in place modification function there, but it won't be documented because it's a BAD IDEA

examples/shared_ptr_modify_in_place.cpp

@@ -0,0 +1,52 @@
+#define SOL_CHECK_ARGUMENTS 1
+#include <sol.hpp>
+
+#include <iostream>
+
+struct ree {
+	int value = 1;
+	ree() {}
+	ree(int v) : value(v) {}
+};
+
+int main() {
+
+	std::cout << "=== special pointers -- modify in place ===" << std::endl;
+
+	sol::state lua;
+
+	auto new_shared_ptr = [](sol::stack_object obj) {
+		// works just fine
+		sol::stack::modify_unique_usertype(obj, [](std::shared_ptr<ree>& sptr) {
+			sptr = std::make_shared<ree>(sptr->value + 1);
+		});
+	};
+
+	auto reset_shared_ptr = [](sol::stack_object obj) {
+		sol::stack::modify_unique_usertype(obj, [](std::shared_ptr<ree>& sptr) {
+			// THIS IS SUCH A BAD IDEA AAAGH
+			sptr.reset();
+			// DO NOT reset to nullptr:
+			// change it to an actual NEW value...
+			// otherwise you will inject a nullptr into the userdata representation...
+			// which will NOT compare == to nil
+		});
+	};
+
+	lua.set_function("f", new_shared_ptr);
+	lua.set_function("f2", reset_shared_ptr);
+	lua.set_function("g", [](ree* r) {
+		std::cout << r->value << std::endl;
+	});
+
+	lua["p"] = std::make_shared<ree>();
+	lua.script("g(p) -- okay");
+	lua.script("f(p)");
+	lua.script("g(p) -- okay");
+	// uncomment the below for
+	// segfault/read-access violation
+	lua.script("f2(p)");
+	//lua.script("g(p) -- kaboom");
+
+	return 0;
+}

single/sol/sol.hpp

@@ -20,8 +20,8 @@
 // CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 // This file was generated with a script.
-// Generated 2018-03-29 13:24:07.253227 UTC
+// Generated 2018-03-30 08:50:17.354157 UTC
-// This header was generated with sol v2.19.5 (revision b879eb0)
+// This header was generated with sol v2.19.5 (revision 035c2f8)
 // https://github.com/ThePhD/sol2
 
 #ifndef SOL_SINGLE_INCLUDE_HPP
@@ -7683,6 +7683,24 @@ namespace sol {
 		void raw_set_field(lua_State* L, Key&& key, Value&& value, int tableindex) {
 			set_field<global, true>(L, std::forward<Key>(key), std::forward<Value>(value), tableindex);
 		}
+
+		template <typename T, typename F>
+		inline void modify_unique_usertype_as(stack_object obj, F&& f) {
+			typedef unique_usertype_traits<T> u_traits;
+			void* raw = lua_touserdata(obj.lua_state(), obj.stack_index());
+			void* ptr_memory = detail::align_usertype_pointer(raw);
+			void* uu_memory = detail::align_usertype_unique<T>(raw);
+			T& uu = *static_cast<T*>(uu_memory);
+			f(uu);
+			*static_cast<void**>(ptr_memory) = static_cast<void*>(u_traits::get(uu));
+		}
+
+		template <typename F>
+		inline void modify_unique_usertype(stack_object obj, F&& f) {
+			typedef meta::bind_traits<meta::unqualified_t<F>> bt;
+			typedef typename bt::template arg_at<0> T;
+			modify_unique_usertype_as<meta::unqualified_t<T>>(obj, std::forward<F>(f));
+		}
 	} // namespace stack
 } // namespace sol
 

single/sol/sol_forward.hpp

@@ -20,8 +20,8 @@
 // CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 // This file was generated with a script.
-// Generated 2018-03-29 13:24:07.463940 UTC
+// Generated 2018-03-30 08:50:17.586274 UTC
-// This header was generated with sol v2.19.5 (revision b879eb0)
+// This header was generated with sol v2.19.5 (revision 035c2f8)
 // https://github.com/ThePhD/sol2
 
 #ifndef SOL_SINGLE_INCLUDE_FORWARD_HPP

sol/stack_core.hpp

@@ -870,6 +870,24 @@ namespace sol {
 		void raw_set_field(lua_State* L, Key&& key, Value&& value, int tableindex) {
 			set_field<global, true>(L, std::forward<Key>(key), std::forward<Value>(value), tableindex);
 		}
+
+		template <typename T, typename F>
+		inline void modify_unique_usertype_as(stack_object obj, F&& f) {
+			typedef unique_usertype_traits<T> u_traits;
+			void* raw = lua_touserdata(obj.lua_state(), obj.stack_index());
+			void* ptr_memory = detail::align_usertype_pointer(raw);
+			void* uu_memory = detail::align_usertype_unique<T>(raw);
+			T& uu = *static_cast<T*>(uu_memory);
+			f(uu);
+			*static_cast<void**>(ptr_memory) = static_cast<void*>(u_traits::get(uu));
+		}
+
+		template <typename F>
+		inline void modify_unique_usertype(stack_object obj, F&& f) {
+			typedef meta::bind_traits<meta::unqualified_t<F>> bt;
+			typedef typename bt::template arg_at<0> T;
+			modify_unique_usertype_as<meta::unqualified_t<T>>(obj, std::forward<F>(f));
+		}
 	} // namespace stack
 } // namespace sol