From 2381f28c734d349b53556e35eeb4351cb74058b2 Mon Sep 17 00:00:00 2001
From: Michael Davidsaver <mdavidsaver@gmail.com>
Date: Thu, 16 Jul 2020 14:05:35 -0700
Subject: [PATCH] allow unselection/clear of Union/Any

---
 src/data.cpp      | 6 ++++++
 src/pvxs/data.h   | 8 +++++++-
 test/testdata.cpp | 7 ++++++-
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/data.cpp b/src/data.cpp
index 1fd8074..0ea3713 100644
--- a/src/data.cpp
+++ b/src/data.cpp
@@ -584,12 +584,18 @@ void Value::copyIn(const void *ptr, StoreType type)
         break;
     }
     case StoreType::Compound:
+        if(type==StoreType::Null) {
+            store->as<Value>() = Value(); // unselect Union or Any
+            break;
+        }
+
         if(desc->code==TypeCode::Any) {
             // assigning variant union.
             auto& val = store->as<Value>();
             if(type==StoreType::Compound) {
                 val = *reinterpret_cast<const Value*>(ptr);
                 break;
+
             } else {
                 val = Value::Helper::build(ptr, type);
                 break;
diff --git a/src/pvxs/data.h b/src/pvxs/data.h
index b2c33c8..41fe11e 100644
--- a/src/pvxs/data.h
+++ b/src/pvxs/data.h
@@ -37,6 +37,8 @@ enum struct StoreType : uint8_t {
     Array,    //!< shared_array<const void>
 };
 
+constexpr struct unselect_t {} unselect;
+
 namespace impl {
 struct FieldStorage;
 struct FieldDesc;
@@ -87,6 +89,10 @@ template<>
 struct StorageMap<Value>
 { typedef Value store_t;   static constexpr StoreType code{StoreType::Compound}; };
 
+template<>
+struct StorageMap<unselect_t>
+{ typedef unselect_t store_t; static constexpr StoreType code{StoreType::Null}; };
+
 template<typename T>
 using StoreAs = StorageMap<typename std::decay<T>::type>;
 
@@ -229,7 +235,7 @@ CASE(std::string, std::string, String);
 
 #undef CASE
 
-} // namespace
+} // namespace impl
 
 //! Definition of a member of a Struct/Union for use with TypeDef
 struct Member {
diff --git a/test/testdata.cpp b/test/testdata.cpp
index 335f6e9..eb3cf96 100644
--- a/test/testdata.cpp
+++ b/test/testdata.cpp
@@ -83,11 +83,16 @@ void testAssignUnion()
 
     testEq(val.nameOf(val["->"]), "s");
 
+    val = unselect;
+
+    testFalse(val["->"].valid());
+
     testThrows<std::invalid_argument>([&val](){
         val["->u16"] = "hello";
     });
 
-    //val = nullptr;
+    // previous selection succeeds, but assignment fails
+    testTrue(val["->"].valid());
 }
 
 void testName()