On Fri, Feb 19, 2016 at 1:07 PM, Richard Smith <rich...@metafoo.co.uk> wrote:
> On Fri, Feb 19, 2016 at 5:35 AM, Michael Matz <m...@suse.de> wrote:
>> Hi,
>>
>> On Thu, 18 Feb 2016, Richard Smith wrote:
>>
>>> >> An empty type is a type where it and all of its subobjects
>>> >> (recursively) are of class, structure, union, or array type. No
>>> >> memory slot nor register should be used to pass or return an object
>>> >> of empty type.
>>> >
>>> > The trivially copyable is gone again. Why is it not necessary?
>>>
>>> The C++ ABI doesn't defer to the C psABI for types that aren't
>>> trivially-copyable. See
>>> http://mentorembedded.github.io/cxx-abi/abi.html#normal-call
>>
>> Hmm, yes, but we don't want to define something for only C and C++, but
>> language independend (so far as possible). And given only the above
>> language I think this type:
>>
>> struct S {
>> S() {something();}
>> };
>>
>> would be an empty type, and that's not what we want.
>
> Yes it is. Did you mean to give S a copy constructor, copy assignment
> operator, or destructor instead?
>
>> "Trivially copyable"
>> is a reasonably common abstraction (if in doubt we could even define it in
>> the ABI), and captures the idea that we need well (namely that a bit-copy
>> is enough).
In this case:
struct dummy0
{
};
struct dummy
{
dummy0 d[20];
dummy0 * foo (int i);
};
dummy0 *
dummy::foo (int i)
{
return &d[i];
}
dummy0 *
bar (dummy d, int i)
{
return d.foo (i);
}
dummy shouldn't be passed as empty type. We need to have a clear
definition for what kinds of member functions are allowed in an empty
type.
--
H.J.
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