Template bug?

[Jarrett Billingsley]

"Artyom Shalkhakov" <artyom.sh at gmail.ru> wrote in message 
news:epmho3$282d$1 at digitaldaemon.com...
> Hello everyone.
>
> I have a problem regarding D templates.
>
> This code doesn't work. I would like to know how do I get it up'n'running?
>
> struct temp_t( type ) {
>    void setOwner( type *newOwner ) {
>        owner = newOwner;
>    }
>
>    type *getOwner() {
>        return owner;
>    }
>
>    protected {
>       temp_t *  head;
>       temp_t *  next;
>       temp_t *  prev;
>       type *      owner;
>    }
> }
>
> class testClass_t {
>    this( int d ) {
>        data = d;
>        tst.setOwner( &this );
>    }
>
>    int                               data;
>    temp_t!( testClass_t )    tst;
> }
>
> void foo() {
>    testClass_t bar = new testClass_t( 0x1234 );
>
>    // what is the difference between '==' and 'is'?
>    assert( ( *bar ).getOwner == bar );  // doesn't work
> }

There are a few things going wrong.

One, when you write "tst.setOwner( &this );", this sets the owner to the 
address of a local variable.  This is a Bad Thing.  Remember that classes 
are reference types, so they are implicitly pointers.  So instead of making 
your struct use pointers, just take all the *s out.

struct temp_t( type ) {
    void setOwner( type newOwner ) {
        owner = newOwner;
    }

    type getOwner() {
        return owner;
    }

    protected {
       temp_t *  head;
       temp_t *  next;
       temp_t *  prev;
       type      owner;
    }
}

Then in your class's constructor, use

tst.setOwner( this );

Lastly, this line:

assert( ( *bar ).getOwner == bar );

Doesn't even compile because (1) you cannot dereference bar because it's not 
a pointer, it's a reference, and (2) there is no .getOwner property for the 
testClass_t class.  Instead, you should use:

assert( bar.tst.getOwner is bar);

And that brings me to my last point, the difference between 'is' and '=='. 
'is' is used to see if two references (or pointers) point to the same 
location.  '==' is used to see if two things are equal.  If you have two 
class references, a and b, and you write

a == b

This is the same as writing

a.opEquals(b)

If a is null, this will get you a segfault.  However, if you just want to 
see if a and b are pointing to the same instance, use

a is b

Which is what you want to do in your example.