tolua++ - Reference Manual

• Support for std::string as a basic type (this can be turned off by a command line option).
• Support for class templates

As well as other features and bugfixes.

tolua is a tool that greatly simplifies the integration of C/C++ code with Lua. Based on a cleaned header file (or extracts from real header files), tolua automatically generates the binding code to access C/C++ features from Lua. Using Lua API and tag method facilities, tolua maps C/C++ constants, external variables, functions, classes, and methods to Lua.

This manual is for tolua++ version 1.0 and is implemented upon Lua 5.0 and based on tolua 5.0. See Compatibility for details on switching from older versions.

The sections below describe how to use tolua. Please contact us with bug reports, suggestions, and comments.

• Shortcuts:
• How tolua works
• How to use tolua
• Basic Concepts
• Binding constants
• Binding external variables
• Binding functions
• Binding struct fields
• Binding classes and methods
• Class Templates
• Module definition
• Renaming constants, variables and functions
• Additional features
• Storing additional fields
• Exported utility functions
• Embedded Lua code
• Compatibility with older versions
• Changes since v3 *
• Changes since v2.*
• Changes since v1.*
• Credits
• Availability

How tolua works

Let's start with some examples. If we specify as input the following C-like header file to tolua:

#define FALSE 0
#define TRUE 1

enum { 
 POINT = 100, 
 LINE, 
 POLYGON
}

Object* createObejct (int type);
void drawObject (Object* obj, double red, double green, double blue);
int isSelected (Object* obj);

...
myLine = createObject(LINE)
...
if isSelected(myLine) == TRUE then
  drawObject(myLine, 1.0, 0.0, 0.0);
else
  drawObject(myLine, 1.0, 1.0, 1.0);
end
...

#define FALSE 0
#define TRUE 1

class Shape
{
  void draw (void);
  void draw (double red, double green, double blue);
  int isSelected (void);
};

class Line : public Shape
{
 Line (double x1, double y1, double x2, double y2);
 ~Line (void);
};

...
myLine = Line:new (0,0,1,1)
...
if myLine:isSelected() == TRUE then
 myLine:draw(1.0,0.0,0.0)
else
 myLine:draw()
end
...
myLine:delete()
...

How to use toLua

tolua -o myfile.c myfile.pkg

The generated code must be compiled and linked with the application to provide the desired access from Lua. Each parsed file represents a package being exported to Lua. By default, the package name is the input file root name (myfile in the example). The user can specify a different name for the package:

tolua -n pkgname -o myfile.c myfile.pkg

The package should also be explicitly initialized. To initialize the package from our C/C++ code, we must declare and call the initialization function. The initialization function is defined as

int tolua_pkgname_open (void);

where pkgname represents the name of the package being bound. If we are using C++, we can opt for automatic initialization:

tolua -a -n pkgname -o myfile.c myfile.pkg

In that case, the initialization function is automatically called. However, if we are planning to use multiple Lua states, automatic initialization does not work, because the order static variables are initialized in C++ is not defined.

Optionally, the prototype of the open function can be outputted to a header file, which name is given by the -H option.

The binding code generated by tolua uses a set of functions defined in the tolua library. Thus, this library also has to be linked with the application. The file tolua.h is also necessary to compile the generated code.

An application can use tolua object oriented framework (see exported utility functions) without binding any package. In that case, the application must call tolua initialization function (this function is called by any package file initialization function):

int tolua_open (void);

Basic Concepts

Including files

$pfile "include_file"

A package file may also include regular C/C++ header files, using the hfile or cfile directive:

$cfile "example.h"

In wich case, tolua will extract the code enclosed between tolua_begin and tolua_end, or or tolua_export for a single line. Consider this C++ header as example:

#ifndef EXAMPLE_H
#define EXAMPLE_H

class Example { // tolua_export

private:

	string name;
	int number;

public:

	void set_number(int number);

	//tolua_begin

	string get_name();
	int get_number();
};
// tolua_end

#endif

In this case, the code that's not supported by tolua (the private part of the class), along with the function set_number is left outside of the package that includes this header.

Finally, lua files can be included on a package file, using $lfile:

$lfile "example.lua"

Basic types

Functions in C/C++ can also manipulate Lua objects explicitly. Thus lua_Object is also considered a basic type. In this case, any Lua value matches it.

