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:

The struct implementation is defined in example.cpp:

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

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:

When run:

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.

SWIG for Lua: Class demonstration

In this entry, we demonstrate how to expose a C++ class to Lua using SWIG. The environment I use is Ubuntu 16.04.

First, we define the C++ class in example.hpp:

The class implementation is defined in example.cpp:

Then, we define the interface file, example.i, to be used by SWIG:

We then use SWIG to generate the wrapper file, example_wrap.cxx:

% swig -lua -c++ example.i

Next, we compile the C++ source files:

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

Then, we create the shared object, example.so:

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

We write a Lua script to test if SWIG works:

When run:

% ./test.lua
Hello, world: x=6 y=7

Hope this helps the world.

Compiling and running SWIG for Lua in Ubuntu 16.04 using C++

This blog entry is quite similar to my previous entry. The difference is that this entry uses C++ source files instead of C.

We first start with the C++ header file:

Notice the use of extern for the declaration of the global variable, since we are dealing with C++ here and not C.

We then create the C++ implementation file:

Then we create the interface file for SWIG:

We then call SWIG to generate the wrapper file:

% swig -lua -c++ example.i

Notice the use of the -c++ argument.

Next, we compile the wrapper file:

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

Notice that the generated wrapper file has the file suffice .cxx

We then compile the C++ implementation file:

% g++ -fPIC -I. -c example.cpp -o example.o

Finally, we create the shared object to be used by our Lua script:

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

We write a Lua script for testing:

When run:

% lua test.lua
3
120
2
Wed Jun 14 16:41:11 2017

Hope this helps the world. :)

Compiling and running SWIG for Lua in Ubuntu 16.04 for the truly lazy

Previously, we were able to compile and run SWIG for Lua in Ubuntu 16.04 using a C source file. In this blog entry, we want to do something very similar except that we use a C header file.

We begin with the files needed starting with the header file, example.h:

Then, we create the C implementation file, example.c:

Finally, we create the interface file, example.i, which is needed by SWIG:

Notice how much simpler this interface file is compared to the previous one. This is why this entry has a subclause "for the truly lazy".

We then call SWIG to create the wrapper file, example_wrap.c:

% swig -lua example.i

Then we compile the generated wrapper file:

% gcc -fPIC -I/usr/include/lua5.2 -c example_wrap.c -o example_wrap.o

We also compile the C source file, example.c:

% gcc -fPIC -I. -c example.c -o example.o

Finally, we create the shared object which is to be used by our Lua script:

% gcc -shared -I/usr/include/lua5.2 example_wrap.o example.o -o example.so

The Lua script is just the same as our previous blog entry. For convenience:

When run:

% lua test.lua
3
120
2
Wed Jun 14 14:56:54 2017

Hope this helps the world. :)

Compiling and running SWIG for Lua in Ubuntu 16.04

As a disclaimer, this blog entry is adapted from:

http://www.swig.org/tutorial.html
http://www.swig.org/Doc2.0/Lua.html#Lua_nn5

Let's begin. first, you need to have SWIG installed:

% sudo apt-get install swig

You also need to have Lua installed. Say, Lua 5.2:

% sudo apt-get install lua5.2

Let us consider the following file, example.c, whose functions and variable we wish to use in Lua:

Then, we need to create an interface file, say, example.i:

After creating the files, we use SWIG to create a wrapper file:

% swig -lua example.i

This creates a wrapper file named example_wrap.c.

Now it is time to compile the C files:

% gcc -fPIC -I/usr/include/lua5.2 -c example_wrap.c -o example_wrap.o
% gcc -fPIC -c example.c -o example.o

Then we create the shared object which shall be used by our Lua script:

% gcc -shared -I/usr/include/lua5.2 example_wrap.o example.o -o example.so

This creates the shared object, example.so.

Finally, we create our Lua script to test if Lua can call the C functions and variable:

When run, this is the output:

% lua test.lua
3
120
2
Wed Jun 14 14:56:54 2017

Hope this helps the world. :)

Calling C++ function from Lua

I tried to follow this example, but compiling failed.

The example was for Lua 5.1, while I was using Lua 5.2, so some changes had to be made.

First I changed the compile command to as follows:

% g++ luaavg.cpp -llua5.2 -I/usr/include/lua5.2 -o luaavg

Then, in the luaavg.cpp file, I changed "L = lua_open()" to "L = luaL_newstate()".

(To install Lua 5.2 in ubuntu, try this.)

boost mpl if demonstration

Consider the following code adapted from Polukhin:

When run:

detail::pre_inc_functor::operator ()
x = 6
detail::plus_assignable_functor::operator ()
cls::operator +=
y = 90
detail::post_inc_functor::operator ()
post::operator ++
z = 141
post::operator ++
alpha = 141
z = 142