On Friday, 16 October 2015 at 10:35:23 UTC, Shriramana Sharma
wrote:
Hello. I still haven't wrapped my mind around the
const/immutable thing yet and am still stuck in C/C++ mode. :-(
A function that takes mutable arguments cannot be called with
immutable input at the call site since it does not promise to
*not* mutate the input. That's of course clear.
Why can't a function that takes an immutable argument be called
with a mutable input at the call site?
The contract of immutable is such that any reference to immutable
data is a guarantee that no reference to the same data will
modify it anywhere in the program. Passing mutable data to a
function where an immutable parameter is declared would break
that contract, since the data could be modified through the
original reference. The compiler can take advantage of such a
strict contract to make optimizations it would otherwise be
unable to.
const only guarantees const data will not be modifed through a
single reference, but says nothing about other references to the
same data. A const parameter can accept const, immutable, and
mutable arguments.