New on tolua++: The C++ type string is also considered a basic type, and is passed as a value to lua (using the c_str() method). This feature can be turned off with the command line option -S.

User defined types

For user defined types, constness is preserved. Thus passing a non constant user defined type to a function that expects constant type generates an type mismatching error.

NULL and nil

Typedefs

typedef double real;

Otherwise, real would be interpreted as a user defined type and would not be mapped to Lua numbers.

Including real header files

/* specify the files to be included */

$#include "header1.h"                 // include first header
$#include "header2.h"                 // include second header

Also note that files included with $cfile or $hfile don't need to be included using this method, this is done automatically by tolua.

In the following sections, we describe how to specify the C/C++ code we want to bind to Lua. The formats are simplified valid C/C++ statements.

Binding constants

#define NAME [ VALUE ]

New on tolua++: All other preprocessor directives are ignored.

For enum's, the general format is:

enum {
  NAME1 [ = VALUE1 ] ,
  NAME2 [ = VALUE2 ] ,
  ...
  NAMEn [ = VALUEn ]
};

Binding external variables

[extern] type var;

double v[10];

New on tolua++: External variables can use the tolua_readonly modifier (see Additional Features)

Binding functions

type funcname (type1 par1[, type2 par2[,...typeN parN]]);

Arrays

The arrays must be pre dimensioned. For instance:

void func (double a[3]);

is a valid function declaration for tolua and calling this function from Lua would be done by, for instance:

p = {1.0,1.5,8.6}
func (p)

The array dimension need not be a constant expression; the dimension can also be specified by any expression that can be evaluated in run time. For instance:

void func (int n, int m, double image[n*m]);

is also valid since the expression n*m is valid in the binding function scope. However, be aware that tolua uses dynamic allocation for binding this function, what can degrade the performance.

Despite the dimension specification, it is important to know that all arrays passed to the actual C/C++ function are in the local scope of the binding function. So, if the C/C++ function being called needs to hold the array pointer for later use, the binding code will not work properly.

Overloaded functions

void func (int a);
void func (double a);

represent two different functions in C++, they are the same function for tolua, because both int and double are mapped to the same Lua type: number.

Another tricky situation occurs when expecting pointers. Suppose:

void func (char* s);
void func (void* p);
void func (Object1* ptr);
void func (Object2* prt);

func(nil)

It is important to know that tolua decides which function will be called in run-time, trying to match each provided function. tolua first tries to call the last specified function; if it fails, tolua then tries the previous one. This process is repeated until one function matches the calling code or the first function is reached. For that reason, the mismatching error message, when it occurs, is based on the first function specification. When performance is important, we can specify the most used function as the last one, because it will be tried first.

tolua allows the use of overloaded functions in C, see Renaming for details.

Default parameter values

type funcname (..., typeN-1 parN-1 [= valueN-1], typeN parN [= valueN]);

toLua implements this feature without using any C++ mechanism; so, it can be used also to bind C functions.

We can also specify default values for the elements of an array (there is no way to specify a default value for the array itself, though). For instance:

void func (int a[5]=0);

sets the default element values to zero, thus the function can be called from Lua with an uninitialized table.

For Lua object types (lua_Object), tolua defines a constant that can be used to specify nil as default value:

void func (lua_Object lo = TOLUA_NIL);

New on tolua++: C++ class constructors are valid as default parameters, but only when the parameter is a value and not a reference. For example:

void set_color(Color& color = Color(0,0,0));

This will work, whereas:

void set_color(const Color& color = Color(0,0,0));

Will probably crash, since the generated bind function will be taking a reference to a temporary object and then use it later. This is a known bug, and will be fixed in a future release.

