Fix std::string in union.

The setup before this commit violates the C++ standard.
If two classes with non-standard constructors and/or
destructors are in a union their constructors and/or
destructors have to be called explicitly when
switching between the value types.

Using a union is also unsafe as it's illegal to
access the "wrong" of the two members. But the API
doesn't have a way to tell which member is the active.

The new solution doesn't use unions but allows clients
to safely access even the wrong value.

sources/include/citygml/externalreference.h

@@ -5,14 +5,23 @@
 
 
 namespace citygml {
-    union LIBCITYGML_EXPORT ExternalObjectReference {
+    class LIBCITYGML_EXPORT ExternalObjectReference {
+    public:
+        ExternalObjectReference() = default;
+        ~ExternalObjectReference() = default;
+
+        const std::string& getName() const;
+        const std::string& getUri() const;
+        void setName(const std::string& name);
+        void setUri(const std::string& uri);
+
+    private:
         PRAGMA_WARN_DLL_BEGIN
-        std::string name;
-        std::string uri;
+        std::string value;
         PRAGMA_WARN_DLL_END
-        
-        ExternalObjectReference();
-        ~ExternalObjectReference();
+
+        enum class ObjectRefType { NAME, URI };
+        ObjectRefType type;
     };
 
     class LIBCITYGML_EXPORT ExternalReference: public Object {

sources/src/citygml/externalreference.cpp

@@ -1,10 +1,34 @@
 #include <citygml/externalreference.h>
 
 namespace citygml {
-    ExternalObjectReference::ExternalObjectReference() {
+    namespace {
+        const std::string EMPTY_STRING("");
+    } // anonymous namespace
+
+    const std::string& ExternalObjectReference::getName() const {
+        if (type == ObjectRefType::NAME) {
+            return value;
+        } else {
+            return EMPTY_STRING;
+        }
+    }
+
+    const std::string& ExternalObjectReference::getUri() const {
+        if (type == ObjectRefType::URI) {
+            return value;
+        } else {
+            return EMPTY_STRING;
+        }
+    }
+
+    void ExternalObjectReference::setName(const std::string& name) {
+        type = ObjectRefType::NAME;
+        value = name;
     }
 
-    ExternalObjectReference::~ExternalObjectReference() {
+    void ExternalObjectReference::setUri(const std::string& uri) {
+        type = ObjectRefType::URI;
+        value = uri;
     }
 
     ExternalReference::ExternalReference(std::string const& id) : Object(id) {

sources/src/parser/externalreferenceparser.cpp

@@ -60,9 +60,9 @@ namespace citygml {
         } else if (node == NodeType::CORE_InformationSystemNode) {
             model->informationSystem = characters;
         } else if (node == NodeType::CORE_NameNode) {
-            model->externalObject.name = characters;
+            model->externalObject.setName(characters);
         } else if (node == NodeType::CORE_UriNode) {
-            model->externalObject.uri = characters;
+            model->externalObject.setUri(characters);
         }
 
         return GMLObjectElementParser::parseChildElementEndTag(node, characters);