method(){...}
> }
> class Foo{
> this(Bar bar){bar.foo.method();}
> }

Lowered it a bit to try to compile, because it seems Foo doesn't have a 
method() :

import std.stdio;

class Bar{
   Foo foo;
   this()
   {
     foo = new Foo(this);
   }
   void method(){ writefln("poo"); }
}
class Foo{
   this(Bar bar){bar.foo.method();}
}

void main()
{
}

And, it doesn't:
main.d(12): Error: no property 'method' for type 'main.Foo'

Though, more to the point:
I would probably forbid "Foo foo = new Foo(this);".  The design that 
leads to this is creating circular dependencies, which is usually bad to 
begin with.  Would we lose much of value?