Multiple returned values

For instance, consider a C function that swaps two values:

void swap (double* x, double* y);

or

void swap (double& x, double& y);

If such a function is declared in the package file, tolua binds it as a function receiving two numbers as input and returning two numbers. So, a valid Lua code would be:

x,y = swap(x,y)

If the input values are not used, the use of default parameter value allows calling the function from Lua without specifying them:

void getBox (double* xmin=0, double* xmax=0, double* ymin=0, double* ymax=0);

In Lua:

xmin, xmax, ymin, ymax = getBox()

With user defined types, we would have for instance:

void update (Point** p);

or

void update (Point*& p);

Binding struct fields

typedef struct [name] {
   type1 fieldname1;
   type2 fieldname2;
   ...
   typeN fieldnameN;
} typename;

Fields that represent arrays of values can also be mapped:

typedef struct {
  int x[10];
  int y[10];
} Example;
 

Binding classes and methods

Specifying inheritance

class classname : public basename
{

 /* class definition */

};

In this case, the definition of basename needs to appear before classname if the inheritance properties are to be taken advantage of from lua.

Specifying exported members and methods

For each bound class, tolua creates a Lua table and stores it at a variable which name is the name of the C++ class. This tables may contain other tables that represent other tables, the way C++ classes may contain other classes and structs. Static exported fields are accessed by indexing this table with the field names (similar to struct fields). Static methods are also called using this table, with a colon. Non static exported fields are accessed by indexing the variable that holds the object. Class methods follow the format of the function declaration showed above. They can be accessed from Lua code using the conventional way Lua uses to call methods, applied of course to a variable that holds the appropriate object or to the class table, for static methods.

There are a few special methods that are also supported by tolua. Constructors are called as static methods, named new or new_local (on tolua++). Destructors are called as a conventional method called delete.

Note that tolua does support overload. This applies even for constructors. Also note that the virtual keyword has no effect in the package file.

The following code exemplifies class definitions that can be interpreted by tolua.

class Point {
   static int n;    // represents the total number of created Points
   static int get_n();    // static method

   double x;        // represents the x coordinate
   double y;        // represents the y coordinate

   static char* className (void);   // returns the name of the class

   Point (void);                          // constructor 1
   Point (double px, double py);          // constructor 2
   ~Point (void);                         // destructor

   Point add (Point& other);              // add points, returning another one
};

class ColorPoint : public Color {
   int red;      // red color component [0 - 255]
   int green;    // green color component [0 - 255]
   int blue;     // blue color component [0 - 255]

   ColorPoint (double px, double py, int r, int g, int b);
};

p1 = Point:new(0.0,1.0)
p2 = ColorPoint:new(1.5,2.2,0,0,255)
print(Point.n)                     -- would print 2
print(Point:get_n())               -- would also print 2
p3 = p1:add(p2)
local p4 = ColorPoint:new_local()
print(p3.x,p3.y)                   -- would print 1.5 and 3.2
print(p2.red,p2.green,p2.blue)     -- would print 0, 0, and 255
print(
p1:delete()                        -- call destructor
p2:delete()                        -- call destructor

New on tolua++: Also note that p4 is created with the new_local constructor, wich also leaves the object to be deleted by the garbaje collector, and it should not be deleted using delete. For each constructor on the pkg, one new and new_local is created.

Of course, we need to specify only the methods and members we want to access from Lua. Sometimes, it will be necessary to declare a class with no member or method just for the sake of not breaking a chain of inheritances.

Overloaded operators

operator+   operator-   operator*   operator/ 
operator<   operator>=  operator==  operator[]

As an example, suppose that in the code above, instead of having:

   Point add (Point& other);              // add points, returning another one

   Point operator+ (Point& other);        // add points, returning another one

p3 = p1 + p2

   double& operator[] (int index);

   double operator[] (int index);

Free functions (i.e., not class members) that overload operators are not supported.

Class Templates

class vector {

	TEMPLATE_BIND(T, int, string, Vector3D, double)

	void clear();
	int size() const;

	const T& operator[](int index) const;
	T& operator[](int index);
	void push_back(T val);

	vector();
	~vector();
};

string_vector = vector_string_:new_local()
string_vector:push_back("hello")
string_vector:push_back("world")
print(string_vector[0].." "..string_vector[1])

class hash_map : public map<K,V> {

	TEMPLATE_BIND(K V, int string, string vector<double>)

	V get_element(K key);
	void set_element(K key, V value);

	hash_map();
	~hash_map();
};

