Create class on stack

[Moritz Maxeiner via Digitalmars-d-learn]

On Monday, 7 August 2017 at 13:40:18 UTC, Moritz Maxeiner wrote:
>
> Thanks, I wasn't aware of this. I tried fooling around scope 
> classes and DIP1000 for a bit and was surprised that this is 
> allowed:
>
> ---
> import core.stdc.stdio : printf;
> import std.algorithm : move;
>
> class A
> {
> 	int i;
>
> 	this() @safe
> 	{
> 		i = 0;
> 	}
> }
>
> void inc(scope A a) @safe
> {
> 	a.i += 1;
> }
>
> void print(scope A a) @trusted
> {
> 	printf("A@%x: %d\n", cast(void*) a, a.i);
> }
>
> auto makeA() @safe
> {
> 	scope a = new A();
> 	a.print();
> 	return move(a);
> }
>
> void main() @safe
> {
> 	auto a = makeA();
> 	foreach (i; 0..10) {
> 		a.print();
> 		a.inc();
> 	}
> }
> ---
>
> You can still create a (scope) class on the stack, escape a 
> reference to it using `move` and use it afterwards, all within 
> the rules of @safe, so I'm not convinced that the reason for 
> deprecating scoped classes is gone yet.
> Compare this to `scoped`, which behaves as expected (since it 
> wraps the reference type object in a value type):
>
> ---
> import std.typecons : scoped;
>
> auto makeA() @trusted
> {
> 	auto a = scoped!A();
> 	a.print();
> 	return move(a);
> }
>
> void main() @trusted
> {
> 	auto a = makeA();
> 	foreach (i; 0..10) {
> 		a.print();
> 		a.inc();
> 	}
> }
> ---

Forgot to add the runtime output after compiling with `dmd a.d 
-dip1000`:

For `scope A`:
A at 198d1568: 0
A at 198d1568: 0
A at 198d1568: 1
A at 198d1568: 2
A at 198d1568: 3
A at 198d1568: 4
A at 198d1568: 5
A at 198d1568: 6
A at 198d1568: 7
A at 198d1568: 8
A at 198d1568: 9

For `scoped!A`:
A at 8de538b8: 0
A at 8de53940: 0
A at 8de53940: 1
A at 8de53940: 2
A at 8de53940: 3
A at 8de53940: 4
A at 8de53940: 5
A at 8de53940: 6
A at 8de53940: 7
A at 8de53940: 8
A at 8de53940: 9