Smart pointers

Smart pointers are provided by the cpp_core crate to make working with C++ objects from Rust easier:

• CppBox: owned, non-null (corresponds to C++ objects passed by value or pointers that semantically own the object)
• Ptr: possibly owned, possibly null (correspond to C++ pointers)
• Ref: not owned, non-null (correspond to C++ references)

Unlike Rust references, these pointers can be freely copied, producing multiple mutable pointers to the same object, which is usually necessary to do when working with C++ libraries.

qt_core provide additional special pointers for working with QObject:

• QPtr: not owned, auto-nullable
• QBox: owned unless has a parent, auto-nullable

All these smart pointers have methods for casting (static_upcast, static_downcast, dynamic_cast) and creating iterators (iter, iter_mut). They also implement operator traits by forwarding to corresponding C++ operators.