feedc0de/boost_variant2
1
0
Fork 0
forked from boostorg/variant2
Code Pull Requests Activity

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

Browse Source
This commit is contained in:
Peter Dimov
2021-12-09 18:34:02 +02:00
parent e4b4b3f02b
commit 3fc7540733
3 changed files with 50 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
include/boost/variant2/variant.hpp
View File

@ -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 } );

1
test/Jamfile
View File

@ -125,3 +125,4 @@ compile variant_derived_construct.cpp ;
run variant_visit_by_index.cpp ;
run variant_ostream_insert.cpp ;
run is_output_streamable.cpp ;

38
test/is_output_streamable.cpp Normal file
View File

@ -0,0 +1,38 @@
// Copyright 2021 Peter Dimov
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt
#include <boost/variant2/variant.hpp>
#include <boost/core/lightweight_test_trait.hpp>
#include <ostream>
#include <string>
struct X
{
};
struct Y
{
};
std::ostream& operator<<( std::ostream& os, Y const& /*y*/ )
{
os << "Y()";
return os;
}
int main()
{
BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, int>));
BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, float>));
BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, std::string>));
BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, void>));
BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, X>));
BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, Y>));
BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, std::string>>));
BOOST_TEST_TRAIT_FALSE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, X>>));
BOOST_TEST_TRAIT_TRUE((boost::variant2::detail::is_output_streamable<std::ostream, boost::variant2::variant<int, float, Y>>));
return boost::report_errors();
}