Minor refactoring of methods to get the underlying column type (#3458)

WIP for #1691

src/core/column.h

@@ -119,14 +119,14 @@ class Column
     size_t nrows() const noexcept;
     size_t na_count() const;
     const dt::Type& type() const noexcept;
-    const dt::Type& data_type() const noexcept;
     dt::LType ltype() const noexcept;
     dt::SType stype() const noexcept;
 
-    // For categorical columns this method will return the stype of the data,
-    // the column is backed up with. For all the other column types,
-    // this method is equivalent to `stype()`.
+    // For categorical columns these methods will return the stype/type
+    // of the data the column is backed up with. For all the other column types,
+    // these methods are equivalent to `stype()`/`type()`.
     dt::SType data_stype() const noexcept;
+    const dt::Type& data_type() const noexcept;
 
     size_t elemsize() const noexcept;
     bool   is_fixedwidth() const noexcept;

src/core/column/column_impl.cc

@@ -234,10 +234,13 @@ size_t ColumnImpl::null_count() const {
   return stats()->nacount();
 }
 
+const Type& ColumnImpl::data_type() const {
+  return type_.is_categorical()? child(0).type()
+                               : type_;
+}
 
 SType ColumnImpl::data_stype() const {
-  return type_.is_categorical()? child(0).stype()
-                               : stype();
+  return data_type().stype();
 }
 
 

src/core/column/column_impl.h

@@ -94,12 +94,14 @@ class ColumnImpl
   public:
     size_t nrows() const noexcept { return nrows_; }
     SType stype() const { return type_.stype(); }
-    SType data_stype() const;
     const Type& type() const { return type_; }
-    const Type& data_type() const {
-      return type_.is_categorical()? child(0).type()
-                                   : type_;
-    }
+
+    // For categorical columns these methods will return the stype/type
+    // of the data the column is backed up with. For all the other column types,
+    // these methods are equivalent to `stype()`/`type()`.
+    SType data_stype() const;
+    const Type& data_type() const;
+
     virtual bool is_virtual() const noexcept = 0;
     virtual bool computationally_expensive() const { return false; }
     virtual size_t memory_footprint() const noexcept = 0;