On 01/24/2013 05:56 PM, Jonathan M Davis wrote:
> On Thursday, January 24, 2013 17:13:36 Ali Çehreli wrote:
>> Normally, const vs. immutable parameters are different in the way that
>> they accept arguments:
>>
>> - immutable is "limiting" because it insists that the argument is
immutable.
>>
>> - const is "welcoming" because it accepts mutable, const, and immutable.
> I've never heard anyone describe it that way before.
We must find easier ways of explaining these semantics. I appreciate the
examples that you have provided but we can't expect newcomers to extract
meaning out of this.
We must be able to come up with a few function design guidelines. My
choice of words like "welcoming" and "limiting" are attempts at finding
sanity.
> const is more generic,
> whereas immutable is more specific. But from the compiler's point of
view,
> passing a non-const object to a const function means doing a
conversion (from
> mutable to const)
That is news to me. There shouldn't be any conversion from non-const to
const because that is a conversion that should have no runtime effect.
As Era Scarecrow put it, it means "this variable cannot be changed by
me". As I understand it, the compiler should merely enforce that
guarantee at compile time without performing any conversions.
>, whereas calling a function of the same type does not
> require a conversion. I believe that _that_ is the core of why ref takes
> precedence over const.
If const really requires a conversion then we have a language design
problem here.
> So, if you have
[...]
These examples are great but as I said, they don't help the programmer
at function signature stage.
immutable is easy: If I am going to need immutable data, then I make the
parameter immutable.
By-value is also understandable: I don't care about the original.
Additionally, when a 'ref' overload exists, by-value gives me the chance
to move because only the rvalue can be bound to by-value. Pretty
indirect way of thinking but we can deal with it...
const should be easy as well: "I am not going to modify". Just like in
C++... Unfortunately that is not the case.
Ali
