[cxx-abi-dev] Non-trivial move constructor

[Richard Smith]

On 24 February 2016 at 05:54, Jason Merrill <jason at redhat.com> wrote:
> On 02/24/2016 05:51 AM, Marc Glisse wrote:
>>
>> in 3.1.1, we use "In the special case where the parameter type has a
>> non-trivial copy constructor or destructor" to force passing by
>> reference. It seems that for C++11, this should also include move
>> constructors, for the same reasons.
>
>
> We talked about adding move constructors to that sentence years ago. Did it
> never make it into the spec?

Looks like it didn't. The rule we ended up with was:

"[Pass an object of class type by value if] every copy constructor and
move constructor is deleted or trivial and at least one of them is not
deleted, and the destructor is trivial.”


However, this seems overly-cautious to me; it would seem sufficient
for there to be at least one copy or move constructor that is trivial
and not deleted, and a trivial destructor. It's not really
particularly plausible for there to be a trivial copy and a
non-trivial move or vice versa, but it *is* plausible for there to be
two non-deleted copy constructors -- a trivial one, and one that takes
a const volatile reference -- and in that case, passing through
registers seems completely reasonable. How about changing the rule in
3.1.1 bullet 1 to:

"In the special case where the parameter type does not have both a
trivial destructor and at least one trivial copy or move constructor
that is not deleted, the caller must allocate space for a temporary
copy, and pass the resulting copy by reference (below). Specifically
[...]"