Add visit_by_index

include/boost/variant2/variant.hpp

@@ -2208,6 +2208,34 @@ void swap( variant<T...> & v, variant<T...> & w )
     v.swap( w );
 }
 
+// visit_by_index
+
+namespace detail
+{
+
+template<class V, class... F> struct visit_by_index_L
+{
+    V&& v;
+    std::tuple<F&&...> tp;
+
+    template<class I> void operator()( I ) const
+    {
+        std::get<I::value>( std::move(tp) )( unsafe_get<I::value>( std::forward<V>(v) ) );
+    }
+};
+
+} // namespace detail
+
+template<class V, class... F> void visit_by_index( V&& v, F&&... f )
+{
+    constexpr auto N = variant_size<V>::value;
+
+    static_assert( N == sizeof...(F), "Incorrect number of function objects" );
+
+    mp11::mp_with_index<N>( v.index(),
+        detail::visit_by_index_L<V, F...>{ std::forward<V>(v), std::tuple<F&&...>( std::forward<F>(f)... ) } );
+}
+
 // hashing support
 
 namespace detail

test/Jamfile

@@ -121,3 +121,5 @@ run variant_visit_r.cpp : : :
     ;
 
 compile variant_derived_construct.cpp ;
+
+run variant_visit_by_index.cpp ;

test/variant_visit_by_index.cpp

@@ -0,0 +1,50 @@
+// Copyright 2017, 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 <boost/config.hpp>
+
+using namespace boost::variant2;
+
+int main()
+{
+    {
+        variant<int, int, float> v;
+
+        visit_by_index( v,
+            []( int& x ){ BOOST_TEST_EQ( x, 0 ); },
+            []( int&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> v( 1 );
+
+        visit_by_index( v,
+            []( int const& x ){ BOOST_TEST_EQ( x, 1 ); },
+            []( int&         ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int, int, float> const v( in_place_index_t<1>(), 2 );
+
+        visit_by_index( v,
+            []( int const&   ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const& x ){ BOOST_TEST_EQ( x, 2 ); },
+            []( float const& ){ BOOST_ERROR( "incorrect alternative" ); } );
+    }
+
+    {
+        variant<int const, int, float const> const v( 3.14f );
+
+        visit_by_index( v,
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( int const&     ){ BOOST_ERROR( "incorrect alternative" ); },
+            []( float const& x ){ BOOST_TEST_EQ( x, 3.14f ); } );
+    }
+
+    return boost::report_errors();
+}