Note that due to the complexity in the definition of some templates, you should be careful on how you declare them. For example, if you create an object with type hash_map<string,vector<double> > and then declare a variable with type hash_map<string, vector<double> > (note the space between string and vector), the type of the variable will not be recognized. The safest way is to declare a typedef, and use that to use each type (this is also a common practice on C++ programming). For example, using the previous declaration of vector:

typedef vector VectorInt;

VectorInt variable;

#define TEMPLATE_BIND(x)

Also, the parameters can have double quotes. Thus, the following uses are valid:

TEMPLATE_BIND((T, int, float)) // to be used inside a real header file
TEMPLATE_BIND("K V", "string string", int double)

Module definition

module name
{
      ... // constant, variable, and function declarations
}

Thus, if we bound the following module declaration:

module mod
{
 #define N
 extern int var;
 int func (...):
}

In Lua we would be able to access such features by indexing the module: mod.N, mod.var, mod.func.

Renaming constants, variables and functions

extern int cvar @ lvar;

#define CNAME @ LNAME

enum {
  CITEM1 @ LITEM1,
  CITEM2 @ LITEM2,
  ...
};

void cfunc @ lfunc (...);

class T
{
   double cfield @ lfield;
   void cmeth @ lmeth (...);
   ...
};

In such a case, the global variable cvar would be identified in Lua by lvar, the constant CNAME by LNAME, and so on. Note that class cannot be renamed, because they represent types in C.

This renaming feature allows function overload in C, because we can choose to export two different C functions with a same Lua name:

void glVertex3d @ glVertex (double x, double y, double z=0.0);
void glVertexdv @ glVertex (double v[3]=0.0);

Renaming Types

$renaming real_name @ new_name

The parameters to renaming can be Lua patterns. For example:

$renaming ^_+ @
$renaming hash_map<string,vector<double> > @ StringHash

Additional features on tolua++

Multiple variable declarations

float x,y,z;

will create 3 different variables of type float. Make sure you don't leave any spaces between the commas, as that will raise a parse error.

tolua_readonly

class Widget {

	tolua_readonly string name;
};

$hfile "node.h"
$#define __operator_arrow operator->()
$#define __get_name get_name()

template class<T>
class Node { // tolua_export

private:
	string name;
	T* value;

public:

	T* operator->() {return value;};
	string get_name() {return name;};

	// tolua_begin

	Node* next;
	Node* prev;

	void set_name(string p_name) {name = p_name;};

	#if 0
	tolua_readonly __operator_arrow @ p;
	tolua_readonly __get_name @ name;

	TEMPLATE_BIND(T, Vector3D)
	#endif

	Node();
};
// tolua_end

node = Node_Vector3D_:new_local()
-- do something with the node here --
print("node name is "..node.name)
print("node value is ".. node.p.x ..", ".. node.p.y ..", ".. node.p.z)

The ability to rename functions as variables might be expanded on future versions.

Storing additional fields

obj = ClassName:new()

obj.myfield = 1  -- even though "myfield" does not represent a field of ClassName

Exported utility functions

module tolua
{
 void tolua_using @ using (lua_Table module);
 char* tolua_type @ type (lua_Object lo);
 void tolua_foreach @ foreach (lua_Object lo, lua_Function f);
 void tolua_class @ class (lua_Table derived, lua_Table base=TOLUA_NIL);
 void tolua_instance @ instance (lua_Table instance, lua_Table classobj);
 lua_Object tolua_base @ base (lua_Object lo);
}

tolua.using (table)

tolua.using(tolua)

all tolua utility functions are mapped to the global environment.

tolua.type (var)

tolua.tag ("type")

tolua.foreach (object)

tolua.cast (object, "typename")

tolua.takeownership (object)

tolua.class (table, base=nil)

tolua.instance (table, class)

-- define a Point class
classPoint = { x=0, y=0 }
tolua.class(classPoint) -- set as a class

-- define print method
function classPoint:print ()
   print(self.x,self.y)
end

-- define add method
function classPoint:add (p2)
   return Point{x=self.x+p2.x,y=self.y+p2.y}
end

-- define a Point constructor
function Point (p)
   tolua.instance(p,classPoint) -- set as an instance of classPoint
return p end

-- define a Color Point class
classColorPoint = { color = 'black' }
tolua.class(classColorPoint,classPoint) -- set as class inheriting from classPoint

-- define class methods
function classColorPoint:print ()
   print(self.x,self.y,self.color)
end

-- define Color Point constructor
function ColorPoint (p)
   tolua.instance(p,classColorPoint) -- set as an instance of classColorPoint
   return p
end

-- Some valid codes would then be
p = Point{x=1}
q = ColorPoint{x=2,y=3,color=2}
r = p:add(q)
r:print() --> would print "3 3"
 

Embedded Lua code

$[

embedded Lua code
...

