LuaBridge 2.8 Reference Manual

Official repository is located at https://github.com/vinniefalco/LuaBridge.
Copyright © 2012 Vinnie Falco. Freely available under the terms of the MIT License.

# 1 - Introduction

LuaBridge is a lightweight and dependency-free library for mapping data, functions, and classes back and forth between C++ and Lua, a powerful, fast, lightweight, embeddable scripting language. LuaBridge has been tested and works with Lua revisions starting from 5.1.5, although it should work in any version of Lua from 5.1.0 and later. It also works transparently with LuaJIT.

LuaBridge offers the following features:

- MIT Licensed, no usage restrictions!
- Headers-only: No Makefile, no .cpp files, just one `#include`!
- Simple, light, and nothing else needed (like Boost).
- No macros, settings, or configuration scripts needed.
- Supports different object lifetime management models.
- Convenient, type-safe access to the Lua stack.
- Automatic function parameter type binding.
- Easy access to Lua objects like tables and functions.
- Written in a clear and easy to debug style.
- C++11 compliant.

LuaBridge is distributed as a a collection of header files. You simply add one line, `#include "LuaBridge/LuaBridge.h"` where you want to pass functions, classes, and variables back and forth between C++ and Lua. There are no additional source files, no compilation settings, and no Makefiles or IDE-specific project files. LuaBridge is easy to integrate.

C++ concepts like variables and classes are made available to Lua through a process called registration. Because Lua is weakly typed, the resulting structure is not rigid. The API is based on C++ template metaprogramming. It contains template code to automatically generate at compile-time the various Lua C API calls necessary to export your program's classes and functions to the Lua environment.

To expose Lua objects to C++, a class called `LuaRef` is provided. The implementation allows C++ code to access Lua objects such as numbers or strings, but more importantly to access things like tables and their values. Using this class makes idioms like calling Lua functions simple and clean.

## 1.1 - Design

LuaBridge tries to be efficient as possible when creating the "glue" that exposes C++ data and functions to Lua. At the same time, the code was written with the intention that it is all as simple and clear as possible, without resorting to obscure C++ idioms, ugly preprocessor macros, or configuration settings. Furthermore, it is designed to be "header-only", making it very easy to integrate into your projects.

Because LuaBridge was written with simplicity in mind there are some features that are not available. Although it comes close to the highest possible performance, LuaBridge is not quite the fastest, OOLua slightly outperforms LuaBridge in some tests. LuaBridge also does not try to implement every possible feature, LuaBind explores every corner of the C++ language (but it requires Boost).

LuaBridge does not support:

- Enumerated constants
- More than 8 parameters on a function or method (although this can be increased by adding more `TypeListValues` specializations).
- Overloaded functions, methods, or constructors.
- Global variables (variables must be wrapped in a named scope).
- Automatic conversion between STL container types and Lua tables (conversion can be enabled for `std::list`, `std::vector`, `std::array`, `std::map`, `std::unordered_map`, `std::pair` or `std::optional` by including `List.h`, `Vector.h`, `Array.h`, `Map.h`, `UnorderedMap.h`, `Pair.h` or `Optional.h` respectively)
- Inheriting Lua classes from C++ classes.
- Passing nil to a C++ function that expects a pointer or reference.
- Standard containers like `std::shared_ptr`.

## 1.2 - Repository

The official repository is located at https://github.com/vinniefalco/LuaBridge.

The master branch contains published library versions. Release versions are marked with tags.

These repositories are also available:

LuaBridgeUnitTests | A stand alone command line application to exercise LuaBridge functionality.
LuaBridgeUnitDemo | A stand alone GUI application that provides an interactive console.

## 1.3 - License and Credits

LuaBridge is published under the terms of the MIT License:

    Permission is hereby granted, free of charge, to any person obtaining a copy
    of this software and associated documentation files (the "Software"), to deal
    in the Software without restriction, including without limitation the rights
    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
    copies of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included in
    all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
    SOFTWARE.

The original version of LuaBridge was written by Nathan Reed. The project has been taken over by Vinnie Falco, who added new functionality and wrote the new documentation. Vinnie also incorporated `LuaRef` and other Lua to C++ binding contributions from Nigel Atkinson.

For questions, comments, or bug reports feel free to open a Github issue or contact Vinnie Falco directly at the email address indicated below.

- Copyright 2019, Dmitry Tarakanov
- Copyright 2012, Vinnie Falco <vinnie.falco@gmail.com>
- Copyright 2008, Nigel Atkinson <suprapilot+LuaCode@gmail.com>
- Copyright 2007, Nathan Reed
- Portions from The Loki Library: Copyright 2001 by Andrei Alexandrescu

Older versions of LuaBridge up to and including 0.2 (available separately) are distributed under the BSD 3-Clause License. See the corresponding license file in those versions (distributed separately) for more details.

# 2 - Accessing C++ from Lua

In order to expose C++ data and functions to Lua, each piece of exported information must be registered. There are five types of objects that LuaBridge can register:

Namespaces | A Lua table that contains other registrations.
Data | Global or static variables, data members, and static data members.
Functions | Regular functions, member functions, and static member functions.
CFunctions | A regular function, member function, or static member function that uses the `lua_CFunction` calling convention.
Properties | Global properties, property members, and static property members. These appear like data to Lua, but are implemented in C++ using functions to get and set the values.

