Simple Cpp Binding

wiki

Description

Here is an example of a Lua binding for a simple C++ class. If you prefer, you can factor out all the nasty bits and put them in a template. SimplerCppBinding shows how to do this using Luna for Lua 5.0

C++ code

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

class Account {
  public:
    Account(double balance)      { m_balance = balance; }
    void deposit(double amount)  { m_balance += amount; }
    void withdraw(double amount) { m_balance -= amount; }
    double balance(void)         { return m_balance; }
  private:
    double m_balance;
};

class LuaAccount {
    static const char className[];
    static const luaL_reg methods[];

    static Account *checkaccount(lua_State *L, int narg) {
      luaL_checktype(L, narg, LUA_TUSERDATA);
      void *ud = luaL_checkudata(L, narg, className);
      if(!ud) luaL_typerror(L, narg, className);
      return *(Account**)ud;  // unbox pointer
    }

    static int create_account(lua_State *L) {
      double balance = luaL_checknumber(L, 1);
      Account *a = new Account(balance);
      lua_boxpointer(L, a);
      luaL_getmetatable(L, className);
      lua_setmetatable(L, -2);
      return 1;
    }
    static int deposit(lua_State *L) {
      Account *a = checkaccount(L, 1);
      double amount = luaL_checknumber(L, 2);
      a->deposit(amount);
      return 0;
    }
    static int withdraw(lua_State *L) {
      Account *a = checkaccount(L, 1);
      double amount = luaL_checknumber(L, 2);
      a->withdraw(amount);
      return 0;
    }
    static int balance(lua_State *L) {
      Account *a = checkaccount(L, 1);
      double balance = a->balance();
      lua_pushnumber(L, balance);
      return 1;
    }
    static int gc_account(lua_State *L) {
      Account *a = (Account*)lua_unboxpointer(L, 1);
      delete a;
      return 0;
    }

  public:
    static void Register(lua_State* L) {
      lua_newtable(L);                 int methodtable = lua_gettop(L);
      luaL_newmetatable(L, className); int metatable   = lua_gettop(L);

      lua_pushliteral(L, "__metatable");
      lua_pushvalue(L, methodtable);
      lua_settable(L, metatable);  // hide metatable from Lua getmetatable()

      lua_pushliteral(L, "__index");
      lua_pushvalue(L, methodtable);
      lua_settable(L, metatable);

      lua_pushliteral(L, "__gc");
      lua_pushcfunction(L, gc_account);
      lua_settable(L, metatable);

      lua_pop(L, 1);  // drop metatable

      luaL_openlib(L, 0, methods, 0);  // fill methodtable
      lua_pop(L, 1);  // drop methodtable

      lua_register(L, className, create_account);
    }
};

const char LuaAccount::className[] = "Account";

#define method(class, name) {#name, class::name}

const luaL_reg LuaAccount::methods[] = {
  method(LuaAccount, deposit),
  method(LuaAccount, withdraw),
  method(LuaAccount, balance),
  {0,0}
};

int main(int argc, char* argv[])
{
  lua_State *L = lua_open();

  luaopen_base(L);
  luaopen_table(L);
  luaopen_io(L);
  luaopen_string(L);
  luaopen_math(L);
  luaopen_debug(L);

  LuaAccount::Register(L);

  if(argc>1) lua_dofile(L, argv[1]);

  lua_close(L);
  return 0;
}

Compiling the Code

This code can be compiled for Lua 5.0 as follows:

g++ -o test  account.cc -L/usr/local/lib -llua -llualib

Lua Test Code

function printf(...) io.write(string.format(unpack(arg))) end

local a = Account(100)
printf("Account balance = $%0.02f\n", a:balance() )
a:deposit(50.30)
printf("Account balance = $%0.02f\n", a:balance() )
a:withdraw(25.10)
printf("Account balance = $%0.02f\n", a:balance() )

Test Code Output

$ ./test account.lua
Account balance = $100.00
Account balance = $150.30
Account balance = $125.20