On Saturday, 17 October 2015 at 02:03:01 UTC, Shriramana Sharma
wrote:
Ali Çehreli wrote:
http://ddili.org/ders/d.en/const_and_immutable.html#ix_const_and_immutable.parameter,
%20const%20vs.%20immutable
Hi Ali – I take this chance to personally thank you sincerely
for your book which provides much-needed hand-holding in my
baby D-steps. I did read that chapter already and IMO you have
given clear instructions as to when to use const and when
immutable.
My question was however to the root of the issue, as to *why*
the compiler cannot consider mutable as immutable just like in
C/C++ any non-const can be taken as const.
It would seem that the answer is one related to optimization.
Obviously, labeling an argument as immutable can be done only
if we are sure that we will have to process only immutable
input, thereby paving the opportunity for the compiler to
optimize some memory access or allocation or such – I'm not
much clear beyond that but that's enough for me now...
It appears that the linked chapter doesn't explain *why* you
would want to receive immutable arguments.
In my experience, the most common motivation is a desire to
escape a reference to the argument. We want to read the data
later, but when we do, we want it to be unchanged from when we
received it:
---
struct S
{
immutable(int)[] numbers;
this(immutable(int)[] numbers)
{
this.numbers = numbers;
}
void printNumbers()
{
import std.stdio;
writeln(numbers);
}
}
immutable numbers = [1, 2, 3];
auto s = S(numbers);
/* ... */
s.printNumers(); // [1, 2, 3]
---
In the above code, *no matter what code is run between
construction and `printNumbers`*, it will always print the same
numbers it received at construction, as the numbers are
immutable. Because of this guarantee, S.numbers can simply alias
the constructor argument as seen in the constructor body, instead
of say, copying the numbers into a new heap-allocated copy of the
array. If we used const instead of immutable, there would be no
such guarantee, as const can refer to mutable data: the numbers
could have been overwritten between construction and the call to
`printNumbers`.
Another common use of immutable is to share data between multiple
threads. As immutable data never changes after initialization, it
can be passed between threads and read freely without worrying
about data races.
const in D simply exists to bridge mutable and immutable data. It
is different from C++'s const, despite sharing the same name.