Both data and properties can be marked as read-only at the time of registration. This is different from `const`; the values of these objects can be modified on the C++ side, but Lua scripts cannot change them. Code samples that follow are in C++ or Lua, depending on context. For brevity of exposition code samples in C++ assume the traditional variable `lua_State* L` is defined, and that a `using namespace luabridge` using-directive is in effect.

## 2.1 - Namespaces

All LuaBridge registrations take place in a namespace. When we refer to a namespace we are always talking about a namespace in the Lua sense, which is implemented using tables. The namespace need not correspond to a C++ namespace; in fact no C++ namespaces need to exist at all unless you want them to. LuaBridge namespaces are visible only to Lua scripts; they are used as a logical grouping tool. To obtain access to the global namespace we write:

    getGlobalNamespace (L);

This returns an object on which further registrations can be performed. The subsequent registrations will go into the global namespace, a practice which is not recommended. Instead, we can add our own namespace by writing:

    getGlobalNamespace (L)
      .beginNamespace ("test");

This creates a table in `_G` called "test". Since we have not performed any registrations, this table will be empty except for some bookkeeping key/value pairs. LuaBridge reserves all identifiers that start with a double underscore. So `__test` would be an invalid name (although LuaBridge will silently accept it). Functions like `beginNamespace` return the corresponding object on which we can make more registrations. Given:

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .beginNamespace ("detail")
        .endNamespace ()
        .beginNamespace ("utility")
        .endNamespace ()
      .endNamespace ();

The results are accessible to Lua as `test`, `test.detail`, and `test.utility`. Here we introduce the `endNamespace` function; it returns an object representing the original enclosing namespace. All LuaBridge functions which create registrations return an object upon which subsequent registrations can be made, allowing for an unlimited number of registrations to be chained together using the dot operator. Adding two objects with the same name, in the same namespace, results in undefined behavior (although LuaBridge will silently accept it).

A namespace can be re-opened later to add more functions. This lets you split up the registration between different source files. These are equivalent:

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .addFunction ("foo", foo)
      .endNamespace ();

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .addFunction ("bar", bar)
      .endNamespace ();

and

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .addFunction ("foo", foo)
        .addFunction ("bar", bar)
      .endNamespace ();

## 2.2 - Properties and Functions

These are registered into a namespace using `addProperty` and `addFunction`. When registered functions are called by scripts, LuaBridge automatically takes care of the conversion of arguments into the appropriate data type when doing so is possible. This automated system works for the function's return value, and up to 8 parameters although more can be added by extending the templates. Pointers, references, and objects of class type as parameters are treated specially, and explained later.

If we have:

    int globalVar;
    static float staticVar;

    std::string stringProperty;
    std::string getString () { return stringProperty; }
    void setString (std::string s) { stringProperty = s; }

    int foo () { return 42; }
    void bar (char const*) { }
    int cFunc (lua_State* L) { return 0; }

These are registered with:

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .addProperty ("var1", &globalVar)
        .addProperty ("var2", &staticVar, false) // read-only
        .addProperty ("prop1", getString, setString)
        .addProperty ("prop2", getString)            // read only
        .addFunction ("foo", foo)
        .addFunction ("bar", bar)
        .addFunction ("cfunc", cFunc)
      .endNamespace ();

Variables can be marked read-only by passing `false` in the second optional parameter. If the parameter is omitted, true is used making the variable read/write. Properties are marked read-only by omitting the set function. After the registrations above, the following Lua identifiers are valid:

    test        -- a namespace
    test.var1   -- a lua_Number property
    test.var2   -- a read-only lua_Number property
    test.prop1  -- a lua_String property
    test.prop2  -- a read-only lua_String property
    test.foo    -- a function returning a lua_Number
    test.bar    -- a function taking a lua_String as a parameter
    test.cfunc  -- a function with a variable argument list and multi-return

Note that `test.prop1` and `test.prop2` both refer to the same value. However, since `test.prop2` is read-only, assignment attempts will generate a run-time error. These Lua statements have the stated effects:

    test.var1 = 5         -- okay
    test.var2 = 6         -- error: var2 is not writable
    test.prop1 = "Hello"  -- okay
    test.prop1 = 68       -- okay, Lua converts the number to a string
    test.prop2 = "bar"    -- error: prop2 is not writable

    test.foo ()           -- calls foo and discards the return value
    test.var1 = foo ()    -- calls foo and stores the result in var1
    test.bar ("Employee") -- calls bar with a string
    test.bar (test)       -- error: bar expects a string not a table

LuaBridge does not support overloaded functions nor is it likely to in the future. Since Lua is dynamically typed, any system that tries to resolve a set of parameters passed from a script will face considerable ambiguity when trying to choose an appropriately matching C++ function signature.

## 2.3 - Class Objects

A class registration is opened using either `beginClass` or `deriveClass` and ended using `endClass`. Once registered, a class can later be re-opened for more registrations using `beginClass`. However, `deriveClass` should only be used once. To add more registrations to an already registered derived class, use `beginClass` on it.

