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Single buffer when a type is nothrow default constructible

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Peter Dimov
2017-05-31 04:59:59 +03:00
parent 4f241bd8b0
commit 08bfd69387
1 changed files with 47 additions and 9 deletions
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56
include/boost/variant2/variant.hpp
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@@ -335,13 +335,13 @@ template<class U, class... T> using resolve_overload_index = mp_find<mp_list<T..
// variant_base
template<class D1, class D2, 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>...>, mp_all<std::is_nothrow_move_constructible<T>...>, T...>;
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_all<std::is_nothrow_move_constructible<T>...>::value || mp_any<std::is_nothrow_default_constructible<T>...>::value, T...>;
struct none {};
// trivially destructible, single buffered
template<class... T> struct variant_base_impl<mp_true, mp_true, T...>
template<class... T> struct variant_base_impl<true, true, T...>
{
int ix_;
variant_storage<none, T...> st1_;
@@ -374,7 +374,9 @@ template<class... T> struct variant_base_impl<mp_true, mp_true, T...>
template<std::size_t I, class... A> void emplace( A&&... a )
{
size_t const J = I+1;
std::size_t const J = I+1;
std::size_t const K = mp_find_if<mp_list<T...>, std::is_nothrow_constructible>::value;
using U = mp_at_c<variant<T...>, I>;
@@ -383,8 +385,25 @@ template<class... T> struct variant_base_impl<mp_true, mp_true, T...>
st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
ix_ = J;
}
else if( K < sizeof...(T) ) // have nothrow destructible
{
try
{
st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
ix_ = J;
}
catch( ... )
{
st1_.emplace( mp_size_t<K>() );
ix_ = K;
throw;
}
}
else
{
assert( std::is_nothrow_move_constructible<U>::value );
U tmp( std::forward<A>(a)... );
st1_.emplace( mp_size_t<J>(), std::move(tmp) );
@@ -394,7 +413,7 @@ template<class... T> struct variant_base_impl<mp_true, mp_true, T...>
};
// trivially destructible, double buffered
template<class... T> struct variant_base_impl<mp_true, mp_false, T...>
template<class... T> struct variant_base_impl<true, false, T...>
{
int ix_;
variant_storage<none, T...> st1_;
@@ -446,7 +465,7 @@ template<class... T> struct variant_base_impl<mp_true, mp_false, T...>
};
// not trivially destructible, single buffered
template<class... T> struct variant_base_impl<mp_false, mp_true, T...>
template<class... T> struct variant_base_impl<false, true, T...>
{
int ix_;
variant_storage<none, T...> st1_;
@@ -501,15 +520,34 @@ template<class... T> struct variant_base_impl<mp_false, mp_true, T...>
{
size_t const J = I+1;
std::size_t const K = mp_find_if<mp_list<T...>, std::is_nothrow_constructible>::value;
using U = mp_at_c<variant<T...>, I>;
if( std::is_nothrow_constructible<U, A...>::value )
{
_destroy();
st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
ix_ = J;
}
else if( K < sizeof...(T) ) // have nothrow destructible
{
_destroy();
try
{
st1_.emplace( mp_size_t<J>(), std::forward<A>(a)... );
ix_ = J;
}
catch( ... )
{
st1_.emplace( mp_size_t<K>() );
ix_ = K;
throw;
}
}
else
{
assert( std::is_nothrow_move_constructible<U>::value );
@@ -517,15 +555,15 @@ template<class... T> struct variant_base_impl<mp_false, mp_true, T...>
U tmp( std::forward<A>(a)... );
_destroy();
st1_.emplace( mp_size_t<J>(), std::move(tmp) );
st1_.emplace( mp_size_t<J>(), std::move(tmp) );
ix_ = J;
}
}
};
// not trivially destructible, double buffered
template<class... T> struct variant_base_impl<mp_false, mp_false, T...>
template<class... T> struct variant_base_impl<false, false, T...>
{
int ix_;
variant_storage<none, T...> st1_;