sol2/documentation/source/api/policies.rst

policies
========
*stack modification right before lua call returns*

``sol::policies`` is an advanced, low-level modification feature allowing you to take advantage of sol3's abstractions before applying your own stack-based modifications at the last moment. They cover the same functionality as `luabind's "return reference to" and "dependency"`_ types. A few pre-rolled policies are defined for your use:

+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| policy                             | usage                                                                                              | modification                                                                                                                                                                |
+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``sol::returns_self``              | ``sol::policies( some_function, sol::returns_self() )``                                            | - takes the argument at stack index 1 (``self`` in member function calls and lambdas that take a specific userdata first) and makes that to be the return value             |
|                                    |                                                                                                    | - rather than creating a new userdata that references the same C++ memory, it copies the userdata, similar to writing ``obj2 = obj1`` just increases the reference count    |
|                                    |                                                                                                    | - saves memory space on top of keeping original memory alive                                                                                                                |
+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``sol::returns_self_with<int...>`` | ``sol::policies( some_function, sol::returns_self_with<2, 3>() )``                                 | - same as above, with the caveat that the ``self`` is returned while also putting dependencies into the ``self``                                                            |
|                                    |                                                                                                    | - can keep external dependencies alive                                                                                                                                      |
+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``sol::self_dependency``           | ``sol::policies( some_function, sol::self_dependency() );``                                        | - this makes the value returned by the bindable take a dependency on the ``self`` argument                                                                                  |
|                                    |                                                                                                    | - useful for returning a reference to a member variable and keeping the parent class of that member variable alive                                                          |
+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``sol::stack_dependencies``        | ``sol::policies( some_function, sol::stack_dependencies( target_index, 2, 1, ... ) );``            | - whatever is at ``target_index`` on the stack is given a special "keep alive" table with the elements on the stack specified by the integer indices after ``target_index`` |
|                                    |                                                                                                    | - allows you to keep arguments and other things alive for the duration of the existence of the class                                                                        |
+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| custom                             | ``sol::policies( some_function, [](lua_State* L, int current_stack_return_count) -> int { ... } )``| - whatever you want, so long as it has the form ``int (lua_State*, int )``                                                                                                  |
|                                    |                                                                                                    | - works with callables (such as lambdas), so long as it has the correct form                                                                                                |
|                                    |                                                                                                    | - expected to return the number of things on the stack to return to Lua                                                                                                     |
+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| - "some_function" can be any callable function, member variable, or similar                                                                                                                                                                                                                                           |
| - dependency additions only work on userdata                                                                                                                                                                                                                                                                          |
|                                                                                                                                                                                                                                                                                                                       |
| - works with ``table::set( ... )``, ``table::set_function( ... );``, and on all usertype bindings                                                                                                                                                                                                                     |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

You can specify multiple policies on the same ``sol::policies`` call, and can also specify custom policies as long as the signature is correct.

.. _luabind's "return reference to" and "dependency": http://www.rasterbar.com/products/luabind/docs.html#dependency