These declarations:

    struct A {
      static int staticData;
      static float staticProperty;

      static float getStaticProperty () { return staticProperty; }
      static void setStaticProperty (float f) { staticProperty = f; }
      static void staticFunc () { }

      static int staticCFunc (lua_State *L) { return 0; }

      std::string dataMember;

      char dataProperty;
      char getProperty () const { return dataProperty; }
      void setProperty (char v) { dataProperty = v; }
      std::string toString () const { return dataMember; }

      void func1 () { }
      virtual void virtualFunc () { }

      int cfunc (lua_State* L) { return 0; }
    };

    struct B : public A {
      double dataMember2;

      void func1 () { }
      void func2 () { }
      void virtualFunc () { }
    };

    int A::staticData;
    float A::staticProperty;

are registered using:

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .beginClass <A> ("A")
          .addStaticProperty ("staticData", &A::staticData)
          .addStaticProperty ("staticProperty", &A::getStaticProperty, &A::setStaticProperty)
          .addStaticFunction ("staticFunc", &A::staticFunc)
          .addStaticFunction ("staticCFunc", &A::staticCFunc)
          .addProperty ("data", &A::dataMember)
          .addProperty ("prop", &A::getProperty, &A::setProperty)
          .addFunction ("func1", &A::func1)
          .addFunction ("virtualFunc", &A::virtualFunc)
          .addFunction ("__tostring", &A::toString)     // Metamethod
          .addFunction ("cfunc", &A::cfunc)
        .endClass ()
        .deriveClass <B, A> ("B")
          .addProperty ("data", &B::dataMember2)
          .addFunction ("func1", &B::func1)
          .addFunction ("func2", &B::func2)
        .endClass ()
      .endNameSpace ();

Method registration works just like function registration. Virtual methods work normally; no special syntax is needed. const methods are detected and const-correctness is enforced, so if a function returns a const object (or a container holding to a const object) to Lua, that reference to the object will be considered const and only const methods can be called on it. It is possible to register Lua metamethods (except `__gc`). Destructors are registered automatically for each class.

As with regular variables and properties, class properties can be marked read-only by passing false in the second parameter, or omitting the set set function. The `deriveClass` takes two template arguments: the class to be registered, and its base class. Inherited methods do not have to be re-declared and will function normally in Lua. If a class has a base class that is **not** registered with Lua, there is no need to declare it as a subclass.

Remember that in Lua, the colon operator '`:`' is used for method call syntax:

    local a = A ()

    a.func1 ()  -- error: func1 expects an object of a registered class
    a.func1 (a) -- okay, verbose, this how OOP works in Lua
    a:func1 ()  -- okay, less verbose, equivalent to the previous

## 2.4 - Property Member Proxies

Sometimes when registering a class which comes from a third party library, the data is not exposed in a way that can be expressed as a pointer to member, there are no get or set functions, or the get and set functions do not have the right function signature. Since the class declaration is closed for changes, LuaBridge allows for a property member proxy. This is a pair of get and set flat functions which take as their first parameter a pointer to the object. This is easily understood with the following example:

    // Third party declaration, can't be changed
    struct Vec
    {
      float coord [3];
    };

Taking the address of an array element, e.g. `&Vec::coord [0]` results in an error instead of a pointer-to-member. The class is closed for modifications, but we want to export Vec objects to Lua using the familiar object notation. To do this, first we add a "helper" class:

    struct VecHelper
    {
      template <unsigned index>
      static float get (Vec const* vec)
      {
        return vec->coord [index];
      }

      template <unsigned index>
      static void set (Vec* vec, float value)
      {
        vec->coord [index] = value;
      }
    };

This helper class is only used to provide property member proxies. `Vec` continues to be used in the C++ code as it was before. Now we can register the `Vec` class with property member proxies for `x`, `y`, and `z`:

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .beginClass <Vec> ("Vec")
          .addProperty ("x", &VecHelper::get <0>, &VecHelper::set <0>)
          .addProperty ("y", &VecHelper::get <1>, &VecHelper::set <1>)
          .addProperty ("z", &VecHelper::get <2>, &VecHelper::set <2>)
        .endClass ()
      .endNamespace ();

It is also possible to use `std::function <>` instances as proxies:

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .beginClass <Vec> ("Vec")
          .addProperty ("x",
            std::function <float (const Vec*)> (
              [] (const Vec* vec) {return vec->coord [0];}),
            std::function <void (Vec*, float)> (
              [] (Vec* vec, float v) {vec->coord [0] = v;}))
          // ... same for "y" and "z"
        .endClass ()
      .endNamespace ();

## 2.5 - Function Member Proxies

Where it is not possible or inconvenient to add a member to be registered, LuaBridge also allows for a function member proxy. This is a flat function which take as its first parameter a pointer to the object:

    // Third party declaration, can't be changed
    struct Vec
    {
      float coord [3];
    };

The class is closed for modifications, but we want to extend Vec objects with our member function. To do this, first we add a "helper" function:

    void scale (float value)
    {
      value->coord [0] *= value;
      value->coord [1] *= value;
      value->coord [2] *= value;
    };

Now we can register the `Vec` class with a member function `scale`:

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .beginClass <Vec> ("Vec")
          .addFunction ("scale", &scale)
        .endClass ()
      .endNamespace ();

It is also possible to use `std::function <>` instances as proxies:

    getGlobalNamespace (L)
      .beginClass <Vec> ("Vec")
        .addFunction ("scaleX",
          std::function <void (Vec*, float)> (
            [] (Vec* vec, float v) {vec->coord [0] *= v;}))
      .endClass ()

## 2.6 - Constructors

A single constructor may be added for a class using `addConstructor`. LuaBridge cannot automatically determine the number and types of constructor parameters like it can for functions and methods, so you must provide them. This is done by specifying the signature of the desired constructor function as the first template parameter to `addConstructor`. The parameter types will be extracted from this (the return type is ignored). For example, these statements register constructors for the given classes:

    struct A
    {
      A ();
    };

    struct B
    {
      explicit B (char const* s, int nChars);
    };

