array as parameter

[Paul via Digitalmars-d-learn]

Dynamic array is really reference. Right? But why modification of 
parameter in this case does not work:

void some_func(string[] s) {
 s ~= "xxx"; s ~= "yyy";
}
but this works:

void some_fun(ref string[] s) {
 s ~= "xxx"; s ~= "yyy";
}

In the 1st case s is reference too, is not it?