Lazily evaluated property pointing to read only object

[mikey via Digitalmars-d-learn]

I'm trying to figure out how to best write a class with a 
property that is only evaluated when it's called for the first 
time. And that returns an object which shouldn't be modifiable a 
part of the owning class.

I've had a go at doing something like this but am not very sure 
if this is how to go about it.


     class Obj {
         string _s;

         this() {
             this("");
         }

         this(string s) {
             _s = s;
         }

         @property string desc() const { return _s; }
         @property void desc(string s) { _s = s; }

         override string toString() const {
             return _s;
         }
     }

     class ConstProp {
         Obj _o;
         string _s;

         this(string s) {
             _s = s;
         }

         const(Obj) lazily() {
             if (_o is null) {
                 _o = new Obj("working " ~ _s);
             }
             return cast(const(Obj)) _o;
         }
     }

     void main() {
         auto c = new ConstProp("test");
         writeln(c.lazily);
         writeln(c.lazily.desc);
     }