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .beginClass <A> ("A")
          .addConstructor <void (*) (void)> ()
        .endClass ()
        .beginClass <B> ("B")
          .addConstructor <void (*) (char const*, int)> ()
        .endClass ()
      .endNamespace ();

Constructors added in this fashion are called from Lua using the fully qualified name of the class. This Lua code will create instances of `A` and `B`.

    a = test.A ()           -- Create a new A.
    b = test.B ("hello", 5) -- Create a new B.
    b = test.B ()           -- Error: expected string in argument 1

## 2.7 - Lua Stack

In the Lua C API, all operations on the `lua_State` are performed through the Lua stack. In order to pass values back and forth between C++ and Lua, LuaBridge uses specializations of this template class concept:

    template <class T>
    struct Stack
    {
      static void push (lua_State* L, T t);
      static T get (lua_State* L, int index);
      static bool isInstance (lua_State* L, int index);
    };

When a specialization of `Stack` exists for a given type `T` we say that the `T` is convertible. Throughout this document and the LuaBridge API, these types can be used anywhere a convertible type is expected.

The Stack template class specializations are used automatically for variables, properties, data members, property members, function arguments and return values. These basic types are supported:

- `bool`
- `char`, converted to a string of length one.
- `char const*` and `std::string` strings.
- Integers, `float`, and `double`, converted to `Lua_number`.

User-defined types which are convertible to one of the basic types are possible, simply provide a `Stack <>` specialization in the `luabridge` namespace for your user-defined type, modeled after the existing types. For example, here is a specialization for a `juce::String`:

    template <>
    struct Stack <juce::String>
    {
      static void push (lua_State* L, juce::String s)
      {
        lua_pushstring (L, s.toUTF8 ());
      }

      static juce::String get (lua_State* L, int index)
      {
        return juce::String (luaL_checkstring (L, index));
      }

      static bool isInstance (lua_State* L, int index)
      {
        return lua_type (L, index) == LUA_TSTRING;
      }
    };

## 2.8 - lua_State

Sometimes it is convenient from within a bound function or member function to gain access to the `lua_State*` normally available to a lua_CFunction. With LuaBridge, all you need to do is add a `lua_State*` as the last parameter of your bound function:

    void useState (lua_State* L);

    getGlobalNamespace (L).addFunction ("useState", &useState);

You can still include regular arguments while receiving the state:

    void useStateAndArgs (int i, std::string s, lua_State* L);

    getGlobalNamespace (L).addFunction ("useStateAndArgs", &useStateAndArgs);

When the script calls `useStateAndArgs`, it passes only the integer and string parameters. LuaBridge takes care of inserting the `lua_State*` into the argument list for the corresponding C++ function. This will work correctly even for the state created by coroutines. Undefined behavior results if the `lua_State*` is not the last parameter.

The same is applicable for properties.

# 3 - Passing Objects

An object of a registered class `T` may be passed to Lua as:

`T` | Passed by value (a copy), with Lua lifetime.
`T const` | Passed by value (a copy), with Lua lifetime.
`T*` | Passed by reference, with C++ lifetime.
`T&` | Passed by reference, with C++ lifetime.
`T const*` | Passed by const reference, with C++ lifetime.
`T const&` | Passed by const reference, with C++ lifetime.

## 3.1 - C++ Lifetime

The creation and deletion of objects with C++ lifetime is controlled by the C++ code. Lua does nothing when it garbage collects a reference to such an object. Specifically, the object's destructor is not called (since C++ owns it). Care must be taken to ensure that objects with C++ lifetime are not deleted while still being referenced by a `lua_State*`, or else undefined behavior results. In the previous examples, an instance of `A` can be passed to Lua with C++ lifetime, like this:

    A a;

    push (L, &a);             // pointer to 'a', C++ lifetime
    lua_setglobal (L, "a");

    push (L, (A const*) &a);   // pointer to 'a const', C++ lifetime
    lua_setglobal (L, "ac");

    push <A const*> (L, &a);  // equivalent to push (L, (A const*) &a)
    lua_setglobal (L, "ac2");

    push (L, new A);          // compiles, but will leak memory
    lua_setglobal (L, "ap");

## 3.2 - Lua Lifetime

When an object of a registered class is passed by value to Lua, it will have Lua lifetime. A copy of the passed object is constructed inside the userdata. When Lua has no more references to the object, it becomes eligible for garbage collection. When the userdata is collected, the destructor for the class will be called on the object. Care must be taken to ensure that objects with Lua lifetime are not accessed by C++ after they are garbage collected, or else undefined behavior results. An instance of `B` can be passed to Lua with Lua lifetime this way:

    B b;

    push (L, b);                    // Copy of b passed, Lua lifetime.
    lua_setglobal (L, "b");

Given the previous code segments, these Lua statements are applicable:

    print (test.A.staticData)       -- Prints the static data member.
    print (test.A.staticProperty)   -- Prints the static property member.
    test.A.staticFunc ()            -- Calls the static method.

    print (a.data)                  -- Prints the data member.
    print (a.prop)                  -- Prints the property member.
    a:func1 ()                      -- Calls A::func1 ().
    test.A.func1 (a)                -- Equivalent to a:func1 ().
    test.A.func1 ("hello")          -- Error: "hello" is not a class A.
    a:virtualFunc ()                -- Calls A::virtualFunc ().

    print (b.data)                  -- Prints B::dataMember.
    print (b.prop)                  -- Prints inherited property member.
    b:func1 ()                      -- Calls B::func1 ().
    b:func2 ()                      -- Calls B::func2 ().
    test.B.func2 (a)                -- Error: a is not a class B.
    test.A.func1 (b)                -- Calls A::func1 ().
    b:virtualFunc ()                -- Calls B::virtualFunc ().
    test.B.virtualFunc (b)          -- Calls B::virtualFunc ().
    test.A.virtualFunc (b)          -- Calls B::virtualFunc ().
    test.B.virtualFunc (a)          -- Error: a is not a class B.

    a = nil; collectgarbage ()      -- 'a' still exists in C++.
    b = nil; collectgarbage ()      -- Lua calls ~B() on the copy of b.

