add Field_t as inheritance of std::varinat<>

src/field.h

@@ -26,7 +26,8 @@ class Field;
 
 using FieldFloat = Field<float>;
 using FieldDouble = Field<double>;
-using Field_t = std::variant<FieldFloat, FieldDouble>;
+class Field_t;
+//using Field_t = std::variant<FieldFloat, FieldDouble>;
 
 template<typename T>
 class Field3D;
@@ -216,6 +217,73 @@ inline Field3D_t Field3D<T>::init(const CdoVar& var) {
   }
 }
 
+/************************************************
+ ************************************************
+ ****************** Field_t  ********************
+ ************************************************
+ ***********************************************/
+
+using _var_ = std::variant<FieldDouble, FieldFloat>;
+
+class Field_t : public _var_ {
+public:
+  // Overwrites of the std::variant<> constructors
+  constexpr Field_t() noexcept;
+
+  Field_t(Field_t& other)
+  : _var_(other) {  }
+
+  template< class T >
+  constexpr Field_t( T&& t ) noexcept
+  : _var_(std::move(t)) {  }
+
+  template< class T,
+            class... Args >
+  constexpr explicit Field_t( std::in_place_type_t<T>,
+                            Args&&... args )
+  : _var_(std::in_place_type_t<T>{}, args...) {  }
+
+  template< class T,
+          class U,
+          class... Args >
+  constexpr explicit Field_t(std::in_place_type_t<T>,
+                            std::initializer_list<U> il,
+                            Args&&... args )
+  : _var_(std::in_place_type_t<T>{}, il, args...) {  }
+
+  template< std::size_t I,
+            class... Args >
+  constexpr explicit Field_t( std::in_place_index_t<I>,
+                            Args&&... args )
+  : _var_(std::in_place_index_t<I>{}, args...) {  }
+
+  template< std::size_t I,
+          class U,
+          class... Args >
+  constexpr explicit Field_t( std::in_place_index_t<I>,
+                            std::initializer_list<U> il,
+                            Args&&... args )
+  : _var_(std::in_place_index_t<I>{}, il, args...) {  }
+
+  // Overwrites of the std::variant<> operator=
+  Field_t& operator=( const Field_t& rhs ) {
+    _var_::operator=(rhs);
+    return *this;
+  }
+
+  Field_t& operator=( Field_t&& rhs ) noexcept {
+    _var_::operator=(std::move(rhs));
+    return *this;
+  }
+
+  template< class T >
+  Field_t& operator=( T&& t ) noexcept {
+    _var_::operator=(std::move(t));
+    return *this;
+  }
+};
+
+
 /************************************************
  ************************************************
  ************* FieldFactory  ********************