Bug 2027 – No way to declare only the reference 'const' or 'invariant' for reference types.

While using D I often want a reference to a class object to be invariant, while the object itself should be mutable. Or, in my current case, I want a delegate variable to be invariant (i.e. the reference may not change), but I do not want that only a invariant function may be assigned to it on initialization... This is obviously an undesirable limitation of the D 2.0 type system. Probably there should be some storage class for these cases. The following piece of code demonstates the problem: class A { private: invariant Object o_; this(Object o) { o_ = o; // error, cannot implicitly convert to invariant } } Because o_ is of type invariant(Object), it cannot be assigned a value of type Object, which is perfecly correct. But I do not intend o_ to be of type invariant(Object), I only want o_ to reference the same instance of Object, throughout the lifetime of the instance of class A, and thus forbid re-assignment...

That's not a bug, that's a request for head-const. :-)

Const and invariant are designed to be fully transitive. There is no way to declare a const reference to mutable data; this is by design. (It has been debated very extensively in the newsgroups.)