SWIG for Lua: Reflection

For this blog entry, we show that it is possible for Lua to know about the fields and methods of a C++ struct. This ability for inspection is known as reflection in the programming world. Only basic reflection is demonstrated here; that is, only basic information is extracted from the C++ struct.

First, we define the C++ struct in the header file, example.hpp:

	struct Point
	{
	int x;
	int y;
	void greet();
	};

view raw example.hpp hosted with ❤ by GitHub

The struct implementation is defined in example.cpp:

	#include "example.hpp"
	#include <cstdio>
	void Point::greet()
	{
	printf("x=%d, y=%d\n", x, y);
	}

view raw example.cpp hosted with ❤ by GitHub

We then define the interface file, example.i, to be used by SWIG:

	%module example
	%{
	/* Includes the header in the wrapper code */
	#include "example.hpp"
	%}
	/* Parse the header file to generate wrappers */
	%include "example.hpp"

view raw example.i hosted with ❤ by GitHub

After which, we use SWIG to generate the wrapper file:

% swig -lua -c++ example.i

We then compile the C++ source files and create the shared object to be used by our Lua script:

% g++ -fPIC -I/usr/include/lua5.2 -c example_wrap.cxx -o example_wrap.o
% g++ -fPIC -c example.cpp -o example.o
% g++ -shared -I/usr/include/lua5.2 example.o example_wrap.o -o example.so

We then write a Lua script, test.lua, for testing:

	#!/usr/bin/lua
	package.loadlib('./example.so', 'luaopen_example')()
	A = example.Point()
	A.x = 101
	A.y = 202
	print(A.x)
	print(A.y)
	A:greet()
	print(swig_type(A.x))
	print(swig_type(A.y))
	print(swig_type(A.greet))
	m = getmetatable(A)
	print('/***** struct metatable *****/')
	for k, v in pairs(m) do
	print(k, v)
	end
	g = m['.get']
	print('/***** struct fields *****/')
	for k, v in pairs(g) do
	print(k, v)
	end
	fn = m['.fn']
	print('/***** struct methods *****/')
	for k, v in pairs(fn) do
	print(k, v)
	end

view raw test.lua hosted with ❤ by GitHub

When run:

	% ./test.lua
	101
	202
	x=101, y=202
	number
	number
	function
	/***** struct metatable *****/
	.type Point
	__gc function: 0x7fced2eaf0a1
	.get table: 0xf14c60
	__tostring function: 0x7fced2eaf140
	__newindex function: 0x7fced2eaef9f
	.static table: 0xf15010
	__eq function: 0x7fced2eaf29b
	__index function: 0x7fced2eaeb93
	.set table: 0xf15060
	.fn table: 0xf150a0
	.bases table: 0xf14c20
	/***** struct fields *****/
	y function: 0x7fced2eb1f9b
	x function: 0x7fced2eb1b2f
	/***** struct methods *****/
	__disown function: 0x7fced2eaf234
	greet function: 0x7fced2eb219a

view raw output.txt hosted with ❤ by GitHub

Lines 20-24 of test.lua print the names of the fields of the struct. Note that the order of the printing of the field names (lines 21-22 of output.txt) may not be the same as the order of declaration of the fields in the C++ struct definition in example.hpp (lines 3-4).

Lines 26-30 print the names of the methods of the struct.

Hope this helps the world.