When Lua script creates an object of class type using a registered constructor, the resulting value will have Lua lifetime. After Lua no longer references the object, it becomes eligible for garbage collection. You can still pass these to C++, either by reference or by value. If passed by reference, the usual warnings apply about accessing the reference later, after it has been garbage collected.

## 3.3 - Pointers, References, and Pass by Value

When C++ objects are passed from Lua back to C++ as arguments to functions, or set as data members, LuaBridge does its best to automate the conversion. Using the previous definitions, the following functions may be registered to Lua:

    void func0 (A a);
    void func1 (A* a);
    void func2 (A const* a);
    void func3 (A& a);
    void func4 (A const& a);

Executing this Lua code will have the prescribed effect:

    func0 (a)   -- Passes a copy of a, using A's copy constructor.
    func1 (a)   -- Passes a pointer to a.
    func2 (a)   -- Passes a pointer to a const a.
    func3 (a)   -- Passes a reference to a.
    func4 (a)   -- Passes a reference to a const a.

In the example above, all functions can read the data members and property members of `a`, or call const member functions of `a`. Only `func0`, `func1`, and `func3` can modify the data members and data properties, or call non-const member functions of `a`.

The usual C++ inheritance and pointer assignment rules apply. Given:

    void func5 (B b);
    void func6 (B* b);

These Lua statements hold:

    func5 (b)   - Passes a copy of b, using B's copy constructor.
    func6 (b)   - Passes a pointer to b.
    func6 (a)   - Error: Pointer to B expected.
    func1 (b)   - Okay, b is a subclass of a.

When a pointer or pointer to const is passed to Lua and the pointer is null (zero), LuaBridge will pass Lua a `nil` instead. When Lua passes a `nil` to C++ where a pointer is expected, a null (zero) is passed instead. Attempting to pass a null pointer to a C++ function expecting a reference results in `lua_error` being called.

## 3.4 - Shared Lifetime

LuaBridge supports a shared lifetime model: dynamically allocated and reference counted objects whose ownership is shared by both Lua and C++. The object remains in existence until there are no remaining C++ or Lua references, and Lua performs its usual garbage collection cycle. A container is recognized by a specialization of the `ContainerTraits` template class. LuaBridge will automatically recognize when a data type is a container when the corresponding specialization is present. Two styles of containers come with LuaBridge, including the necessary specializations.

### 3.4.1 - Class RefCountedObjectPtr

This is an intrusive style container. Your existing class declaration must be changed to be also derived from `RefCountedObject`. Given `class T`, derived from `RefCountedObject`, the container `RefCountedObjectPtr <T>` may be used. In order for reference counts to be maintained properly, all C++ code must store a container instead of the pointer. This is similar in style to `std::shared_ptr` although there are slight differences. For example:

    // A is reference counted.
    struct A : public RefCountedObject
    {
      void foo () { }
    };

    struct B
    {
      RefCountedObjectPtr <A> a; // holds a reference to A
    };

    void bar (RefCountedObjectPtr <A> a)
    {
      a->foo ();
    }

### 3.4.2 - Class RefCountedPtr

This is a non intrusive reference counted pointer. The reference counts are kept in a global hash table, which does incur a small performance penalty. However, it does not require changing any already existing class declarations. This is especially useful when the classes to be registered come from a third party library and cannot be modified. To use it, simply wrap all pointers to class objects with the container instead:

    struct A
    {
      void foo () { }
    };

    struct B
    {
      RefCountedPtr <A> a;
    };

    RefCountedPtr <A> createA ()
    {
      return new A;
    }

    void bar (RefCountedPtr <A> a)
    {
      a->foo ();
    }

    void callFoo ()
    {
      bar (createA ());

      // The created A will be destroyed
      // when we leave this scope
    }

### 3.4.3 - User-defined Containers

If you have your own container, you must provide a specialization of `ContainerTraits` in the `luabridge` namespace for your type before it will be recognized by LuaBridge (or else the code will not compile):

    template <class T>
    struct ContainerTraits <CustomContainer <T> >
    {
      typedef typename T Type;

      static T* get (CustomContainer <T> const& c)
      {
        return c.getPointerToObject ();
      }
    };

Standard containers like `std::shared_ptr` or `boost::shared_ptr` will not work. This is because of type erasure; when the object goes from C++ to Lua and back to C++, there is no way to associate the object with the original container. The new container is constructed from a pointer to the object instead of an existing container. The result is undefined behavior since there are now two sets of reference counts.

