|Posted by Andrew Fedoniouk|
in reply to Kramer
Posted in reply to Kramer
final and const are different concepts.
'final' means: you cannot change anything final.
You cannot change final value
E.g. you cannot override final class. MyString: String is impossible
in Java. Strings are final there. As char and struct in D - you cannot
derive from them.
const in C++ is attribute of type - type modifier and not so restrictive.
const is just an instruction to compiler - warn me if you see an attempt
to change const value.
const is especially useful (and really make sense) for simple (atomic,
types: scalars, arrays and pointers. In classes you can always
do protection of values by wrapping them in getters/setters.
But for e.g. arrays there is no way to say 'I want this array to be
Correct me if I am wrong: Internally D runtime has a concept of
e.g. char s = "static string"; // s points to memory mapped data section.
s = 'x'; // this assignment should create copy of the string in the heap
it is impossible to write into data section of executable.
I think that it is relatively easy to extend type system of D to support e.g. arrays and const arrays. - these are just two similar but distinct types:
const array has opIndex but not opIndexAssign, has uint length() but
not uint length(uint).
I am not familiar with D compiler internals but from syntax/grammar point of view I think introduction of const will not change basics of D. But it will definitely increase the quality.
"Kramer" <Kramer_member@pathlink.com> wrote in message news:email@example.com...
> What is the difference between final and const in D? I'm only asking in
> relation to several disscussions on "constness" and comments Matthew, Kris
> others have made. Since there is no D documentation on final but there is
> const, this is how I'm currently interpreting it. Please correct me
> and/or fill
> in where I've left a hole or mis-represented, but I'd really like to
> this issue as it pertains to D.
> From Java: "The value of a final variable cannot change after it has been
> initialized. Such variables are similar to constants in other programming
> The above description is how I imagine it's implemented in D. Sounds a
> bit like
> a constant (be it a variable, method, class, etc...).
> Not sure what it means in Java (I don't think it's actually implemented,
> just a
> reserved word), but in D, the docs say "The const attribute declares
> that can be evaluated at compile time."
> The way I read that is, the compiler can do some optimizing to it since it
> the variable will never change. But, isn't that what final says as well?
> could be totally off on the optimizing bit, but hopefully you get my
> I certainly don't have a complete understanding on all the technical
> behind "constness", but I have heard it asked for on this forum and it
> feels like something that should exist in the language. "constness" kind
> feels like a contract saying that if I pass a variable that I don't want
> changed, the compiler should let me know if somewhere in my code, I've
> that by trying to modify it. The same goes for if I want to declare a
> parameter as constant; it's a contract saying "I won't change this, but if
> try, please scream at me!".
> So maybe there's already a way of doing "constness" in D and maybe that
> way is
> final, maybe it's const. Maybe it should be the "read-only" keyword I've
> people mention. But whatever it is, I believe it should be a keyword of
> sort and not a "best practice" by the programmer. If it's a keyword, then
> that much easier to express what you want to say, or rather how the
> should behave. And after all, isn't that what Contract Programming is
> P.S. If I've mis-represented anything or anyone, I apologize; and I'm not
> trying to beat a dead horse. I think I must not be getting some obvious
> that someone has given on this subject, and I'd just really like to fully
> understand this concept as it exists in D, as I think it's an important