[Issue 8409] New: Proposal: implement arr.dup in library

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8409

           Summary: Proposal: implement arr.dup in library
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: druntime
        AssignedTo: nobody at puremagic.com
        ReportedBy: k.hara.pg at gmail.com


--- Comment #0 from Kenji Hara <k.hara.pg at gmail.com> 2012-07-22 05:38:09 PDT ---
If issue 8408 is fixed, we can use following dup function definition.

template hasMutableIndirection(T)
{
    enum hasMutableIndirection = !is(typeof({ Unqual!T t = void; immutable T u
= t; }));
}
static assert(!hasMutableIndirection!(int));
static assert(!hasMutableIndirection!(int[3]));
static assert( hasMutableIndirection!(Object));

@property auto dup(E)(inout(E)[] arr) pure @trusted
{
    static if (hasMutableIndirection!E)
    {
        auto copy = new E[](arr.length);
        copy[] = cast(E[])arr[];        // assume constant
        return cast(inout(E)[])copy;    // assume constant
    }
    else
    {
        auto copy = new E[](arr.length);
        copy[] = arr[];
        return copy;
    }
}

In above dup function,
- If E has some mutable indirections, returns inout(E) and has constant purity.
- If E has no mutable indirections, return E[] and has string purity.
  That means: return value is implicitly convertible to immutable(E)[] with
  issue 5081, and can make a immutable duplication from mutable array.

Finally, we can remove the duplication of built-in dup/idup.

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