$]

As an example consider the following .pkg excerpt:

/* Bind a Point class */
class Point
{
 Point (int x, int y);
 ~Point ();
 void print ();
 ...
} CPoint;

$[

-- Create a Point constructor
function Point (self)
 local cobj = CPoint:new(self.x or 0, self.y or 0)
 tolua.takeownership(cobj)
 return cobj
end

$]

Binding such a code would allow us to write the following Lua code:

p = Point{ x=2, y=3 }
p:print()
...
 

Compatibility with older versions.

toLua 4

Retrieving Objects

Users switching from tolua v4 should know that tolua++ stores the objects as void** on Lua, so when retrieving an object from the luastate using lua_touserdata, the pointer should be dereferenced. The library function tolua_tousertype should work as expected. Example:

lua_pushglobal(lua_state, "get_Object");
lua_call(lua_state, 0, 1);  // calling a function that returns an Object

Object *new_object = (Object*)(*lua_touserdata(lua_state, -1));
or
Object *new_object = (Object*)tolua_tousertype(lua_state, -1);

tolua++ binds the c++ type std::string as a basic type, passing it to Lua as a regular string (using the method c_str()). This feature can be turned off with the command line option -S.

Operators

The list of supported operators has changed, see Binding classes and methods for more information.

toLua 5

operator[] index

Users switching from tolua v5 should know that tolua 5 substracts 1 from the index on operator[] functions, for compatibility with lua's method for indexing arrays (1 is the first element). This feature is turned off by default on tolua++ (making it compatible with tolua 4). It can be turned back on with the command line option -1

C++ Strings

(see c++ strings on tolua 4 below)

Changes since v. 3.0

• Support for binding arrays as variables and struct/class fields;
• Support for embedding Lua code into the generated binding code;
• New utility functions: cast and takeownership;
• Option to create the corresponding header file of the binding code;
• New "close" package function;
• Fixed bug on cloning objects in C++;
• Fixed bug on enum and struct parsing;

Changes since v. 2.0

• There is a new executable parser;
• Support for multiple Lua states is provided;
• Support for module definition is provided;
• Global variables is now directly bound to Lua global variables;
• Constness of user defined types is preserved in Lua;
• Support for multiple returned values from C/C++ is provided (simulating parameters passed by reference);
• Constants, variables, and functions bound to Lua can have different names from their C/C++ counterparts;
• Object-oriented framework (and other utility functions) used in tolua is now exported for Lua programmers;

Incompatibilities

Lua code based on tolua v2.* should run with no change on tolua v3.0. Although, it may be necessary to change the .pkg file in order to get the same behavior. The following incompatibilities exist:
• Parameters defined as pointer to basic types are no longer converted to arrays of dimension one; they are now considered parameters passed by reference.
• Automatic initialization for C++ code must be explicitly requested when using the new parser;
• Global variables are no longer mapped to a table; the definition of a module including the global variables may be used to simulate the old behavior;
• The initialization function is no longer toLua_package_open but tolua_package_open, without the capital letter (sorry!).

Changes since v. 1.*

• The binding code should run much faster;
• The cleaned header file extension should now be .pkg instead of .L;
• Type modifiers is now accepted (though the current version ignores const's);
• Returning object by value is accepted and memory allocation is controlled by Lua garbage collection;
• Overloaded functions/methods are accepted;
• Parameters with default values are accepted;
• Some overloaded operators are automatically bound.

Credits

While working at NGD Studios, Ariel Manzur made some changes to tolua4 for their game engine. After the release of tolua5, having left NGD, enough changes were made to tolua to justify a separate release (with Waldemar's blessing :-)
 

Availability