out_value
----------------

Motivation
~~~~~~~~~~

This policy makes it possible to wrap functions that take non-const references
or pointer to non-const as it's parameters with the intention to write return 
values to them. Since it's impossible to pass references to primitive types
in lua, this policy will add another return value with the value after the
call. If the function already has one return value, one instance of this
policy will add another return value (read about multiple return values in
the lua manual).

Defined in
~~~~~~~~~~

.. parsed-literal::

    #include <luabind/out_value_policy.hpp>

Synopsis
~~~~~~~~

.. parsed-literal::

    out_value(index, policies = none)


Parameters
~~~~~~~~~~

=============== =============================================================
Parameter       Purpose
=============== =============================================================
``index``       The index of the parameter to be used as an out parameter.
``policies``    The policies used internally to convert the out parameter
                to/from Lua. ``_1`` means **to** C++, ``_2`` means **from**
                C++.
=============== =============================================================

Example
~~~~~~~

.. parsed-literal::

    void f1(float& val) { val = val + 10.f; }
    void f2(float\* val) { \*val = \*val + 10.f; }

    module(L)
    [
        def("f", &f, **out_value(_1)**)
    ];

    Lua 5.0  Copyright (C) 1994-2003 Tecgraf, PUC-Rio
    > print(f1(10))
    20
    > print(f2(10))
    20