trait for hooking special handles / pointers

template <typename T>
struct unique_usertype_traits {
        typedef T type;
        typedef T actual_type;
        static const bool value = false;

        static bool is_null(const actual_type&) {...}

        static type* get (const actual_type&) {...}

This is a customization point for users who need to work with special kinds of pointers/handles. The traits type alerts the library that a certain type is to be pushed as a special userdata with special deletion / destruction semantics, like many smart pointers / custom smart pointers / handles. It is already defined for std::unique_ptr<T, D> and std::shared_ptr<T> and works properly with those types (see shared_ptr here and unique_ptr here for examples). You can specialize this to get unique_usertype_traits semantics with your code. For example, here is how boost::shared_ptr<T> would look:

namespace sol {
        template <typename T>
        struct unique_usertype_traits<boost::shared_ptr<T>> {
                typedef T type;
                typedef boost::shared_ptr<T> actual_type;
                static const bool value = true;

                static bool is_null(const actual_type& value) {
                        return value == nullptr;

                static type* get (const actual_type& p) {
                        return p.get();

This will allow the library to properly handle boost::shared_ptr<T>, with ref-counting and all. The type is the type that lua and sol will interact with, and will allow you to pull out a non-owning reference / pointer to the data when you just ask for a plain T* or T& or T using the getter functions and properties of Sol. The actual_type is just the “real type” that controls the semantics (shared, unique, CComPtr, ComPtr, OpenGL handles, DirectX objects, the list goes on).


If is_null triggers (returns true), a nil value will be pushed into Lua rather than an empty structure.