On Thursday, 10 April 2014 at 05:54:52 UTC, monarch_dodra wrote:
On Wednesday, 9 April 2014 at 23:53:04 UTC, dnspies wrote:
Does concatenation always create a new array? What if the
type of the original is immutable (or even const)? In that
case, wouldn't it be worthwhile to check first if rhs can be
copied to the right of lhs?
Yes, it *always* creates a new array. First:
The resulting array is guaranteed new. Consider:
1. If "rhs" was copied to the right of "lhs", then "lhs" and
"result" would be aliasing data:
a = [1];
b = [1];
c = a ~ b;
c[0] = 2;
assert(a[0] == 1); //Could fail! A got clobbered!
2. If "lhs" has some capacity, then it is guaranteed its next
concatenation will be non allocating.
a = [1];
a.reserve(2);
p = a.ptr;
b =a ~ 2; //Uses a's capacity;
a ~= 2; //relocates.
assert(a.ptr == p); //Fails!
If you *really* wanted to "make concatenation maybe append",
you can do it this way:
a = [1, 2, 3];
b = [...];
c = a; c ~= b; //c has the same contents as "a ~ b", but may
actually contain a part of "a".
The aliasing thing isn't a problem if the data-type is const or
immutable, because you can't change it any way. (Your "A got
clobbered" example doesn't apply)
