From 67a12199e65e37efcd96b69ab403eee58b559218 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Tue, 18 Oct 2022 06:08:18 +0300
Subject: [PATCH] Add JSON support (refs #28)

---
 include/boost/variant2/variant.hpp | 89 +++++++++++++++++++++++++++++-
 test/Jamfile                       |  5 ++
 test/variant_json_value_from.cpp   | 41 ++++++++++++++
 test/variant_json_value_to.cpp     | 42 ++++++++++++++
 4 files changed, 175 insertions(+), 2 deletions(-)
 create mode 100644 test/variant_json_value_from.cpp
 create mode 100644 test/variant_json_value_to.cpp

diff --git a/include/boost/variant2/variant.hpp b/include/boost/variant2/variant.hpp
index e2bbc3a..b474643 100644
--- a/include/boost/variant2/variant.hpp
+++ b/include/boost/variant2/variant.hpp
@@ -15,8 +15,9 @@
 
 #include <boost/mp11.hpp>
 #include <boost/assert.hpp>
+#include <boost/assert/source_location.hpp>
 #include <boost/config.hpp>
-#include <boost/detail/workaround.hpp>
+#include <boost/config/workaround.hpp>
 #include <boost/cstdint.hpp>
 #include <cstddef>
 #include <type_traits>
@@ -24,8 +25,9 @@
 #include <initializer_list>
 #include <utility>
 #include <functional> // std::hash
-#include <cstdint>
 #include <iosfwd>
+#include <cstdint>
+#include <cerrno>
 
 //
 
@@ -2402,6 +2404,89 @@ template<> struct hash< ::boost::variant2::monostate >
 
 } // namespace std
 
+// JSON support
+
+namespace boost
+{
+namespace json
+{
+
+class value;
+
+struct value_from_tag;
+
+template<class T>
+void value_from( T&& t, value& jv );
+
+template<class T>
+struct value_to_tag;
+
+template<class T>
+T value_to( value const & v );
+
+template<class T>
+struct try_value_to_tag;
+
+template<class T1, class T2>
+struct result_for;
+
+template<class T>
+typename result_for<T, value>::type
+try_value_to( value const & jv );
+
+template<class T>
+typename result_for<T, value>::type
+result_from_errno( int e, boost::source_location const* loc ) noexcept;
+
+template<class T, class E> struct is_null_like;
+
+template<> struct is_null_like< variant2::monostate, void >: std::true_type
+{
+};
+
+} // namespace json
+
+namespace variant2
+{
+
+template<class... T>
+    void tag_invoke( boost::json::value_from_tag const&, boost::json::value& v, variant<T...> const & w )
+{
+    visit( [&](auto const& t){
+
+        boost::json::value_from( t, v );
+
+    }, w );
+}
+
+template<class... T>
+    typename boost::json::result_for<variant<T...>, boost::json::value>::type
+    tag_invoke( boost::json::try_value_to_tag<variant<T...>> const&, boost::json::value const& v )
+{
+    static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
+    auto r = boost::json::result_from_errno< variant<T...> >( EINVAL, &loc );
+
+    mp11::mp_for_each<mp11::mp_iota_c<sizeof...(T)>>( [&](auto I){
+
+        if( !r )
+        {
+            using Ti = mp11::mp_at_c<variant<T...>, I>;
+            auto r2 = boost::json::try_value_to<Ti>( v );
+
+            if( r2 )
+            {
+                r.emplace( in_place_index<I>, *r2 );
+            }
+        }
+
+    });
+
+    return r;
+}
+
+} // namespace variant2
+} // namespace boost
+
 #undef BOOST_VARIANT2_CX14_ASSERT
 
 #if defined(_MSC_VER) && _MSC_VER < 1910
diff --git a/test/Jamfile b/test/Jamfile
index 2fbb975..3c67277 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -126,3 +126,8 @@ run variant_visit_by_index.cpp ;
 
 run variant_ostream_insert.cpp ;
 run is_output_streamable.cpp ;
+
+local JSON = <library>/boost//json/<warnings>off "<toolset>msvc-14.0:<build>no" "<toolset>msvc-14.2:<cxxflags>-wd5104" ;
+
+run variant_json_value_from.cpp : : : $(JSON) ;
+run variant_json_value_to.cpp : : : $(JSON) ;
diff --git a/test/variant_json_value_from.cpp b/test/variant_json_value_from.cpp
new file mode 100644
index 0000000..9734c74
--- /dev/null
+++ b/test/variant_json_value_from.cpp
@@ -0,0 +1,41 @@
+// Copyright 2022 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/json/value_from.hpp>
+#include <boost/json/serialize.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <string>
+
+using namespace boost::variant2;
+namespace json = boost::json;
+
+int main()
+{
+    {
+        monostate m;
+        json::value w = json::value_from( m );
+        BOOST_TEST_EQ( w, json::value( nullptr ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v;
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( nullptr ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v( 17 );
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( 17 ) );
+    }
+
+    {
+        variant<monostate, int, std::string> v( "test" );
+        json::value w = json::value_from( v );
+        BOOST_TEST_EQ( w, json::value( "test" ) );
+    }
+
+    return boost::report_errors();
+}
diff --git a/test/variant_json_value_to.cpp b/test/variant_json_value_to.cpp
new file mode 100644
index 0000000..1263931
--- /dev/null
+++ b/test/variant_json_value_to.cpp
@@ -0,0 +1,42 @@
+// Copyright 2022 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/json/value_to.hpp>
+#include <boost/json/serialize.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::variant2;
+namespace json = boost::json;
+
+int main()
+{
+    {
+        json::value v;
+        auto r = json::try_value_to<monostate>( v );
+        BOOST_TEST( r.has_value() );
+    }
+
+    using V = variant<monostate, int, std::string>;
+
+    {
+        json::value v;
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V() );
+    }
+
+    {
+        json::value v( 12 );
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V(12) );
+    }
+
+    {
+        json::value v( "test" );
+        auto r = json::try_value_to<V>( v );
+        BOOST_TEST( r.has_value() ) && BOOST_TEST_EQ( *r, V("test") );
+    }
+
+    return boost::report_errors();
+}