BUG: DataSet assignment operator is missing

* Some compilers complain if the copy constructor is given explicitly
  but the assignment operator is implicitly set to default.

* Explicitly defining the assignment operator allows us to properly
  handle reference counters for shared resources.

c++/src/H5DataSet.cpp

@@ -76,6 +76,27 @@ DataSet::DataSet(const DataSet &original) : H5Object(), AbstractDs(), id(origina
     incRefCount(); // increment number of references to this id
 }
 
+//--------------------------------------------------------------------------
+// Function:    DataSet assignment operator
+///\brief       Assignment operator: same HDF5 object as \a original
+///\param       original - IN: DataSet instance to copy
+// Programmer   Lee Newberg - 2021
+//--------------------------------------------------------------------------
+DataSet &
+DataSet::operator=(const DataSet &original)
+{
+    // Wrap the changes to `*this` by an incremented reference count for
+    // `original` to prevent trouble in the case that `*this` and
+    // `original` share reference counts or memory in any relevant
+    // sense.
+    original.incRefCount();
+    decRefCount(); // for old value of id
+    id = original.id;
+    incRefCount(); // for new value of id
+    original.decRefCount();
+    return *this;
+}
+
 //--------------------------------------------------------------------------
 // Function:    DataSet overload constructor - dereference
 ///\brief       Given a reference, ref, to an hdf5 location, creates a

c++/src/H5DataSet.h

@@ -117,6 +117,9 @@ class H5_DLLCPP DataSet : public H5Object, public AbstractDs {
     // Copy constructor - same as the original DataSet.
     DataSet(const DataSet &original);
 
+    // Assignment operator
+    DataSet &operator=(const DataSet &original);
+
     // Creates a copy of an existing DataSet using its id.
     DataSet(const hid_t existing_id);