Make json value type prepend non-overriding for consistency with map

3 files changed, 26 insertions, 14 deletions

diff --git a/libbuild2/json.cxx b/libbuild2/json.cxx
index 6295b59..4ed1631 100644
--- a/libbuild2/json.cxx
+++ b/libbuild2/json.cxx
@@ -511,7 +511,7 @@ namespace build2
   }
 
   void json_value::
-  append (json_value&& v)
+  append (json_value&& v, bool override)
   {
     if (type == json_type::null)
     {
@@ -584,10 +584,10 @@ namespace build2
                                {
                                  return m.name == o.name;
                                }));
-              if (i != object.end ())
-                i->value = move (m.value);
-              else
+              if (i == object.end ())
                 object.push_back (move (m));
+              else if (override)
+                i->value = move (m.value);
             }
           }
 
@@ -602,7 +602,7 @@ namespace build2
   }
 
   void json_value::
-  prepend (json_value&& v)
+  prepend (json_value&& v, bool override)
   {
     if (type == json_type::null)
     {
@@ -675,10 +675,10 @@ namespace build2
                                {
                                  return m.name == o.name;
                                }));
-              if (i != object.end ())
-                i->value = move (m.value);
-              else
+              if (i == object.end ())
                 object.insert (object.begin (), move (m));
+              else if (override)
+                i->value = move (m.value);
             }
           }
 
diff --git a/libbuild2/json.hxx b/libbuild2/json.hxx
index 538c87d..96596e3 100644
--- a/libbuild2/json.hxx
+++ b/libbuild2/json.hxx
@@ -170,16 +170,22 @@ namespace build2
     // values are incompatible. Note that for numbers this can also lead to
     // the change of the value type.
     //
-    // Append/prepend to an object overrides existing members. Append/prepend
-    // an array to an array splices in the array elements rather than adding
-    // an element of the array type.
+    // Append/prepend an array to an array splices in the array elements
+    // rather than adding an element of the array type.
+    //
+    // By default, append to an object overrides existing members while
+    // prepend does not. In a sense, whatever appears last is kept, which is
+    // consistent with what we expect to happen when specifying the same name
+    // repeatedly (provided it's not considered invalid) in a text
+    // representation (e.g., {"a":1,"a":2}). Position-wise, both append and
+    // prepend retain the positions of existing members with append inserting
+    // new ones at the end while prepend -- at the beginning.
     //
     void
-    append (json_value&&);
+    append (json_value&&, bool override = true);
 
     void
-    prepend (json_value&&);
-
+    prepend (json_value&&, bool override = false);
 
     // Array element access.
     //
diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index b157806..d754edf 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -1237,6 +1237,12 @@ namespace build2
   // of what looks like harmless looseness in a few contexts (such as the
   // $json.member_*() functions).
   //
+  // Note that similar to map, JSON object append/+= is overriding while
+  // prepend/=+ is not. In a sense, whatever appears last (from left to right)
+  // is kept, which is consistent with what we expect to happen when
+  // specifying the same name repeatedly (provided it's not considered
+  // invalid) in a representation (e.g., {"a":1,"a":2}).
+  //
   template <>
   struct LIBBUILD2_SYMEXPORT value_traits<json_value>
   {