iox-eclipse-iceoryx#1104 Add release notes for non nullable function

Signed-off-by: Christian Eltzschig <[email protected]>

doc/website/release-notes/iceoryx-unreleased.md

@@ -87,6 +87,7 @@
 - Extract `iceoryx_hoofs/platform` into separate package `iceoryx_platform` [\#1615](https://github.com/eclipse-iceoryx/iceoryx/issues/1615)
 - `cxx::unique_ptr` is no longer nullable [\#1104](https://github.com/eclipse-iceoryx/iceoryx/issues/1104)
 - Use builder pattern in mutex [\#1036](https://github.com/eclipse-iceoryx/iceoryx/issues/1036)
+- `cxx::function` is no longer nullable [\#1104](https://github.com/eclipse-iceoryx/iceoryx/issues/1104)
 
 **Workflow:**
 
@@ -461,3 +462,26 @@
         .create(myMutex);
     myMutex->lock();
     ```
+
+24. `cxx::function` is no longer nullable.
+
+    ```cxx
+    // before
+    cxx::function<void()> helloFunc = []{ std::cout << "hello world\n"; };
+    cxx::function<void()> emptyFunction;
+
+    if (helloFunc) { // required since the object could always be null
+        helloFunc();
+    }
+
+    // after
+    cxx::function<void()> helloFunc = []{ std::cout << "hello world\n"; };
+    cxx::optional<cxx::function<void()>> emptyPtr(cxx::nullopt); // if function shall be nullable use cxx::optional
+
+    // no more null check required since it is no longer nullable
+    helloFunc();
+    ```
+
+    Compilers like ``gcc-12>`` and `clang>14` as well as static code analysis tools like `clang-tidy`
+    will warn the user with a used after move warning when one accesses a moved object. Accessing
+    a moved `unique_ptr` is well defined and behaves like dereferencing a `nullptr`.