On 1/20/22 15:01, forkit wrote:
> On Thursday, 20 January 2022 at 22:31:17 UTC, Steven Schveighoffer wrote:
>>
>> Because it would allow altering const data.
>>
>
> I'm not sure I understand. At what point in this function is valuesArray
> modified, and thus preventing it being passed in with const?
>
> // ---
>
> int[][int][] CreateDataSet
> ref const int[] idArray, ref int[][] valuesArray, const int numRecords)
> {
> int[][int][] records;
Elements of records are mutable.
> records.reserve(numRecords);
>
> foreach(i, const id; idArray)
> records ~= [ idArray[i] : valuesArray[i] ];
If that were allowed, you could mutate elements of record and would
break the promise to your caller.
Aside: There is no reason to pass arrays and associative arrays as 'ref
const' in D as they are already reference types. Unlike C++, there is no
copying of the elements. When you pass by value, just a couple of
fundamental types are copied.
Furthermore and in theory, there may be a performance penalty when an
array is passed by reference because elements would be accessed by
dereferencing twice: Once for the parameter reference and once for the
.ptr property of the array. (This is in theory.)
void foo(ref const int[]) {} // Unnecessary
void foo(const int[]) {} // Idiomatic
void foo(in int[]) {} // Intentful :)
Passing arrays by reference makes sense when the function will mutate
the argument.
Ali
