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Add monostate, valueless; opt-into single-buffer is now 'valueless', not arbitrary nothrow default constructible

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Peter Dimov
2017-06-13 21:48:14 +03:00
parent 3bb14bf93b
commit 236193114c
1 changed files with 32 additions and 6 deletions
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38
include/boost/variant2/variant.hpp
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@ -45,7 +45,33 @@ public:
} }
}; };
// // monostate
struct monostate
{
};
constexpr bool operator<(monostate, monostate) noexcept { return false; }
constexpr bool operator>(monostate, monostate) noexcept { return false; }
constexpr bool operator<=(monostate, monostate) noexcept { return true; }
constexpr bool operator>=(monostate, monostate) noexcept { return true; }
constexpr bool operator==(monostate, monostate) noexcept { return true; }
constexpr bool operator!=(monostate, monostate) noexcept { return false; }
// valueless
struct valueless
{
};
constexpr bool operator<(valueless, valueless) noexcept { return false; }
constexpr bool operator>(valueless, valueless) noexcept { return false; }
constexpr bool operator<=(valueless, valueless) noexcept { return true; }
constexpr bool operator>=(valueless, valueless) noexcept { return true; }
constexpr bool operator==(valueless, valueless) noexcept { return true; }
constexpr bool operator!=(valueless, valueless) noexcept { return false; }
// variant forward declaration
template<class... T> class variant; template<class... T> class variant;
@ -434,7 +460,7 @@ template<class U, class... T> using resolve_overload_index = mp_find<mp_list<T..
// variant_base // variant_base
template<bool is_trivially_destructible, bool is_single_buffered, class... T> struct variant_base_impl; // trivially destructible, single buffered template<bool is_trivially_destructible, bool is_single_buffered, class... T> struct variant_base_impl; // trivially destructible, single buffered
template<class... T> using variant_base = variant_base_impl<mp_all<std::is_trivially_destructible<T>...>::value, mp_any<mp_all<std::is_nothrow_move_constructible<T>...>, std::is_nothrow_default_constructible<T>...>::value, T...>; template<class... T> using variant_base = variant_base_impl<mp_all<std::is_trivially_destructible<T>...>::value, mp_any<mp_all<std::is_nothrow_move_constructible<T>...>, std::is_same<T, valueless>...>::value, T...>;
struct none {}; struct none {};
@ -491,9 +517,9 @@ template<class... T> struct variant_base_impl<true, true, T...>
template<std::size_t J, class U, class... A> void emplace_impl( mp_false, mp_false, A&&... a ) template<std::size_t J, class U, class... A> void emplace_impl( mp_false, mp_false, A&&... a )
{ {
std::size_t const K = mp_find_if<mp_list<T...>, std::is_nothrow_constructible>::value; std::size_t const K = mp_find<mp_list<T...>, valueless>::value;
if( K < sizeof...(T) ) // have nothrow destructible if( K < sizeof...(T) ) // have valueless
{ {
try try
{ {
@ -644,7 +670,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
{ {
size_t const J = I+1; size_t const J = I+1;
std::size_t const K = mp_find_if<mp_list<T...>, std::is_nothrow_constructible>::value; std::size_t const K = mp_find<mp_list<T...>, valueless>::value;
using U = mp_at_c<variant<T...>, I>; using U = mp_at_c<variant<T...>, I>;
@ -655,7 +681,7 @@ template<class... T> struct variant_base_impl<false, true, T...>
st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... ); st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
ix_ = J; ix_ = J;
} }
else if( K < sizeof...(T) ) // have nothrow destructible else if( K < sizeof...(T) ) // have valueless
{ {
_destroy(); _destroy();