On 4/9/2015 5:05 AM, Michel Fortin wrote:
Why should it not have an opAssign marked @system?
"Andrei's idea was to not do the copy for @system opAssign's, thus providing C++
equivalence for those folks that need it and don't care about guaranteed memory
safety."
And what happens if the struct has a postblit but it is @disabled? Will the
compiler forbid you from passing it by ref in cases where it'd need to make a
copy, or will it just not be a RCO?
It wouldn't be an RCO.
More generally, is it right to add implicit copying just because a struct has a
postblit and a destructor? If someone implemented a by-value container in D
(such as those found in C++), this behaviour of the compiler would trash the
performance by silently doing useless unnecessary copies. You won't even get
memory-safety as a benefit: if the container allocates from the GC it's safe
anyway, otherwise you're referencing deallocated memory with your ref parameter
(copying the struct would just make a copy elsewhere, not retain the memory of
the original).
The only real purpose to a postblit is to support ref counting. Why would a
by-value container use a postblit and not ref count?
I think you're assuming too much from the presence of a postblit and a
destructor. This implicit copy behaviour should not be trigged by seemingly
unrelated clues. Instead of doing that:
auto tmp = rc;
the compiler should insert this:
auto tmp = rc.opPin();
RCArray can implement opPin by returning a copy of itself. A by-value container
can implement opPin by returning a dummy struct that retains the container's
memory until the dummy struct's destructor is called. Alternatively someone
could make a dummy "void opPin() @system {}" to signal it isn't safe to pass
internal references around (only in system code would the implicit call to opPin
compile). If you were writing a layout-compatible D version of std::vector,
you'd likely have to use a @system opPin because there's no way you can "pin"
that memory and guaranty memory-safety when passing references around.
My first impression is that's too complicated for the user to get right.
