char[][] join ==> string

[Ali Çehreli]

On 04/07/2011 09:01 AM, Simen kjaeraas wrote:
 > On Thu, 07 Apr 2011 02:13:16 +0200, bearophile
 > <bearophileHUGS at lycos.com> wrote:
 >
 >> Given an array of strings std.string.join() returns a single string:
 >>
 >> import std.string;
 >> void main() {
 >> string[] a1 = ["hello", "red"];
 >> string j1 = join(a1, " "); // OK
 >> }
 >>
 >>
 >> But in a program I need an array of mutable arrays of chars. If I join
 >> the arrays I get a mutable array of chars. But I need a string:
 >>
 >> import std.string;
 >> void main() {
 >> char[][] a2 = ["hello".dup, "red".dup];
 >> string j2 = join(a2, " "); // error
 >> }
 >>
 >> Error: cannot implicitly convert expression (join(a," ")) of type
 >> char[] to string
 >>
 >> .idup avoids the error:
 >>
 >> string j3 = join(a2, " ").idup; // OK
 >>
 >> Given the low efficiency of the D GC it's better to reduce memory
 >> allocations as much as possible.
 >> Here join() creates a brand new array, so idup performs a useless
 >> copy. To avoid this extra copy do I have to write another joinString()
 >> function?
 >>
 >> Bye,
 >> bearophile
 >
 > Isn't this a prime case for std.exception.assumeUnique?

Almost. assumeUnique is too eager and tries to null its reference 
parameter. Copying from std/exception.d:

immutable(T)[] assumeUnique(T)(ref T[] array) pure nothrow
{
     auto result = cast(immutable(T)[]) array;
     array = null;
     return result;
}

And that fails as join's return type is not an lvalue.

We need a simplyAssumeUnique() that doesn't null the reference parameter 
:); and it would have no value over casting other than communicating the 
intent.

Ali