Any type.
Regular const object references cannot be reassigned.
class Widget { int x; int y() @safe const { return x; } } const a = new Widget; // Fine a.y(); // error! can't modify const a // a.x = 5; // error! can't modify const a // a = new Widget;
However, Rebindable!(Widget) does allow reassignment, while otherwise behaving exactly like a const Widget.
class Widget { int x; int y() const @safe { return x; } } auto a = Rebindable!(const Widget)(new Widget); // Fine a.y(); // error! can't modify const a // a.x = 5; // Fine a = new Widget;
Rebindable!(T) is a simple, efficient wrapper that behaves just like an object of type T, except that you can reassign it to refer to another object. For completeness, Rebindable!(T) aliases itself away to T if T is a non-const object type.
You may want to use Rebindable when you want to have mutable storage referring to const objects, for example an array of references that must be sorted in place. Rebindable does not break the soundness of D's type system and does not incur any of the risks usually associated with cast.