automate tuple creation

[Ali Çehreli]

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