更简单的 Cpp 绑定

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描述

这是来自 SimpleCppBinding 的示例，但这次使用的是 Lua 5.0 版本的 Luna [1]。

将您想要从 C++ 导出到 Lua 的方法放在 Account::methods 表中。当使用 obj:method(...) 语法时，userdata 的 *index* 事件将在方法表中查找方法。

方法表存储在名为 Account 的全局变量中，以便 Lua 脚本可以添加用 Lua 编写的函数。

新的 Account 对象可以在 Lua 脚本中使用 Account:new(...) 或 Account(...) 语法创建。后者是使用方法表的 *call* 事件实现的。由 Lua 脚本创建的任何 C++ 对象都将在 userdata 的 *gc* 事件中被删除。

userdata 元表通过将 __metatable 字段设置为方法表来隐藏在 Lua 脚本中。请注意 getmetatable 如何返回方法表，该方法表也存储在全局变量 Account 中。

方法表有一个元表，使继承更容易。为了让 Account 继承 parent 的方法，请像这样设置方法表的 *index* 事件：getmetatable(Account).__index = parent

一个 [UML 图] 可能有助于可视化表关系。引用组合链接表示元表关系。

如果您想从 C++ 代码调用 Lua 函数，请参阅 CallingLuaFromCpp，并参阅 CppBindingWithLunar 以了解 Luna 5 的改进版本。

account.cc C++ 代码

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

#include "luna.h"

class Account {
  lua_Number m_balance;
public:
  static const char className[];
  static Luna<Account>::RegType methods[];

  Account(lua_State *L)      { m_balance = luaL_checknumber(L, 1); }
  int deposit (lua_State *L) { m_balance += luaL_checknumber(L, 1); return 0; }
  int withdraw(lua_State *L) { m_balance -= luaL_checknumber(L, 1); return 0; }
  int balance (lua_State *L) { lua_pushnumber(L, m_balance); return 1; }
  ~Account() { printf("deleted Account (%p)\n", this); }
};

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

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

Luna<Account>::RegType Account::methods[] = {
  method(Account, deposit),
  method(Account, withdraw),
  method(Account, 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);

  Luna<Account>::Register(L);

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

  lua_setgcthreshold(L, 0);  // collected garbage
  lua_close(L);
  return 0;
}

适用于 Lua 5.0 的 luna.h

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

template <typename T> class Luna {
  typedef struct { T *pT; } userdataType;
public:
  typedef int (T::*mfp)(lua_State *L);
  typedef struct { const char *name; mfp mfunc; } RegType;

  static void Register(lua_State *L) {
    lua_newtable(L);
    int methods = lua_gettop(L);

    luaL_newmetatable(L, T::className);
    int metatable = lua_gettop(L);

    // store method table in globals so that
    // scripts can add functions written in Lua.
    lua_pushstring(L, T::className);
    lua_pushvalue(L, methods);
    lua_settable(L, LUA_GLOBALSINDEX);

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

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

    lua_pushliteral(L, "__tostring");
    lua_pushcfunction(L, tostring_T);
    lua_settable(L, metatable);

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

    lua_newtable(L);                // mt for method table
    int mt = lua_gettop(L);
    lua_pushliteral(L, "__call");
    lua_pushcfunction(L, new_T);
    lua_pushliteral(L, "new");
    lua_pushvalue(L, -2);           // dup new_T function
    lua_settable(L, methods);       // add new_T to method table
    lua_settable(L, mt);            // mt.__call = new_T
    lua_setmetatable(L, methods);

    // fill method table with methods from class T
    for (RegType *l = T::methods; l->name; l++) {
    /* edited by Snaily: shouldn't it be const RegType *l ... ? */
      lua_pushstring(L, l->name);
      lua_pushlightuserdata(L, (void*)l);
      lua_pushcclosure(L, thunk, 1);
      lua_settable(L, methods);
    }

    lua_pop(L, 2);  // drop metatable and method table
  }

  // get userdata from Lua stack and return pointer to T object
  static T *check(lua_State *L, int narg) {
    userdataType *ud =
      static_cast<userdataType*>(luaL_checkudata(L, narg, T::className));
    if(!ud) luaL_typerror(L, narg, T::className);
    return ud->pT;  // pointer to T object
  }

private:
  Luna();  // hide default constructor

  // member function dispatcher
  static int thunk(lua_State *L) {
    // stack has userdata, followed by method args
    T *obj = check(L, 1);  // get 'self', or if you prefer, 'this'
    lua_remove(L, 1);  // remove self so member function args start at index 1
    // get member function from upvalue
    RegType *l = static_cast<RegType*>(lua_touserdata(L, lua_upvalueindex(1)));
    return (obj->*(l->mfunc))(L);  // call member function
  }

  // create a new T object and
  // push onto the Lua stack a userdata containing a pointer to T object
  static int new_T(lua_State *L) {
    lua_remove(L, 1);   // use classname:new(), instead of classname.new()
    T *obj = new T(L);  // call constructor for T objects
    userdataType *ud =
      static_cast<userdataType*>(lua_newuserdata(L, sizeof(userdataType)));
    ud->pT = obj;  // store pointer to object in userdata
    luaL_getmetatable(L, T::className);  // lookup metatable in Lua registry
    lua_setmetatable(L, -2);
    return 1;  // userdata containing pointer to T object
  }

