Question

D discards top level array’s const during implicit functions instantiation and leaves it in case of explicit one.
Consider code:

// main.d
import std.stdio;
void foo( T )( T val )
{
  writeln( typeid( T ) );
}
void main()
{

  const int[] arr;
  writeln( typeid( arr ) ); // actual type
  foo( arr ); // implicit instantiation
  foo!( typeof( arr ) )( arr ); // explicit instantiation
}

…and output:

$ dmd main.d && ./main
const(const(int)[])
const(int)[]
const(const(int)[])

As you can see, top level const was lost in case of implicit instantiation. Is this bug, feature or my misunderstanding ?

Answer 1

What’s lost is the constness of the array pointer – not the constness of the array itself.

const int[] in D protects both the array pointer(you can’t point it to a different array) and the array data(you can’t change the elements). That’s why the first and third outputs have 2 consts in them. However, when you pass the array to the function, there is no need to keep the constness of the pointer – if you change val inside foo to a different array, it won’t effect the content of arr in the main function.