Merge pull request #17 from rob100/unique_ptr

If possible prefer unique_ptr to shared_ptr because it is more efficient

07-Considering_Performance.md

@@ -205,6 +205,14 @@ std::shared_ptr<ModelObject_Impl>(new ModelObject_Impl());
 std::make_shared<ModelObject_Impl>(); // (it's also more readable and concise)
 ```
 
+### Prefer `unique_ptr` to `shared_ptr`
+
+If possible use `unique_ptr` instead of `shared_ptr`. The `unique_ptr` does not need to keep track of its copies because it is not copyable. Because of this it is more efficient than the `shared_ptr`. Equivalent to `shared_ptr` and `make_shared` you should use `make_unique` to create the `unique_ptr`:
+
+```cpp
+std::make_unique<ModelObject_Impl>();
+```
+
 ### Get rid of std::endl
 
 `std::endl` implies a flush operation. It's equivalent to `"\n" << std::flush`.