  // garbage collection metamethod
  static int gc_T(lua_State *L) {
    userdataType *ud = static_cast<userdataType*>(lua_touserdata(L, 1));
    T *obj = ud->pT;
    delete obj;  // call destructor for T objects
    return 0;
  }

  static int tostring_T (lua_State *L) {
    char buff[32];
    userdataType *ud = static_cast<userdataType*>(lua_touserdata(L, 1));
    T *obj = ud->pT;
    sprintf(buff, "%p", obj);
    lua_pushfstring(L, "%s (%s)", T::className, buff);
    return 1;
  }
};

编译代码

此代码可以为 Lua 5.0 编译如下

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

Lua 测试代码

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

function Account:show()
  printf("Account balance = $%0.02f\n", self:balance())
end

a = Account(100)
b = Account:new(30)

print('a =', a)
print('b =', b)
print('metatable =', getmetatable(a))
print('Account =', Account)
table.foreach(Account, print)

a:show() a:deposit(50.30) a:show() a:withdraw(25.10) a:show()

parent = {}

function parent:rob(amount)
  amount = amount or self:balance()
  self:withdraw(amount)
  return amount
end

getmetatable(Account).__index = parent

debug.debug()

测试代码输出

$ ./test account.lua
a =     Account (0xa041d98)
b =     Account (0xa045390)
metatable =     table: 0xa044f28
Account =       table: 0xa044f28
show    function: 0xa046760
balance function: 0xa0455f8
withdraw        function: 0xa045300
deposit function: 0xa045508
new     function: 0xa044fe8
Account balance = $100.00
Account balance = $150.30
Account balance = $125.20
lua_debug> a:show()
Account balance = $125.20
lua_debug> b:show()
Account balance = $30.00
lua_debug> print(a:rob(20))
20
lua_debug> a:show()
Account balance = $105.20
lua_debug> b:deposit(a:rob())
lua_debug> a:show()
Account balance = $0.00
lua_debug> b:show()
Account balance = $135.20
lua_debug> cont
deleted Account (0xa045390)
deleted Account (0xa041d98)

传递对象

如果您想将对象传递给另一个 C++ 函数，可以使用 check 函数来验证 userdata 是否为正确的类型，并获取指向该对象的指针。

在本例中，C 函数 rob 从 Account 中扣除 20 美元。

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

#include "luna.h"

class Account {
  lua_Number m_balance;
public:
  Account(double balance=0)    : m_balance(balance) { }
  void deposit(double amount)  { m_balance += amount; }
  void withdraw(double amount) { m_balance -= amount; }
  double balance(void)         { return m_balance; }
  ~Account() { printf("deleted Account (%p)\n", this); }

  // Lua interface
  Account(lua_State *L) :     m_balance(luaL_checknumber(L, 1)) { }
  int deposit (lua_State *L) { deposit (luaL_checknumber(L, 1)); return 0; }
  int withdraw(lua_State *L) { withdraw(luaL_checknumber(L, 1)); return 0; }
  int balance (lua_State *L) { lua_pushnumber(L, balance()); return 1; }

  static const char className[];
  static Luna<Account>::RegType methods[];
};

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

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

Luna<Account>::RegType Account::methods[] = {
  method(Account, deposit),
  method(Account, withdraw),
  method(Account, balance),
  {0,0}
};

static int report (lua_State *L, int status)
{
  if (status) {
    const char *msg = lua_tostring(L, -1);
    if (msg == NULL) msg = "(error with no message)";
    fprintf(stderr, "ERROR: %s\n", msg);
    lua_pop(L, 1);
  }
  return status;
}

static int application (lua_State *L)
{
  lua_settop(L, 0);
  lua_pushliteral(L, "_TRACEBACK");
  lua_rawget(L, LUA_GLOBALSINDEX);   // get traceback function
  int tb = lua_gettop(L);

  lua_pushliteral(L, "main");
  lua_gettable(L, LUA_GLOBALSINDEX);
  report(L, lua_pcall(L, 0, 1, tb));
  Account *a = Luna<Account>::check(L, -1);
  printf("the balance of 'a' is $%.2lf\n", a->balance());

  return 0;
}

static int rob (lua_State *L)
{
  Account *b = Luna<Account>::check(L, 1);
  b->withdraw(20.00);
  printf("take $20.00 from 'b'. the balance of 'b' is $%.2lf\n", b->balance());
  return 1;
}

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);

  Luna<Account>::Register(L);

  lua_register(L, "rob", rob);

  if (argc>1) {
    printf("loading '%s'\n", argv[1]);
    if (report(L, luaL_loadfile(L, argv[1]) || lua_pcall(L, 0, 0, 0)) == 0) {
      printf("running application\n");
      if (report(L, lua_cpcall(L, &application, 0)) == 0) {
        printf("okay\n");
      }
    }
  }

  lua_setgcthreshold(L, 0);  // collected garbage
  lua_close(L);
  return 0;
}

Lua 测试代码

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

function Account:show()
  printf("Account balance = $%0.02f\n", self:balance())
end

b = Account(30)
print('b =', b)
b:show()
rob(b)
b:show()

function main()
  a = Account(100)
  print('a =', a)
  a:show() a:deposit(50.30) a:show()
  return a
end

测试代码输出

$ ./test account.lua
loading 'account4.lua'
b =     Account (0xa041d98)
Account balance = $30.00
take $20.00 from 'b'. the balance of 'b' is $10.00
Account balance = $10.00
running application
a =     Account (0xa045390)
Account balance = $100.00
Account balance = $150.30
the balance of 'a' is $150.30
okay
deleted Account (0xa045390)
deleted Account (0xa041d98)

另请参阅

• 将代码绑定到 Lua