Add ability to take non-const & shared_ptr params

cheatsheet.md

@@ -163,6 +163,28 @@ chaiscript::Boxed_Number(chai.eval("5.3 + 2.1")).get_as<int>(); // works with an
 static_cast<int>(chai.eval<double>("5.3+2.1")); // this version only works if we know that it's a double
 ```
 
+### Conversion Caveats
+
+Conversion to `std::shared_ptr<T> &` is supported for function calls, but if you attempt to keep a reference to a `shared_ptr<>` you might invoke undefined behavior
+
+```cpp
+// ok this is supported, you can register it with chaiscript engine
+void nullify_shared_ptr(std::shared_ptr<int> &t) {
+  t == nullptr
+}
+```
+
+```cpp
+int main()
+{
+  // do some stuff and create a chaiscript instance
+  std::shared_ptr<int> &ptr = chai.eval<std::shared_ptr<int> &>(somevalue);
+  // DO NOT do this. Taking a non-const reference to a shared_ptr is not 
+  // supported and causes undefined behavior in the chaiscript engine
+}
+```
+
+
 ## Sharing Values
 
 ```

include/chaiscript/dispatchkit/boxed_cast_helper.hpp

@@ -173,6 +173,18 @@ namespace chaiscript
       {
       };
 
+    template<typename Result>
+      struct Cast_Helper_Inner<std::shared_ptr<Result> &>
+      {
+        typedef Boxed_Value::Sentinel<Result> Result_Type;
+
+        static Result_Type cast(const Boxed_Value &ob, const Type_Conversions_State *)
+        {
+          std::shared_ptr<Result> &res = ob.get().cast<std::shared_ptr<Result> >();
+          return ob.pointer_sentinel(res);
+        }
+      };
+
     /// Cast_Helper_Inner for casting to a const std::shared_ptr<const> & type
     template<typename Result>
       struct Cast_Helper_Inner<const std::shared_ptr<const Result> > : Cast_Helper_Inner<std::shared_ptr<const Result> >

include/chaiscript/dispatchkit/boxed_value.hpp

@@ -231,6 +231,50 @@ namespace chaiscript
         return m_data->m_type_info.bare_equal(ti);
       }
 
+      template<typename T>
+      struct Sentinel {
+        Sentinel(std::shared_ptr<T> &ptr, Data &data)
+          : m_ptr(ptr), m_data(data)
+        {
+        }
+
+        ~Sentinel()
+        {
+          // save new pointer data
+          m_data.get().m_data_ptr = m_ptr.get().get();
+          m_data.get().m_const_data_ptr = m_ptr.get().get();
+        }
+
+        Sentinel& operator=(Sentinel&&s) {
+          m_ptr = std::move(s.m_ptr);
+          m_data = std::move(s.m_data);
+        }
+
+        Sentinel(Sentinel &&s)
+          : m_ptr(std::move(s.m_ptr)),
+            m_data(std::move(s.m_data))
+        {
+        }
+
+        operator std::shared_ptr<T>&() const
+        {
+          return m_ptr.get();
+        }
+
+        Sentinel &operator=(const Sentinel &) = delete;
+        Sentinel(Sentinel&) = delete;
+
+        std::reference_wrapper<std::shared_ptr<T>> m_ptr;
+        std::reference_wrapper<Data> m_data;
+      };
+
+
+      template<typename T>
+      Sentinel<T> pointer_sentinel(std::shared_ptr<T> &ptr) const
+      {
+        return Sentinel<T>(ptr, *(m_data.get()));
+      }
+
       bool is_null() const CHAISCRIPT_NOEXCEPT
       {
         return (m_data->m_data_ptr == nullptr && m_data->m_const_data_ptr == nullptr);