add Session::is_closed method (eclipse-zenoh#282)

include/zenoh/api/session.hxx

@@ -1038,6 +1038,7 @@ class Session : public Owned<::z_owned_session_t> {
     /// @param err if not null, the result code will be written to this location, otherwise ZException exception will be
     /// thrown in case of error.
     /// @return declared ``zenoh::ext::PublicationCache`` instance.
+    /// @note Zenoh-c only.
     [[nodiscard]] ext::PublicationCache declare_publication_cache(
         const KeyExpr& key_expr, PublicationCacheOptions&& options = PublicationCacheOptions::create_default(),
         ZResult* err = nullptr) {
@@ -1081,7 +1082,10 @@ class Session : public Owned<::z_owned_session_t> {
                                                                 interop::as_loaned_c_ptr(key_expr), &opts);
         __ZENOH_RESULT_CHECK(res, err, "Failed to declare Background Publication Cache");
     }
-
 #endif
+
+    /// @brief Check if session is closed.
+    /// @return ``true`` if session is closed, ``false`` otherwise.
+    bool is_closed() const { return ::z_session_is_closed(interop::as_loaned_c_ptr(*this)); }
 };
 }  // namespace zenoh