Added limitations

Author: cschreib

README.md

@@ -53,3 +53,13 @@ int main() {
     return 0;
 }
 ```
+
+## Limitation
+
+ - Because of the unique ownership, weak pointers locking cannot extend the lifetime of the pointed object, hence `observable_unique_ptr` provides less thread-safety compared to `std::shared_ptr`.
+ - `observable_unique_ptr` does not support pointers to arrays, but `std::unique_ptr` and `std::shared_ptr` both do.
+ - `observable_unique_ptr` does not support custom allocators, but `std::shared_ptr` does.
+ - `observable_unique_ptr` does not have a `release()` function to let go of the ownership, which `std::unique_ptr` has.
+ - `observable_unique_ptr` allows moving from other `observable_unique_ptr` only if the deleter type is exactly the same, while `std::unique_ptr` allows moving from a convertible deleter.
+ - `observable_unique_ptr may or may not own a deleter instance; if in doubt, check `has_deleter()`` before calling `get_deleter()`, or use `try_get_deleter()`.
+ - a moved-from `observable_unique_ptr` will not own a deleter instance.