36 lines
8.3 KiB
ReStructuredText
36 lines
8.3 KiB
ReStructuredText
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policies
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========
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*stack modification right before lua call returns*
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``sol::policies`` is an advanced, low-level modification feature allowing you to take advantage of sol2'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:
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+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| policy | usage | modification |
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+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| ``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 |
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| | | - 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 |
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| | | - saves memory space on top of keeping original memory alive |
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+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| ``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`` |
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| | | - can keep external dependencies alive |
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+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| ``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 |
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| | | - useful for returning a reference to a member variable and keeping the parent class of that member variable alive |
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+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| ``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`` |
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| | | - allows you to keep arguments and other things alive for the duration of the existence of the class |
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+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| 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 )`` |
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| | | - works with callables (such as lambdas), so long as it has the correct form |
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| | | - expected to return the number of things on the stack to return to Lua |
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+------------------------------------+----------------------------------------------------------------------------------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| - "some_function" can be any callable function, member variable, or similar |
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| - dependency additions only work on userdata |
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| - works with ``table::set( ... )``, ``table::set_function( ... );``, and on all usertype bindings |
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+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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You can specify multiple policies on the same ``sol::policies`` call, and can also specify custom policies as long as the signature is correct.
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.. _luabind's "return reference to" and "dependency": http://www.rasterbar.com/products/luabind/docs.html#dependency
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