Associative Array of Const Objects?

[bitwise via Digitalmars-d-learn]

On Sunday, 29 March 2015 at 19:04:30 UTC, anonymous wrote:
>
> Notice how you have that '*' there that allows you to 
> distinguish the data from the reference.
>
> You can have a mutable pointer to const data in D, too:
>
> struct Test {}
> const(Test)* test1 = null;
> test1 = new Test; /* fine */
> const(Test*) test2 = null;
> /* equivalent variant: const Test* test2 = null; */
> test2 = new Test; /* Error: cannot modify const expression 
> test2 */
>
> You cannot have a mutable class object reference to const data, 
> because syntactically that distinction isn't made in D. There 
> is std.typecons.Rebindable, though:
>
> import std.typecons: Rebindable;
> class Test {}
> Rebindable!(const Test) test = null;
> test = new Test; /* fine */
>
>> I would have suggested that I got things backward, but this 
>> doesn't work either:
>>
>> const Test test = new Test();
>> test = new Test(); // error: cannot modify const expression
>
> `const Test test` is the same as `const(Test) test`.


Interesting, but I still don't understand why D doesn't have 
something like this:

const Test test;    // or const(Test) test;
test = new Test()  // fine, underlaying data is const, the 
reference is not

Test const test = new Test();
test.a = 5;              // fine, test is read-only but 
underlaying data is not const
test = new Test();  // error: test is read-only

const(Test) const test = new Test();
test.a = 5;              // error, underlaying data is const
test = new Test();  // error: read-only