Add operator<< for variant

include/boost/variant2/variant.hpp

@@ -25,6 +25,7 @@
 #include <utility>
 #include <functional> // std::hash
 #include <cstdint>
+#include <iosfwd>
 
 //
 
@@ -2236,6 +2237,30 @@ template<class R = detail::deduced, class V, class... F> constexpr auto visit_by
         detail::visit_by_index_L<R, V, F...>{ std::forward<V>(v), std::tuple<F&&...>( std::forward<F>(f)... ) } );
 }
 
+// output streaming
+
+namespace detail
+{
+
+template<class Ch, class Tr, class... T> struct ostream_insert_L
+{
+    std::basic_ostream<Ch, Tr>& os;
+    variant<T...> const& v;
+
+    template<class I> std::basic_ostream<Ch, Tr>& operator()( I ) const
+    {
+        return os << unsafe_get<I::value>( v );
+    }
+};
+
+} // namespace detail
+
+template<class Ch, class Tr, class... T> std::basic_ostream<Ch, Tr>& operator<<( std::basic_ostream<Ch, Tr>& os, variant<T...> const& v )
+{
+    return mp11::mp_with_index<sizeof...(T)>( v.index(),
+        detail::ostream_insert_L<Ch, Tr, T...>{ os, v } );
+}
+
 // hashing support
 
 namespace detail

test/Jamfile

@@ -123,3 +123,5 @@ run variant_visit_r.cpp : : :
 compile variant_derived_construct.cpp ;
 
 run variant_visit_by_index.cpp ;
+
+run variant_ostream_insert.cpp ;

test/variant_ostream_insert.cpp

@@ -0,0 +1,36 @@
+// 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.hpp>
+#include <sstream>
+#include <string>
+
+using namespace boost::variant2;
+
+template<class T> std::string to_string( T const& t )
+{
+    std::ostringstream os;
+
+    os << t;
+
+    return os.str();
+}
+
+int main()
+{
+    variant<int, float, std::string> v( 1 );
+
+    BOOST_TEST_EQ( to_string( v ), to_string( 1 ) );
+
+    v = 3.14f;
+
+    BOOST_TEST_EQ( to_string( v ), to_string( 3.14f ) );
+
+    v = "test";
+
+    BOOST_TEST_EQ( to_string( v ), to_string( "test" ) );
+
+    return boost::report_errors();
+}