how to declare an immutable class?

[Mike Parker via Digitalmars-d-learn]

On Friday, 12 August 2016 at 04:49:46 UTC, Charles Hixson wrote:

> It works, it's just not the syntax that I'd prefer.  And it 
> leaves me wondering exactly what
> immutable class Msg {...}
> was declaring.

This should demonstrate:

```
immutable class iMsg {
     int getX() { return 10; }
}

class Msg {
     int getX() { return 20; }
}

void main() {
     auto msg1 = new immutable iMsg;
     assert(msg1.getX() == 10);

     auto msg2 = new immutable Msg;
     assert(msg2.getX() == 20);
}
```

The line with msg2.getX() will fail to compile, because it's 
calling a non-immutable method on an immutable object. Change the 
declaration of Msg to the following and it compiles:

```
class Msg {
     int getX() immutable { return 20; }
}
```

immutable class Foo { ... } is the same as declaring every member 
of Foo as immutable, just as final class Foo { ... } makes every 
method final.