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);
    }