Using Lua with C++: A short tutorial

Using Lua with C++: A short tutorial

Written by Christian Stigen Larsen <csl@sublevel3.org>

Using Lua is easy! In this short tutorial we'll show how to write a fully working host-program in C++ with Lua callbacks.

Since the static Lua libraries are written in C, you must import them as such:

extern "C" {
#include "lua.h"
}

int main() {
lua_State *L = lua_open();
lua_close(L);
return 0;
}

Compiling and linking with GNU g++:

g++ host.cpp -o host -Ilua-5.0.2/include/ -Llua-5.0.2/lib/ -llua

Including lualib.h and lauxlib.h makes it easy to write a fully working host:

#include <iostream>

extern "C" {
#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"
}

void report_errors(lua_State *L, int status) {
if ( status!=0 ) {
std::cerr << "-- " << lua_tostring(L, -1) << std::endl;
lua_pop(L, 1); // remove error message
}
}

int main(int argc, char** argv) {
for ( int n=1; n<argc; ++n ) {
const char* file = argv
;

lua_State *L = lua_open();

luaopen_io(L); // provides io.*
luaopen_base(L);
luaopen_table(L);
luaopen_string(L);
luaopen_math(L);
luaopen_loadlib(L);

std::cerr << "-- Loading file: " << file << std::endl;

int s = luaL_loadfile(L, file);

if ( s==0 ) {
// execute Lua program
s = lua_pcall(L, 0, LUA_MULTRET, 0);
}

report_errors(L, s);
lua_close(L);
std::cerr << std::endl;
}

return 0;
}

Compilation and linking:

g++ host.cpp -o host -Ilua-5.0.2/include/ -Llua-5.0.2/lib/ -llua -llualib

Let's test this with some Lua programs. The files here are from the distribution, hello.lua is simply:

-- the first program in every language
io.write("Hello world, from ",_VERSION,"!/n")

Executing a couple of Lua programs with our host program produces:

[csl@eris:~/dev/lua/lua-5.0.2]$ ./host test/hello.lua test/printf.lua
-- Loading file: test/hello.lua
Hello world, from Lua 5.0.2!

-- Loading file: test/printf.lua
Hello csl from Lua 5.0.2 on Wed Mar  2 13:13:05 2005

It gets very interesting when Lua programs call your own functions. In the following program, we define a function my_function() and register it with the Lua environment using lua_register(). Our function prints its arguments as strings and returns the integer value of 123.

#include <iostream>

extern "C" {
#include "lua.h"
#include "lualib.h"
#include "lauxlib.h"
}

int my_function(lua_State *L) {
int argc = lua_gettop(L);

std::cerr << "-- my_function() called with " << argc
<< " arguments:" << std::endl;

for ( int n=1; n<=argc; ++n ) {
std::cerr << "-- argument " << n << ": "
<< lua_tostring(L, n) << std::endl;
}

lua_pushnumber(L, 123); // return value
return 1; // number of return values
}

void report_errors(lua_State *L, int status) {
if ( status!=0 ) {
std::cerr << "-- " << lua_tostring(L, -1) << std::endl;
lua_pop(L, 1); // remove error message
}
}

int main(int argc, char** argv) {
for ( int n=1; n<argc; ++n ) {
const char* file = argv
;

lua_State *L = lua_open();

luaopen_io(L); // provides io.*
luaopen_base(L);
luaopen_table(L);
luaopen_string(L);
luaopen_math(L);
luaopen_loadlib(L);

// make my_function() available to Lua programs
lua_register(L, "my_function", my_function);

std::cerr << "-- Loading file: " << file << std::endl;

int s = luaL_loadfile(L, file);

if ( s==0 ) {
// execute Lua program
s = lua_pcall(L, 0, LUA_MULTRET, 0);
}

report_errors(L, s);
lua_close(L);
std::cerr << std::endl;
}

return 0;
}

Let's write a small Lua program test.lua to call my_function():

io.write("Running ", _VERSION, "/n")
a = my_function(1, 2, 3, "abc", "def")
io.write("my_function() returned ", a, "/n")