Re: 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@198d1568: 0
A@198d1568: 0
A@198d1568: 1
A@198d1568: 2
A@198d1568: 3
A@198d1568: 4
A@198d1568: 5
A@198d1568: 6
A@198d1568: 7
A@198d1568: 8
A@198d1568: 9

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