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

boost mpl manipulating a vector

Consider the following code copied from Polukhin:


When run, this code simply prints "Hello World! 78"

Luabind tostring demonstration

Consider the following code:

When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/tostring$ cat test.lua
package.loadlib('./testclass.so', 'init')()
a = testclass(101)
print(tostring(a))
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/tostring$ cat commands.bash
#!/bin/bash
g++ testclass.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o testclass.so testclass.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/tostring$ ./commands.bash
101
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/tostring$

Luabind operator other() demonstration

Consider the following code:


When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator_other$ cat test.lua
package.loadlib('./testclass.so', 'init')()
a = testclass(5)
b = testclass(9)
c = a + b
a:print()
b:print()
c:print()
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator_other$ cat commands.bash
#!/bin/bash
g++ testclass.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o testclass.so testclass.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator_other$ ./commands.bash
5
9
14
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator_other$

Luabind operator demonstration

Consider the following code:


When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator$ cat test.lua
package.loadlib('./testclass.so', 'init')()
a = testclass(5)
a:print()
c = a + 6
c:print()
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator$ cat commands.bash 
#!/bin/bash
g++ testclass.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o testclass.so testclass.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator$ ./commands.bash 
5
11
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/operator$ 

Luabind: enums demonstration

Consider the following code:

When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/enums$ cat test.lua 
package.loadlib('./testclass.so', 'init')()
print(A.first)
print(A.second)
print(A.third)
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/enums$ cat ./commands.bash 
#!/bin/bash
g++ testclass.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o testclass.so testclass.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/enums$ ./commands.bash 
1
2
3
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/enums$ 

You actually don't have to define the enum in the C++ struct A. Try removing it and the test.lua will still work.

boost mpl vector demonstration

Consider the following code copied from Polukhin's book:


When run, this code prints nothing. All checking is done at compile time.

boost enable_if_c example

Consider the following code adapted from Polukhin's book:


When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/cpp/boost/polukhin/ch04/enable_if_c$ g++ -I~/dkuyu/bin/boost_1_60_0 c.cpp && ./a.out
double data_processor<T, typename boost::enable_if_c<boost::is_integral<T>::value>::type>::process(const T&, const T&, const T&) [with T = int; typename boost::enable_if_c<boost::is_integral<T>::value>::type = void]
double data_processor<T, typename boost::enable_if_c<boost::is_integral<T>::value>::type>::process(const T&, const T&, const T&) [with T = short int; typename boost::enable_if_c<boost::is_integral<T>::value>::type = void]
double data_processor<T, typename boost::enable_if_c<boost::is_float<T>::value>::type>::process(const T&, const T&, const T&) [with T = double; typename boost::enable_if_c<boost::is_float<T>::value>::type = void]
double data_processor<T, typename boost::enable_if_c<boost::is_float<T>::value>::type>::process(const T&, const T&, const T&) [with T = float; typename boost::enable_if_c<boost::is_float<T>::value>::type = void]
double data_processor<T, enable>::process(const T&, const T&, const T&) [with T = const char*; enable = void]

Luabind: properties demonstration

Consider the following code:

When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/properties$ cat commands.bash 
#!/bin/bash
g++ testclass.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o testclass.so testclass.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/properties$ ./commands.bash 
7
89
5
lua: property 'b' is read only
stack traceback:
[C]: in function '__newindex'
test.lua:7: in main chunk
[C]: in ?
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/properties$ 

Luabind: Binding an overloaded class function

Consider the following code:


When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/overloaded_function$ cat test.lua 
package.loadlib('./overloaded_function.so', 'init')()
a = overloaded_function()
a:print_string(6)
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/overloaded_function$ cat commands.bash 
#!/bin/bash
g++ overloaded_function.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o overloaded_function.so overloaded_function.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/overloaded_function$ ./commands.bash 
print_string: 6

Consider the following code which registers a free function (plus()) as a member function of class A:

