Immutable objects and constructor ?

[chmike via Digitalmars-d-learn]

I'm implementing the flyweight pattern. It means that I have a 
set of object instances representing all the possible values. 
This allows me to manipulate "values" by simply manipulating 
references to the instance. Testing "value" equality boils down 
to simply compare reference value. I hope I can use these 
immutable instances as case argument of a switch, but I'm not 
there yet.

I have declared an immutable class.

----
immutable interface SomeInfo { ... }

immutable class Info : SomeInfo {
   @disable this();
   this(int codeValue, string codeName)
   {
       codeValue_ = (codeValue * 10) - 113; // some random 
example computation
       codeName_ = "Info."~codeName_;
   }
   private:
      int codeValue_;
      string codeName_;
}
----

But when I try to instantiate the class I get an dramatic 
compilation error:

"none of the overloads of '__ctor' are callable using a mutable 
object, candidates are: "

Adding immutable to the constructor doesn't help. How can I 
initialize the immutable instances of a class ?


I have seen it is possible to cast away the immutability. Does it 
also work for immutable classes ? Could I use an object factory ?

How should I do if I would like to use the lazy pattern for 
initializing some member variables of the instance ? Something 
like :

immutable class Info : SomeInfo
{
   ...
   string toString()
   {
      if (!toString_)
          synchronize { // make it thread safe
             if (!toString_)
                toString_ = format("bla bla %s (%d)", codeName_, 
codeValue_);
          }
      return toString_;
   }
   ...
   private:
      string toString_;
}