### 3.4.4 - Container Constructors

When a constructor is registered for a class, there is an additional optional second template parameter describing the type of container to use. If this parameter is specified, calls to the constructor will create the object dynamically, via operator new, and place it a container of that type. The container must have been previously specialized in `ContainerTraits`, or else a compile error will result. This code will register two objects, each using a constructor that creates an object with Lua lifetime using the specified container:

    class C : public RefCountedObject
    {
      C () { }
    };

    class D
    {
      D () { }
    };

    getGlobalNamespace (L)
      .beginNamespace ("test")
        .beginClass <C> ("C")
          .addConstructor <void (*) (void), RefCountedObjectPtr <C> > ()
        .endClass ()
        .beginClass <D> ("D")
          .addConstructor <void (*) (void), RefCountedPtr <D> > ()
        .endClass ();
      .endNamespace ()

## 3.5 - Mixing Lifetimes

Mixing object lifetime models is entirely possible, subject to the usual caveats of holding references to objects which could get deleted. For example, C++ can be called from Lua with a pointer to an object of class type; the function can modify the object or call non-const data members. These modifications are visible to Lua (since they both refer to the same object). An object store in a container can be passed to a function expecting a pointer. These conversion work seamlessly.

## 3.6 - Convenience Functions

The `setGlobal` function can be used to assign any convertible value into a global variable.

# 4 - Accessing Lua from C++

Because Lua is a dynamically typed language, special consideration is required to map values in Lua to C++. The following sections describe the classes and functions used for representing Lua types. Only the essential operations are explained; To gain understanding of all available functions, please refer to the documentation comments in the corresponding source files.

## 4.1 - Class LuaRef

The `LuaRef` class is a container which references any Lua type. It can hold anything which a Lua variable can hold: nil, number, boolean, string, table, function, thread, userdata, and lightuserdata. Because `LuaRef` uses the `Stack` template specializations to do its work, classes, functions, and data exported to Lua through namespace registrations can also be stored (these are instances of userdata). In general, a `LuaRef` can represent any convertible C++ type as well as all Lua types.

A `LuaRef` variable constructed with no parameters produces a reference to nil:

    LuaRef v (L); // References nil

To construct a `LuaRef` to a specific value, the two parameter constructor is used:

    LuaRef v1 (L, 1);                   // A LUA_TNUMBER
    LuaRef v2 (L, 1.1);                 // Also a LUA_TNUMBER
    LuaRef v3 (L, true);                // A LUA_TBOOLEAN
    LuaRef v4 (L, "string");            // A LUA_TSTRING

The functions `newTable` and `getGlobal` create references to new empty table and an existing value in the global table respectively:

    LuaRef v1 = newTable (L);           // Create a new table
    LuaRef v2 = getGlobal (L, "print")  // Reference to _G ["print"]

A `LuaRef` can hold classes registered using LuaBridge:

    class A;
    //...
    LuaRef v (L, new A); // A LuaBridge userdata holding a pointer to A

Any convertible type may be assigned to an already-existing `LuaRef`:

    LuaRef v (L);         // Nil
    v = newTable (L);     // An empty table
    v = "string"          // A string. The previous value becomes
                          // eligible for garbage collection.

A `LuaRef` is itself a convertible type, and the convertible type `Nil` can be used to represent a Lua nil.

    LuaRef v1 (L, "x");   // assign "x"
    LuaRef v2 (L, "y");   // assign "y"
    v2 = v1;              // v2 becomes "x"
    v1 = "z";             // v1 becomes "z", v2 is unchanged
    v1 = newTable (L);    // An empty table
    v2 = v1;              // v2 references the same table as v1
    v1 = Nil ();          // v1 becomes nil, table is still
                          // referenced by v2.

Values stored in a `LuaRef` object obey the same rules as variables in Lua: tables, functions, threads, and full userdata values are objects. The `LuaRef` does not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.

### 4.1.1 - Type Conversions

A universal C++ conversion operator is provided for implicit conversions which allow a `LuaRef` to be used where any convertible type is expected. These operations will all compile:

    void passInt (int);
    void passBool (bool);
    void passString (std::string);
    void passObject (A*);

    LuaRef v (L);
    //...
    passInt (v);        // implicit conversion to int
    passBool (v);       // implicit conversion to bool
    passString (v);     // implicit conversion to string
    passObject (v);     // must hold a registered LuaBridge class or a
                        // lua_error() will be called.

Since Lua types are dynamic, the conversion is performed at run time using traditional functions like `lua_toboolean` or `lua_tostring`. In some cases, the type information may be incorrect especially when passing objects of registered class types. When performing these conversions, LuaBridge may raise a Lua error by directly or indirectly calling `lua_error` To be bullet-proof, such code must either be wrapped in a `lua_pcall`, or you must install a Lua panic function that throws an exception which you can catch.

When an explicit conversion is required (such as when writing templates), use the `cast` template function or an explicit C++ style cast.

    void passString (std::string);

    LuaRef v (L);

    // The following are all equivalent:

    passString (std::string (v));
    passString ((std::string)v);
    passString (static_cast <std::string> (v));
    passString (v.cast <std::string> ());

## 4.2 - Table Proxies

As tables are the sole data structuring mechanism in Lua, the `LuaRef` class provides robust facilities for accessing and manipulating table elements using a simple, precise syntax. Any convertible type may be used as a key or value. Applying the array indexing operator `[]` to a `LuaRef` returns a special temporary object called a table proxy which supports all the operations which can be performed on a `LuaRef`. In addition, assignments made to table proxies change the underlying table. Because table proxies are compiler-created temporary objects, you don't work with them directly. A LuaBridge table proxy should not be confused with the Lua proxy table technique described in the book "Programming in Lua"; the LuaBridge table proxy is simply an intermediate C++ class object that works behind the scenes to make table manipulation syntax conform to C++ idioms. These operations all invoke table proxies:

    LuaRef v (L);
    v = newTable (L);

    v ["name"] = "John Doe";      // string key, string value
    v [1] = 200;                  // integer key, integer value
    v [2] = newTable (L);         // integer key, LuaRef value
    v [3] = v [1];                // assign 200 to integer index 3
    v [1] = 100;                  // v[1] is 100, v[3] is still 200
    v [3] = v [2];                // v[2] and v[3] reference the same table
    v [2] = Nil ();               // Removes the value with key = 2. The table
                                  //   is still referenced by v[3].

