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.)

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

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

Running the Lua (5.2) C API in Ubuntu 14

Took me some time to figure this out trying out this example.

First you need to install Lua:

ubuntu> sudo apt-get install lua5.2
ubuntu> sudo apt-get install liblua5.2-dev

Then compile the source file:

ubuntu> gcc a.c -I/usr/include/lua5.2 -L/usr/lib/x86_64-linux-gnu -llua5.2

I had to make some adjustments in the source file. The adjusted source file is as follows:

Inheritance in Lua

Consider this code:

This is the output when run:

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/oop$ ./inheritance.lua
Calling constructor for table: 0x2414180
Accessing key = new for table: 0x2412f40; self = table: 0x2414180
Calling constructor for table: 0x2412f40
Accessing key = deposit for table: 0x2411eb0; self = table: 0x2412f40
Accessing key = deposit for table: 0x2412f40; self = table: 0x2414180
Account:deposit(): self = table: 0x2411eb0
Accessing key = balance for table: 0x2411eb0; self = table: 0x2412f40
Accessing key = balance for table: 0x2412f40; self = table: 0x2414180
Accessing key = withdraw for table: 0x2411eb0; self = table: 0x2412f40
SpecialAccount:withdraw(): self = table: 0x2411eb0
Accessing key = getLimit for table: 0x2411eb0; self = table: 0x2412f40
SpecialAccount:getLimit(): self = table: 0x2411eb0
Accessing key = get_balance for table: 0x2411eb0; self = table: 0x2412f40
Accessing key = get_balance for table: 0x2412f40; self = table: 0x2414180
Account:get_balance(): self = table: 0x2411eb0
-100
kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/oop$

Notice how searching for a key is recursive. When a key is not found in s, it is searched in SpecialAccount. When the key is not found in SpecialAccount, it is searched in Account.

Classes in Lua

This is an adaptation of the code found here:

When run, the output looks like this:

kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/oop$ ./l.lua 
Calling constructor for table: 0x8a2160
Accessing key = deposit for table: 0x8a1df0; self = table: 0x8a2160
Account:deposit(): self = table: 0x8a1df0
Accessing key = get_balance for table: 0x8a1df0; self = table: 0x8a2160
Account:get_balance(): self = table: 0x8a1df0
a's balance: 100
Calling constructor for table: 0x8a2160
Accessing key = deposit for table: 0x89a4a0; self = table: 0x8a2160
Account:deposit(): self = table: 0x89a4a0
Accessing key = balance for table: 0x89a4a0; self = table: 0x8a2160
Account:balance = 0
Accessing key = withdraw for table: 0x89a4a0; self = table: 0x8a2160
Account:withdraw(): self = table: 0x89a4a0
Accessing key = get_balance for table: 0x89a4a0; self = table: 0x8a2160
Account:get_balance(): self = table: 0x89a4a0
b's balance: 60
kuyu@castor-ub:~/dkuyu/Dropbox/practice/lua/oop$