2012/11/2 Dave Abrahams <dave@boostpro.com>:
on Fri Nov 02 2012, Mathias Gaunard <mathias.gaunard-AT-ens-lyon.org> wrote:
On 01/11/12 14:37, Dave Abrahams wrote:
As far as I can tell, putting sizeof() inside a layer of template instantiation doesn't solve anything. But maybe I'm missing something.
The idea would be that the user of variant<T0, T1, ..., Tn> would be able to provide incomplete types Ti if size_forward<Ti> is specialized.
Yes, that is what I mean.
I suppose one could arrange for a later compilation error should the size turn out to be wrong.
Yes, BOOST_STATIC_ASSERT((size_forward<Ti>::value == sizeof(Ti))); must be put somevere in the code. But, sizeof() for incomplete types is not the only problem. It looks like we would also need has_nothrow_constructor_forward<Ti> (and may be some more "_forward" versions of meta functions), because Boost.Variant uses some heuristics to detect fallback_type and dynamic allocation requirements. Here is a small example: struct some_pod_t; template <> struct size_forward<some_pod_t> {enum ENU {value = 1}; }; struct some_nonpod_type { some_nonpod_type(); some_nonpod_type(const some_nonpod_type&){} some_nonpod_type& operator=(const some_nonpod_type&){} }; // Will not detect fallback type or will be a compilation failure in has_nothrow_constructor<some_pod_t> boost::variant<some_nonpod_type, some_pod_t> v; // If there was no compilation error... struct some_pod_t {}; // Will detect some_pod_t as fallback type boost::variant<some_nonpod_type, some_pod_t> v2; ... v = v2; // Strange things may happen -- Best regards, Antony Polukhin