## 4.3 - Calling Lua

Table proxies and `LuaRef` objects provide a convenient syntax for invoking `lua_pcall` on suitable referenced object. This includes C functions, Lua functions, or Lua objects with an appropriate `__call` metamethod set. The provided implementation supports up to eight parameters (although more can be supported by adding new functions). Any convertible C++ type can be passed as a parameter in its native format. The return value of the function call is provided as a `LuaRef`, which may be nil.

    LuaRef same = getGlobal (L, "same");

    // These all evaluate to true
    same (1,1);
    !same (1,2);
    same ("text", "text");
    !same (1, "text");
    same (1, 1, 2); // third param ignored

    function same (arg1, arg)
      return arg1 == arg2
    end

Table proxies support all of the Lua call notation that `LuaRef` supports, making these statements possible:

    LuaRef v = getGlobal (L, "t");

    t[1]();
    t[2]("a", "b");
    t[2](t[1]); // Call t[3] with the value in t[2]
    t[4]=t[3]();   // Call t[3] and store the result in t[4].

    t [t[5]()] = "wow"; // Store "wow" at the key returned by
                        //   the call to t[5]

    t = {}
    t[1] = function () print ("hello") end
    t[2] = function (u, v) print (u, v) end
    t[3] = "foo"

### 4.3.1 - Class LuaException

When `LuaRef` is used to call into Lua using the `()` operator it issues a protected call using `lua_pcall`. LuaBridge uses the C++ exception handling mechanism, throwing a `LuaException` object:

    LuaRef f (L) = getGlobal (L, "fail");

    try {
      f ();
    }
    catch (LuaException const& e) {
      std::cerr && e.what ();
    }

    function fail ()
      error ("A problem occurred")
    end

# 5 - Security

The metatables and userdata that LuaBridge creates in the `lua_State*` are protected using a security system, to eliminate the possibility of undefined behavior resulting from scripted manipulation of the environment. The security system has these components:

- Class and const class tables use the table proxy technique. The corresponding metatables have `__index` and `__newindex` metamethods, so these class tables are immutable from Lua.
- Metatables have `__metatable` set to a boolean value. Scripts cannot obtain the metatable from a LuaBridge object.
- Classes are mapped to metatables through the registry, which Lua scripts cannot access. The global environment does not expose metatables
- Metatables created by LuaBridge are tagged with a lightuserdata key which is unique in the process. Other libraries cannot forge a LuaBridge metatable.

This security system can be easily bypassed if scripts are given access to the debug library (or functionality similar to it, i.e. a raw `getmetatable`). The security system can also be defeated by C code in the host, either by revealing the unique lightuserdata key to another module or by putting a LuaBridge metatable in a place that can be accessed by scripts.

When a class member function is called, or class property member accessed, the `this` pointer is type-checked. This is because member functions exposed to Lua are just plain functions that usually get called with the Lua colon notation, which passes the object in question as the first parameter. Lua's dynamic typing makes this type-checking mandatory to prevent undefined behavior resulting from improper use.

If a type check error occurs, LuaBridge uses the `lua_error` mechanism to trigger a failure. A host program can always recover from an error through the use of `lua_pcall`; proper usage of LuaBridge will never result in undefined behavior.

# Appendix - API Reference

## Free Functions

    /// Gets a global Lua variable reference.
    ///
    LuaRef getGlobal(lua_State* L, const char* name);

    /// Sets a global Lua variable.
    ///
    template<class V>
    void setGlobal(lua_State* L, V* varPtr, const char* name);

    /// Gets the global namespace registration object.
    ///
    Namespace getGlobalNamespace(lua_State* L);

## Namespace Registration - Namespace

    /// Begins or continues class registration, returns this class object.
    ///
    template<class T>
    Class<T> beginClass(const char* name);

    /// Begins derived class registration, returns this class object.
    ///
    template<class T, class Base>
    Class<T> deriveClass(const char* name);

    /// Begin or continues namespace registration, returns this namespace object.
    ///
    template<class T>
    Namespace beginNamespace(const char* name);

    /// Ends namespace registration, returns the parent namespace object.
    ///
    template<class T>
    Namespace endNamespace();

    /// Registers a function.
    ///
    template<class R, class... Params>>
    Namespace addFunction(const char* name,
                          R (*fn)(Params...));

    /// Registers a function.
    ///
    template<class R, class... Params>>
    Namespace addFunction(const char* name,
                          std::function<R (Params...)> fn);

    /// Registers a function with an extra Lua state parameter.
    ///
    template<class R, class... Params>>
    Namespace addFunction(const char* name,
                          R (*fn)(Params..., lua_State*))

    /// Registers a C-function.
    ///
    Namespace addFunction(const char* name,
                          int (*fn)(lua_State*));

    /// Registers a property with a getter and setter.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          V (*getFn)(),
                          void (*setFn)(V));

