add OptionalReference::has_value

Author: thelfer

include/MGIS/Utilities/OptionalReference.hxx

@@ -22,7 +22,7 @@ namespace mgis {
    * \brief a class that may contain a reference to a class
    *
    * \note this class was introduced by of the lack of optional
-   * references befor to C++-26.
+   * references prior to C++-26.
    * \note the design of the class is loosely modeled by the
    * std::experimental::observer_ptr proposal
    */
@@ -66,6 +66,10 @@ namespace mgis {
 
     constexpr pointer operator->() const noexcept { return this->ptr; }
 
+    [[nodiscard]] constexpr bool has_value() const noexcept {
+      return this->ptr != nullptr;
+    }
+
     [[nodiscard]] constexpr operator bool() const noexcept {
       return this->ptr != nullptr;
     }