Newbie: copy, assignment of class instances

[Larry Luther]

"bearophile" <bearophileHUGS at lycos.com> wrote in message 
news:ht42el$6c9$1 at digitalmars.com...
  ...
| There is no standard way to do this. In general you don't need to copy 
classes or their contents, because they are on the heap and managed by the 
GC.

I'm nonplussed.

Could you expand on why D class instances don't need to copy their contents 
and
instances of D structs do?  While migrating C++ code to D I've had to 
convert
"struct"s to "class"es because of the need for inheritance.  Why would the 
need
to copy an instance's contents to another instance disappear?

Larry