    /// Registers a property with a getter and setter.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          std::function<V ()> getFn,
                          std::function<void (V)> setFn);

    /// Registers a property with a C-function getter and setter.
    ///
    Namespace addProperty(const char* name,
                          int (*getFn)(lua_State*),
                          int (*setFn)(lua_State*));

    /// Registers a read-only property with a getter function.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          V (*getFn)());

    /// Registers a read-only property with a getter function.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          std::function<V ()> getFn);

    /// Registers a read-only property with a C-function getter.
    ///
    Namespace addProperty(const char* name,
                          int (*getFn)(lua_State*));

    /// Registers a variable, writable or read-only.
    ///
    template<class V>
    Namespace addVariable(const char* name,
                          V* varPtr,
                          bool isWritable = true);

## Class Registration - Class<T>

    /// Ends class registration, returns the parent namespace object.
    ///
    template<class T>
    Namespace endClass();

### Member Function Registration

    /// Registers a member function.
    ///
    template<class R, class... Params>>
    Namespace addFunction(const char* name,
                          R (T::* fn)(Params...));

    /// Registers a function.
    ///
    template<class R, class... Params>>
    Namespace addFunction(const char* name,
                          std::function<R (Params...)> fn);

    /// Registers a function with an extra Lua state parameter.
    ///
    template<class R, class... Params>>
    Namespace addFunction(const char* name,
                          R (T::* fn)(Params..., lua_State*))

    /// Registers a C-function.
    ///
    Namespace addFunction(const char* name,
                          int (*fn)(lua_State*));

### Member Property Registration

    /// Registers a property with a getter and setter.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          V (T::* getFn)(),
                          void (T::* setFn)(V));

    /// Registers a property with a getter and setter.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          std::function<V ()> getFn,
                          std::function<void (V)> setFn);

    /// Registers a property with a C-function getter and setter.
    ///
    Namespace addProperty(const char* name,
                          int (*getFn)(lua_State*),
                          int (*setFn)(lua_State*));

    /// Registers a read-only property with a getter member function.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          V (T::* getFn)());

    /// Registers a read-only property with a getter function.
    ///
    template<class V>
    Namespace addProperty(const char* name,
                          std::function<V ()> getFn);

    /// Registers a read-only property with a C-function getter.
    ///
    Namespace addProperty(const char* name,
                          int (*getFn)(lua_State*));

    /// Registers a member variable, writable or read-only.
    ///
    template<class V>
    Namespace addData(const char* name,
                      V T::* varPtr,
                      bool isWritable = true);

### Static Function Registration

    /// Registers a function.
    ///
    template<class R, class... Params>>
    Namespace addStaticFunction(const char* name,
                                R (*fn)(Params...));

    /// Registers a function.
    ///
    template<class R, class... Params>>
    Namespace addStaticFunction(const char* name,
                                std::function<R (Params...)> fn);

    /// Registers a function with an extra Lua state parameter.
    ///
    template<class R, class... Params>>
    Namespace addStaticFunction(const char* name,
                                R (*fn)(Params..., lua_State*))

    /// Registers a C-function.
    ///
    Namespace addStaticFunction(const char* name,
                                int (*fn)(lua_State*));

### Static Property Registration

    /// Registers a property with a getter and setter.
    ///
    template<class V>
    Namespace addStaticProperty(const char* name,
                                V (*getFn)(),
                                void (*setFn)(V));

    /// Registers a property with a getter and setter.
    ///
    template<class V>
    Namespace addStaticProperty(const char* name,
                                std::function<V ()> getFn,
                                std::function<void (V)> setFn);

    /// Registers a property with a C-function getter and setter.
    ///
    Namespace addStaticProperty(const char* name,
                                int (*getFn)(lua_State*),
                                int (*setFn)(lua_State*));

    /// Registers a read-only property with a getter function.
    ///
    template<class V>
    Namespace addStaticProperty(const char* name,
                                V (*getFn)());

    /// Registers a read-only property with a getter function.
    ///
    template<class V>
    Namespace addStaticProperty(const char* name,
                                std::function<V ()> getFn);

    /// Registers a read-only property with a C-function getter.
    ///
    Namespace addStaticProperty(const char* name,
                                int (*getFn)(lua_State*));

    /// Registers a variable, writable or read-only.
    ///
    Namespace addStaticData(const char* name,
                            T* varPtr,
                            bool isWritable = true);

## Lua Variable Reference - LuaRef

    /// Creates a nil reference.
    ///
    LuaRef(lua_State* L);

    /// Returns native Lua string representation.
    ///
    std::string tostring() const;

    /// Dumps reference to a stream.
    ///
    void print(std::ostream& stream) const;

    /// Returns the Lua state.
    ///
    lua_State* state() const;

    /// Place the object onto the Lua stack.
    ///
    void push(lua_State* L);

    /// Return the lua_type.
    ///
    int type() const;

    /// Perform the explicit type conversion.
    ///
    template<class T>
    T cast() const;

    /// Check if the Lua value is convertible to the type T.
    ///
    template<class T>
    bool isInstance() const;

## Stack Traits - Stack<T>

    /// Converts the C++ value into the Lua value at the top of the Lua stack.
    ///
    void put (lua_State* L, T value);

    /// Converts the Lua value at the index into the C++ value of the type T.
    ///
    T get (lua_State* L, int index);

    /// Checks if the Lua value at the index is convertible into the C++ value of the type T.
    ///
    bool isInstance (lua_State* L, int index);