Question

I have modified std.array.array so that it will always work at compile time, and it looks like this:

ForeachType!Range[] array(Range)(Range r)
if (isIterable!Range && !isNarrowString!Range)
{
  auto a = appender!(ForeachType!Range[])();
  foreach (e; r)
  {
    a.put(e);
  }
  return a.data;
}

And a usage:

struct Type
{
  int[] xs;
  this(int[] r) { this.xs = r.array; }
}

enum Type t1 = Type([]);
static if (t1.xs.length) { }  // Error: expression null.length is not constant

Base on my understanding, when r is an empty range, array() returns a null. In this regard, is there supposed to be a difference between null and an empty array?

Replacing return a.data; in array() with return a.data.length ? a.data : []; does fix the problem, so I suppose there is a difference?

The thing is that this only happens with constructors. The following doesn’t produce the error:

enum int[] t1 = iota(0,0).array;
static if (t1.length) { }

So I suppose there is no difference between null and []? I’m confused.

Answer 1

It’s true that[] compares equal to null. [] == null and [] is null are both true. The same goes for an unitialized array or one initialized with null.

However, an empty array is not always null. For example:

int[] a;
assert(a is null);
a = new int[2];
a = a[0..0];
assert(a !is null);

See The D Programming Language, p. 95.

Edit:
In light of the fact that compile-time .length seems to be malfunctioning, I suggest using std.array.empty for the check, which should be more reliable than a null comparison. On DMD 2.061, static if (!t1.xs.empty) { } works where static if (t1.xs.length) { } failed.