On Friday, 28 November 2014 at 10:55:27 UTC, bearophile wrote:
In D code it's a good idea to set as const/immutable (where
possible) all variables that don't need to change, for both
safety and compiler-enforced code documentation.
In my D functions sometimes I create dynamic arrays that later
don't have to change length nor to be reassigned, but I have to
mutate or assign their items. So their length and ptr can be
const/immutable, unlike the array contents. The D type system
doesn't allow this.
So is it a good idea to try to add to Phobos a headConst
function similar to this (only for built-in dynamic arrays)
that tries to enforce those constraints?
import std.traits, std.range;
struct HeadConst(T) {
T[] data;
alias data this;
@property size_t length() const pure nothrow @safe @nogc {
return data.length;
}
@disable void opAssign();
}
HeadConst!(ElementType!R) headConst(R)(R arr)
if (isDynamicArray!R) {
return typeof(return)(arr);
}
void main() {
import std.stdio;
auto arr = new int[2].headConst;
arr[1] = 10;
arr[1].writeln;
arr.length.writeln;
//arr.length = arr.length + 1; // error
auto b = new int[2];
//arr = b; // error
arr[] = b[];
b = arr;
arr[] = 1;
HeadConst!int c = arr;
HeadConst!int d;
arr = d; // fail
}
Bye,
bearophile
I wouldn't call it HeadConst, as that is a very general term.
A common situation for me is where I know the length at
compile-time, I want to make use of that knowledge (either in a
template somewhere or in hope of a compiler optimisation) but I
don't want the memory on the stack. That doesn't necessarily mean
I don't want to change the pointer though.
There's quite a lot of different possibilities that are useful, a
small set of utilities to help manage them would be great.
