Hi.
It seems boost::is_empty uses Empty Base Optimisation to detect whether a
class type is stateless or not. This predictably fails on final types, as
they cannot be inherited from:
#include <iostream>
#include
class Test final {};
int main() {
std::cout << boost::is_empty<Test>::value;
return 0;
}
Fails on gcc 4.8.1 with
In file included from /usr/local/include/boost/type_traits.hpp:54:0,
from main.cpp:2:
/usr/local/include/boost/type_traits/is_empty.hpp: In
instantiation of 'struct boost::detail::empty_helper_t1<Test>':
/usr/local/include/boost/type_traits/is_empty.hpp:79:5: required
from 'const bool boost::detail::empty_helper::value'
/usr/local/include/boost/type_traits/is_empty.hpp:88:5: required
from 'const bool boost::detail::is_empty_impl<Test>::value'
/usr/local/include/boost/type_traits/is_empty.hpp:220:1:
required from 'struct boost::is_empty<Test>'
main.cpp:7:39: required from here
/usr/local/include/boost/type_traits/is_empty.hpp:52:8: error:
cannot derive from 'final' base 'Test' in derived type
'boost::detail::empty_helper_t1<Test>'
struct empty_helper_t1 : public T
^
Yet std::is_empty works just fine (al least in gcc), and appears to be
implemented via an intrinsic:
template<typename _Tp>
struct is_empty
: public integral_constant
{};
Now, is it at all possible to implement conforming is_empty without
resorting to compiler magic that are intrinsics?
