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.