session and insert query builder progress

include/matador/object/object_ptr.hpp

@@ -8,24 +8,39 @@
 #include <memory>
 
 namespace matador::object {
+struct null_object_ptr_t {
+  constexpr null_object_ptr_t() = default;
+};
+
+inline constexpr null_object_ptr_t nullobj{};
+
 template <typename Type>
 class object_ptr {
 public:
   object_ptr()
   : proxy_(std::make_shared<object_proxy<Type>>()) {}
+  object_ptr(null_object_ptr_t) {}
   explicit object_ptr(std::shared_ptr<Type> obj)
   : proxy_(std::make_shared<object_proxy<Type>>(obj)) {}
   explicit object_ptr(std::shared_ptr<object_proxy<Type>> obj)
   : proxy_(std::move(obj)) {}
   object_ptr(const object_ptr &other) = default;
   object_ptr(object_ptr &&other) noexcept = default;
-  object_ptr &operator=(const object_ptr &other) = default;
-  object_ptr &operator=(object_ptr &&other) = default;
+  object_ptr& operator=(const object_ptr &other) = default;
+  object_ptr& operator=(object_ptr &&other) = default;
+  object_ptr& operator=(null_object_ptr_t) {
+    proxy_.reset();
+    return *this;
+  }
 
   bool operator==(const object_ptr &other) const {
     return get() == other.get();
   }
+  bool operator==(null_object_ptr_t) const {
+    return empty();
+  }
   bool operator!=(const object_ptr &other) const { return !operator==(other); }
+  bool operator!=(null_object_ptr_t) const { return !empty(); }
 
   using value_type = Type;