[container.vector-interface] add back operator[]

dune/xt/la/container/common.hh

@@ -285,6 +285,16 @@ protected:
   }
 
 public:
+  inline ScalarType& operator[](const size_t ii)
+  {
+    return get_entry_ref(ii);
+  }
+
+  inline const ScalarType& operator[](const size_t ii) const
+  {
+    return get_entry_ref(ii);
+  }
+
   /// \}
   /// \name These methods override default implementations from VectorInterface.
   /// \{

dune/xt/la/container/vector-interface.hh

@@ -136,6 +136,26 @@ public:
     return true;
   } // ... valid()
 
+  /**
+   * \brief Get writable reference to the iith entry.
+   * \note \attention The returned reference may be invalid once (any entry of) the vector is changed!
+   */
+  inline ScalarType& operator[](const size_t ii)
+  {
+    CHECK_CRTP(this->as_imp()[ii]);
+    return this->as_imp()[ii];
+  }
+
+  /**
+   * \brief Get read-only reference to the iith entry.
+   * \note \attention The returned reference may be invalid once (any entry of) the vector is changed!
+   */
+  inline const ScalarType& operator[](const size_t ii) const
+  {
+    CHECK_CRTP(this->as_imp()[ii]);
+    return this->as_imp()[ii];
+  }
+
   /**
    * \brief   The dimension of the vector.
    * \return  The dimension of the vector.