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.