If you find that the higher level abstractions are not meeting your needs, you may want to delve into the ``stack`` namespace to try and get more out of Sol. ``stack.hpp`` and the ``stack`` namespace define several utilities to work with Lua, including pushing / popping utilities, getters, type checkers, Lua call helpers and more. This namespace is not thoroughly documented as the majority of its interface is mercurial and subject to change between releases to either heavily boost performance or improve the Sol :doc:`api<top>`.
There are, however, a few :ref:`template customization points<extension_points>` that you may use for your purposes and a handful of potentially handy functions. These may help if you're trying to slim down the code you have to write, or if you want to make your types behave differently throughout the Sol stack. Note that overriding the defaults **can** throw out many of the safety guarantees Sol provides: therefore, modify the :ref:`extension points<extension_points>` at your own discretion.
Retrieves the value of the object at ``index`` in the stack. The return type varies based on ``T``: with primitive types, it is usually ``T``: for all unrecognized ``T``, it is generally a ``T&`` or whatever the extension point :ref:`stack::getter\<T><getter>` implementation returns. The type ``T`` has top-level ``const`` qualifiers and reference modifiers removed before being forwarded to the extension point :ref:`stack::getter\<T><getter>` struct. ``stack::get`` will default to forwarding all arguments to the :ref:`stack::check_get<stack-check-get>` function with a handler of ``type_panic`` to strongly alert for errors, if you ask for the :ref:`safety<safety>`.
Checks if the object at ``index`` is of type ``T``. If it is not, it will call the ``handler`` function with ``lua_State*``, ``int index``, ``type`` expected, and ``type`` actual as arguments.
int push( lua_State* L, Arg&& arg, Args&&... args )
// recursively call the the above "push" with T inferred, one for each argument
template <typename... Args>
int multi_push( lua_State* L, Args&&... args )
Based on how it is called, pushes a variable amount of objects onto the stack. in 99% of cases, returns for 1 object pushed onto the stack. For the case of a ``std::tuple<...>``, it recursively pushes each object contained inside the tuple, from left to right, resulting in a variable number of things pushed onto the stack (this enables multi-valued returns when binding a C++ function to a Lua). Can be called with ``sol::stack::push<T>( L, args... )`` to have arguments different from the type that wants to be pushed, or ``sol::stack::push( L, arg, args... )`` where ``T`` will be inferred from ``arg``. The final form of this function is ``sol::stack::multi_push``, which will call one ``sol::stack::push`` for each argument. The ``T`` that describes what to push is first sanitized by removing top-level ``const`` qualifiers and reference qualifiers before being forwarded to the extension point :ref:`stack::pusher\<T><pusher>` struct.
auto check_get( lua_State* L, int index, Handler&& handler )
Retrieves the value of the object at ``index`` in the stack, but does so safely. It returns an ``optional<U>``, where ``U`` in this case is the return type deduced from ``stack::get<T>``. This allows a person to properly check if the type they're getting is what they actually want, and gracefully handle errors when working with the stack if they so choose to. You can define ``SOL_CHECK_ARGUMENTS`` to turn on additional :ref:`safety<safety>`, in which ``stack::get`` will default to calling this version of the function with a handler of ``type_panic`` to strongly alert for errors and help you track bugs if you suspect something might be going wrong in your system.
template <bool global = false, typename Key, typename Value>
void set_field( lua_State* L, Key&& k, Value&& v );
template <bool global = false, typename Key, typename Value>
void set_field( lua_State* L, Key&& k, Value&& v, int objectindex);
Sets the field referenced by the key ``k`` to the given value ``v``, by pushing the key onto the stack, pushing the value onto the stack, and then doing the equivalent of ``lua_setfield`` for the object at the given ``objectindex``. Performs optimizations and calls faster verions of the function if the type of ``Key`` is considered a c-style string and/or if its also marked by the templated ``global`` argument to be a global.
..code-block:: cpp
:caption: function: get_field
template <bool global = false, typename Key>
void get_field( lua_State* L, Key&& k [, int objectindex] );
Gets the field referenced by the key ``k``, by pushing the key onto the stack, and then doing the equivalent of ``lua_getfield``. Performs optimizations and calls faster verions of the function if the type of ``Key`` is considered a c-style string and/or if its also marked by the templated ``global`` argument to be a global.
This function leaves the retrieved value on the stack.
.._extension_points:
objects (extension points)
--------------------------
The structs below are already overriden for a handful of types. If you try to mess with them for the types ``sol`` has already overriden them for, you're in for a world of thick template error traces and headaches. Overriding them for your own user defined types should be just fine, however.
This is an SFINAE-friendly struct that is meant to expose static function ``get`` that returns a ``T``, or something convertible to it. The default implementation assumes ``T`` is a usertype and pulls out a userdata from Lua before attempting to cast it to the desired ``T``. There are implementations for getting numbers (``std::is_floating``, ``std::is_integral``-matching types), getting ``std::string`` and ``const char*``, getting raw userdata with :doc:`userdata_value<types>` and anything as upvalues with :doc:`upvalue_index<types>`, getting raw `lua_CFunction`_ s, and finally pulling out Lua functions into ``std::function<R(Args...)>``. It is also defined for anything that derives from :doc:`sol::reference<reference>`. It also has a special implementation for the 2 standard library smart pointers (see :doc:`usertype memory<usertype_memory>`).
This is an SFINAE-friendly struct that is meant to expose static function ``push`` that returns the number of things pushed onto the stack. The default implementation assumes ``T`` is a usertype and pushes a userdata into Lua with a :ref:`usertype_traits\<T><usertype-traits>` metatable associated with it. There are implementations for pushing numbers (``std::is_floating``, ``std::is_integral``-matching types), getting ``std::string`` and ``const char*``, getting raw userdata with :doc:`userdata<types>` and raw upvalues with :doc:`upvalue<types>`, getting raw `lua_CFunction`_ s, and finally pulling out Lua functions into ``sol::function``. It is also defined for anything that derives from :doc:`sol::reference<reference>`. It also has a special implementation for the 2 standard library smart pointers (see :doc:`usertype memory<usertype_memory>`).
This is an SFINAE-friendly struct that is meant to expose static function ``check`` that returns the number of things pushed onto the stack. The default implementation simply checks whether the expected type passed in through the template is equal to the type of the object at the specified index in the Lua stack. The default implementation for types which are considered ``userdata`` go through a myriad of checks to support checking if a type is *actually* of type ``T`` or if its the base class of what it actually stored as a userdata in that index. Down-casting from a base class to a mroe derived type is, unfortunately, impossible to do.
