parse_into clears sequences before filling them

example/parse_into.cpp

@@ -65,6 +65,10 @@ struct options
 
     iterator
     end();
+
+    void
+    clear()
+    { }
 };
 
 struct coordinate
@@ -112,6 +116,10 @@ struct accumulator
 
     iterator
     end() { return nullptr; }
+
+    void
+    clear()
+    { }
 };
 
 struct coordinates2

include/boost/json/detail/parse_into.hpp

@@ -502,6 +502,32 @@ std::true_type check_inserter( It1, It2 )
     return {};
 }
 
+template<class T>
+void
+clear_container(
+    T&,
+    mp11::mp_int<2>)
+{
+}
+
+template<class T>
+void
+clear_container(
+    T& target,
+    mp11::mp_int<1>)
+{
+    target.clear();
+}
+
+template<class T>
+void
+clear_container(
+    T& target,
+    mp11::mp_int<0>)
+{
+    target.clear();
+}
+
 template< class V, class P >
 class converting_handler<sequence_conversion_tag, V, P>
     : public composite_handler<
@@ -553,14 +579,11 @@ public:
     bool on_array_begin( error_code& ec )
     {
         if( this->inner_active_ )
-        {
             return this->inner_.on_array_begin( ec );
-        }
+
-        else
+        this->inner_active_ = true;
-        {
+        clear_container( *value_, inserter_implementation<V>() );
-            this->inner_active_ = true;
+        return true;
-            return true;
-        }
     }
 
     bool on_array_end( error_code& ec )
@@ -605,6 +628,7 @@ public:
         if( this->inner_active_ )
             return this->inner_.on_object_begin(ec);
 
+        clear_container( *value_, inserter_implementation<V>() );
         return true;
     }
 

test/parse_into.cpp

@@ -252,6 +252,13 @@ public:
             error::size_mismatch, {1, 2, 3} );
 
         testParseInto< std::vector<std::array<int, 4>> >( {arr,arr,arr} );
+
+        std::vector<int> v;
+        parse_into(v, "[1,2,3,4]");
+        BOOST_TEST( v.size() == 4 );
+
+        parse_into(v, "[5,6,7]");
+        BOOST_TEST( v.size() == 3 );
     }
 
     void testMap()
@@ -272,6 +279,13 @@ public:
             error::not_object, { "1", 1, "2", 2} );
         testParseIntoErrors< std::map<std::string, std::map<std::string, int>> >(
             error::not_object, { {"1", {}}, {"2", {"3", 4}} } );
+
+        std::map<std::string, int> m;
+        parse_into(m, R"( {"1": 1, "2": 2, "3": 3} )");
+        BOOST_TEST( m.size() == 3 );
+
+        parse_into(m, R"( {"4": 4, "5": 5} )");
+        BOOST_TEST( m.size() == 2 );
     }
 
     void testTuple()