diff --git a/compiler/cpp/src/parse/t_struct.h b/compiler/cpp/src/parse/t_struct.h
index b82933d..ffe2af3 100644
--- a/compiler/cpp/src/parse/t_struct.h
+++ b/compiler/cpp/src/parse/t_struct.h
@@ -44,24 +44,59 @@
     t_type(program),
     is_xception_(false),
     is_union_(false),
+    members_validated(false),
+    members_with_value(0),
     xsd_all_(false) {}
 
   t_struct(t_program* program, const std::string& name) :
     t_type(program, name),
     is_xception_(false),
     is_union_(false),
+    members_validated(false),
+    members_with_value(0),
     xsd_all_(false) {}
 
   void set_name(const std::string& name) {
     name_ = name;
+    validate_union_members();
   }
 
   void set_xception(bool is_xception) {
     is_xception_ = is_xception;
   }
 
+  void validate_union_member( t_field * field) {
+    if( is_union_ && (! name_.empty())) {
+
+      // unions can't have required fields
+      if( field->get_req() == t_field::T_REQUIRED) {
+        pwarning(  1, "Required field %s of union %s set to optional.\n", field->get_name().c_str(), name_.c_str());
+        field->set_req( t_field::T_OPTIONAL);
+      }
+
+      // unions may have up to one member defaulted, but not more
+      if( field->get_value() != NULL) {
+        if( 1 < ++members_with_value) {
+          throw "Error: Field "+field->get_name()+" provides another default value for union "+name_;
+        }
+      }
+    }
+    
+  }
+
+  void validate_union_members() {
+    if( is_union_ && (! name_.empty()) && (!members_validated)) {
+      members_type::const_iterator m_iter;
+      for (m_iter = members_in_id_order_.begin(); m_iter != members_in_id_order_.end(); ++m_iter) {
+        validate_union_member( *m_iter);
+      }
+      members_validated = true;          
+    }
+  }
+
   void set_union(bool is_union) {
     is_union_ = is_union;
+    validate_union_members();
   }
 
   void set_xsd_all(bool xsd_all) {
@@ -83,6 +118,7 @@
       return false;
     }
     members_in_id_order_.insert(bounds.second, elem);
+    validate_union_member( elem);
     return true;
   }
 
@@ -141,6 +177,8 @@
   members_type members_in_id_order_;
   bool is_xception_;
   bool is_union_;
+  bool members_validated;
+  int  members_with_value;
 
   bool xsd_all_;
 };