Extending "scope"

[Jarrett Billingsley]

"Brian White" <bcwhite at pobox.com> wrote in message 
news:fta21v$1ane$1 at digitalmars.com...
>I wrote something about "GC vs delete", but after thinking about it some 
>more, how about this...
>
> There seems to be a fundamental conflict between RAII and the garbage 
> collector.  You can assign a new object to a "scope" variable to cause it 
> to get destructed without waiting for the GC, but what about objects 
> within that?  For example:
>
> class tBar {
>     this()
>     {
>         writeln("a bar has been constructed");
>     }
>     ~this()
>     {
>         writeln("a bar has been destructed");
>     }
> }
>
> class tFoo {
>     scope tBar bar;
>
>     this()
>     {
>         bar = new tBar();
>         writeln("a foo has been constructed");
>     }
>     ~this()
>     {
>         delete bar;
>         writeln("a foo has been destructed");
>     }
> }
>
> unittest {
>     auto foo = new tFoo();
> }
>
>
> If you run this, you'll get:
>
> a bar has been constructed
> a foo has been constructed
>
> At some point, you _may_ also get:
>
> a bar has been destructed
> a foo has been destructed
>
>
> When it comes to RAII, that's not enough.  Walter has allowed us to 
> specify a class as being "scope" which requires you to create them with 
> "scope auto foo = new tFoo()".  However, if I also add that property to 
> "tBar", it won't compile.
>
> scope class tBar {
>     this()
>     {
>         writeln("a bar has been constructed");
>     }
>     ~this()
>     {
>         writeln("a bar has been destructed");
>     }
> }
>
> scope class tFoo {
>     tBar bar;
>
>     this()
>     {
>         bar = new tBar();
>         writeln("a foo has been constructed");
>     }
>     ~this()
>     {
>         delete bar;
>         writeln("a foo has been destructed");
>     }
> }
>
> unittest {
>     scope auto foo = new tFoo();
> }
>
> $ dmd ...
> test.d(32): variable socket_test.tFoo.bar globals, statics, fields, 
> manifest constants, ref and out parameters cannot be auto
> test.d(32): variable socket_test.tFoo.bar reference to scope class must be 
> scope
>
> Nor can I declare a member variable "scope tBar bar" within class tFoo. In 
> other words, it's not possible to embed one enforced RAII object within 
> another.
>
>
> What do others think about extending "scope" to allow it to be applied to 
> member data?  They would by default be set to "null" but have the 
> requirement that they must be set exactly once in the constructor and 
> nowhere else.  They would also be deleted automatically during the 
> destructor.
>
> Or...  Is there a different way to accomplish this?
>
> -- Brian

I think you're the 318th person to suggest this.  I might be talking out of 
you-know-where but I think I remember Walter mentioning that this is in the 
cards for D2.