On Thu, Feb 11, 2016 at 2:47 AM, Matthijs van Duin
<matthijsvand...@gmail.com> wrote:
> On 8 February 2016 at 22:40, H.J. Lu <hjl.to...@gmail.com> wrote:
>> "empty type". An empty type is either an array of empty types or a
>> class type where every member is of empty type.
>
> Note that the term "empty type" is commonly used in type theory to
> denote a (or the) type with no values. The closest thing C has would be
> an empty enum when using -fstrict-enums. (Declaring it as return type
> implies [[noreturn]] or undefined behaviour.)
>
> A type with a unique value (such as void or an empty struct) is usually
> known as a unit type.
>
> BTW, being standard layout is not sufficient (nor required afaict) for
> zero-register passing of a unit type. The requirement you need is
> trivially-copyable. Example:
>
> #include <map>
> #include <iostream>
> #include <type_traits>
>
> using namespace std;
>
> class EmptyInt {
> static map< const EmptyInt *, int > values;
>
> public:
> EmptyInt() = default;
> EmptyInt( int x ) { values[this] = x; }
> ~EmptyInt() { values.erase(this); }
>
> operator int () const { return values[this]; }
> };
>
> typeof( EmptyInt::values ) EmptyInt::values;
>
> EmptyInt foo() {
> return 42;
> }
>
> int main() {
> cout << is_standard_layout<EmptyInt>{} << endl;
> cout << foo() << endl;
> return 0;
> }
My current proposal is
1. "class type". A class type is a structure, union or C++ class.
2. "empty type". An empty type is Plain Old Data (POD) for the
purposes of layout, and.a type where it and all of its subobjects
are of class or array type.
> This evil contraption satisfies all POD-requirements except for not
> being trivially-copyable. On the other hand taking this example from
> http://en.cppreference.com/w/cpp/concept/StandardLayoutType
>
> struct Q {};
> struct S : Q {};
> struct T : Q {};
> struct U : S, T {}; // not a standard-layout class
>
> Even though U is not standard-layout, it is trivially-copyable and I see
> no reason to allocate a register to pass it.
>
Since this isn't Plain Old Data (POD) for the purposes of layout, it
isn't covered by my proposal for psABI. I leave this to C++ ABI.
--
H.J.
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