When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/register_free$ cat test.lua
package.loadlib('./register_free.so', 'init')()
x = A(3)
print(x:plus(5))
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/register_free$ cat commands.bash
#!/bin/bash
g++ register_free.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o register_free.so register_free.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/register_free$ ./commands.bash
8
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/register_free$

Luabind class demonstration

Consider this code that exposes a C++ class to Lua:

When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/testclass$ cat test.lua 
package.loadlib('./testclass.so', 'init')()
a = testclass('a string')
a:print_string()
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/testclass$ cat commands.bash 
#!/bin/bash
g++ testclass.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o testclass.so testclass.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/testclass$ ./commands.bash 
a string

Implementing a type trait

I copied/adapted this code from Polukhin's Boost book:

When run:

kuyu@ub16:~/dkuyu/Dropbox/practice/cpp/polukhin/ch04/implement_type_trait$ g++ -std=c++11 -I/home/kuyu/dkuyu/bin/boost_1_60_0 1.cpp && ./a.out
int=false
vector<int>=true
vector<char*>=true

How to make a Windows folder always available in Ubuntu in Oracle Virtualbox

First share your Windows folder to your Ubuntu virtual machine in Oracle Virtualbox:

In your Ubuntu machine, add a similar line to your /etc/fstab file:

dkuyu   /home/kuyu/dkuyu    vboxsf  uid=kuyu,gid=kuyu,rw,dmode=700,fmode=700,_netdev      0 0

(Be sure to create the Linux folder afterwards. You can do this via mkdir command. In this example, make sure that /home/kuyu/dkuyu exists. If not, create it.)

Reboot your Ubuntu virtual machine.

After reboot, the Windows folder should be available in the Ubuntu folder /home/kuyu/dkuyu (for this example).

Luabind for sin math function

First we register the cmath sin() function as a function named sin() in Lua:

We write a simple script to test the Lua sin() function:

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/luabind/sin$ cat test.lua 
package.loadlib('./c.so', 'init')()
print(sin(1.57))

We compile and test it as follows:

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/luabind/sin$ cat commands.bash 

#!/bin/bash

g++ c.cpp -I/usr/include/lua5.2/ -c -fPIC

g++ -shared -Wl,--whole-archive -o c.so c.o -lluabind -Wl,--no-whole-archive

lua test.lua

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/luabind/sin$ ./commands.bash 

0.99999970197678

(1.57 is very close to pi/2 and the sin of pi/2 is 1.)

Making luabind work in ubuntu

To be able to run Luabind in Ubuntu, you need to install libluabind-dev and lua:
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/hellobind$ sudo apt-get install libluabind-dev
kuyu@ub16:~/dkuyu/Dropbox/practice/lua/luabind/hellobind$ sudo apt-get install lua

Consider this hellobind.cpp code:
Then consider this test.lua code:

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/luabind/hellobind$ cat test.lua
package.loadlib('./hellobind.so', 'init')()
greet()

To run the test.lua code:

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/luabind/hellobind$ cat commands.bash 
#!/bin/bash
g++ hellobind.cpp -I/usr/include/lua5.2/ -c -fPIC
g++ -shared -Wl,--whole-archive -o hellobind.so hellobind.o -lluabind -Wl,--no-whole-archive
lua test.lua
kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/luabind/hellobind$ ./commands.bash 
hello world!

Sometimes, loading the C++ library in Lua can be problematic. To see what the problems are when loading the library, you can execute this lua file (execute "lua test1.lua"):

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/luabind/hellobind$ cat test1.lua
local initfunction, errormessage = package.loadlib('/home/kuyu/dkuyu/Dropbox/practice/lua/luabind/hellobind.so','init')
if errormessage then
    print('Error loading hello_world:', errormessage)
end

boost::mpl::map demonstration

When run:

kuyu@castor-ub:~/dkuyu/Dropbox/practice/cpp/boost/mpl$ g++ -I/home/kuyu/dkuyu/bin/boost_1_60_0 map.cpp && ./a.out

x=5