Auto objects and scope

[Bill Baxter]

genie wrote:
> Forgive my ignorance, guys, but there is something in GC for D I
> can't spin my head about - look at this bit of code:
> 
> Test tst=new Test;
> 
> ...
> if(b)
> {
>    auto Test t1=new Test;
>    ...
>    tst=t1;
> }
> ...
> tst.func();
> 
> auto modifier forces the object to be deleted even though it was
> assigned to something else before leaving the scope - is this
> behaviour correct?
> 
> Gene

Yep.  auto (now 'scope') is not a reference counter.  It just says 
delete this one variable when this one variable goes out of scope.

I think it's more or less equivalent to "scope(exit) delete t1;".

In fact it will merrily delete whatever t1 happens to point to at the 
end of the scope:
---------------------------
import std.stdio : writefln;

class C {
     this(char[]n) { name=n; }
     ~this() { writefln("Bye bye: ", name); }

     char [] name;
}

void main()
{
     auto a = new C("a");
     {
         auto scope b = new C("b");

         b=a;
         writefln("Leaving scope...");
     }
     writefln("Left scope.");

     writefln("Ending main");
}
/*
Output:
Leaving scope...
Bye bye: a
Left scope.
Ending main
Bye bye: b
*/

a gets deleted at the end of the scope instead of b, and b only gets 
cleaned up later by the final garbage collection sweep.

So it seems it really is just syntactic sugar for the scope(exit) statement.

I think some sort of reference-counting construct is needed for the type 
of cases your example illustrates.  For things like File objects, you 
want to be able to return them, but you want them to be cleaned up 
immediately when no longer in use.  'scope' is too limited I think.

I'm not sure how to do it though.  It's not clear how to do in a clean 
way as a library (like a shared_ptr!()) without an overloadable opAssign.


--bb