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Only define operator<< when all contained types are streamable (refs #31)

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Peter Dimov
2021-12-09 18:34:02 +02:00
parent e4b4b3f02b
commit 3fc7540733
3 changed files with 50 additions and 1 deletions
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12
include/boost/variant2/variant.hpp
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@@ -2253,9 +2253,19 @@ template<class Ch, class Tr, class... T> struct ostream_insert_L
}
};
template<class Os, class T, class E = void> struct is_output_streamable: std::false_type
{
};
template<class Os, class T> struct is_output_streamable<Os, T, decltype( std::declval<Os&>() << std::declval<T const&>(), (void)0 )>: std::true_type
{
};
} // namespace detail
template<class Ch, class Tr, class T1, class... T> std::basic_ostream<Ch, Tr>& operator<<( std::basic_ostream<Ch, Tr>& os, variant<T1, T...> const& v )
template<class Ch, class Tr, class T1, class... T,
class E = typename std::enable_if< mp11::mp_all< detail::is_output_streamable<std::basic_ostream<Ch, Tr>, T>... >::value >::type >
std::basic_ostream<Ch, Tr>& operator<<( std::basic_ostream<Ch, Tr>& os, variant<T1, T...> const& v )
{
return mp11::mp_with_index<1 + sizeof...(T)>( v.index(),
detail::ostream_insert_L<Ch, Tr, T1, T...>{ os, v } );