To elaborate... what's the point of making everything cheap to
copy?
Copying of an object is _not_ an operation like swap() or "move",
which are essential to many algorithms.
Indeed, an object might not want to be copyable at all, or it
might need to perform some expensive operation (as in Jon's
example) in order for the copy to behave like a copy is supposed
to.
The only thing that the arbitrary requirement "copies are
supposed to be cheap" would do is that it would slow down
everything else, forcing checks on operations that shouldn't need
to be checked. And it's completely unnecessary unless you're
working with reference types, in which case copying is already
cheap anyways.
So basically, algorithms should _expect_ copying of arbitrary
objects to be expensive, and there's no need for them to be
otherwise. C++'s swap() illustrates the lack of the need for
copying beautifully -- often times the only objects I copy
"generically" in C++ are iterators. I never find the need to copy
other objects... and I believe D has no such need, either.
