Reversing a string

[rikki cattermole]

So strings in D are Unicode. This is both a great thing and a horrible 
thing.
To reverse a Unicode string correctly you need to take into account BiDi 
and graphemes, in other words it gets rather complex. However I suspect 
that this isn't what you want.

Now a (w/d)string is defined as:

alias string  = immutable(char)[];
alias wstring = immutable(wchar)[];
alias dstring = immutable(dchar)[];

Note the immutable, it means you cannot modify individual values. Which 
is a problem for reverse because it modifies in place.

Which means:

writeln("Hello D".reverse);

Won't work, but:

writeln("Hello D".dup.reverse);

Will. A simple duplication (char[]) makes it work.

Finally, arrays in D are absolutely brilliant. They are what we call 
slices. A slice is a pointer + a length. That is it. Hence they cannot 
be reversed in place. Of course this is great for interacting with e.g. 
C, since its just a matter of slicing any data back to get your bounds 
checking ext.