Associative Array of Const Objects?
bitwise via Digitalmars-d-learn
digitalmars-d-learn at puremagic.com
Sun Mar 29 12:13:30 PDT 2015
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
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