在 C/C++ 中实现 __index 元函数

Question

我有一个 C++ 回调/仿函数系统的脚本，它可以使用字符串和/或变体调用任何“注册的”C++ 函数。

//REMOVED ERROR CHECKS AND ERRONEOUS STUFF FOR THIS POST
int LuaGameObject::LuaCallFunction( lua_State *luaState )
{
    if ( lua_isuserdata( luaState, 1 ) == 1 )
    {
        int nArgs = lua_gettop( luaState );

        //Get GameObject
        OGameObject* pGameObject = static_cast<OGameObject*>(lua_touserdata( luaState, 1 ));
        if ( pGameObject )
        {
            //Get FunctionName
            const char* functionNameString = lua_tostring( luaState, 2 );

            //Get Args
            std::vector<OVariant> args;
            for ( int i = 3; i <= nArgs; ++i )
            {
                OVariant variant;
                variant.SetFromLua( luaState, i );
                args.push_back( variant );
            }

            //Call it!
            CallGameObjectFunction( luaState, pGameObject, functionNameString, args );

            return 1;
        }
    }

    return 0;
}

OVariant LuaGameObject::ExecuteLua()
{
    lua_State *lState = luaL_newstate();

    luaL_openlibs( lState );
    lua_register( lState, "Call", LuaCallFunction );

    luaL_loadstring( lState, m_pScript );

    //now run it
    lua_pcall( lState, 0, 1, 0 );

    //process return values
    OVariant result;
    result.SetFromLua( lState, -1 );

    lua_close( lState );

    return result;
}

在 lua 中我可以做这样的事情......

local king = Call("EmpireManager","GetKing")
Call("MapCamera","ZoomToActor",king)

不过，我感觉可以使用__index元方法来简化lua...

local king = EmpireManager:GetKing()
MapCamera:ZoomToActor(king)

我希望通过使用__index元方法的以下实现来实现简化的lua

这是我注册 __index 元函数的方法...（主要是从在线示例中复制的）

void LuaGameObject::Register( lua_State * l )
{
    luaL_Reg sRegs[] =
    {
        { "__index", &LuaGameObject::LuaCallFunction },
        { NULL, NULL }
    };

    luaL_newmetatable( l, "luaL_EmpireManager" );

    // Register the C functions into the metatable we just created.
    luaL_setfuncs( l, sRegs, 0 );
    lua_pushvalue( l, -1 );

    // Set the "__index" field of the metatable to point to itself
    // This pops the stack
    lua_setfield( l, -1, "__index" );

    // Now we use setglobal to officially expose the luaL_EmpireManager metatable 
    // to Lua. And we use the name "EmpireManager".
    lua_setglobal( l, "EmpireManager" );
}

不幸的是，我似乎无法正确设置回调。 Lua 正确调用了我的 LuaGameObject::LuaCallFunction，但堆栈不包含我想要的内容。在 LuaGameObject::LuaCallFunction 中，我可以在堆栈上找到函数名称和 EmpireManager 对象。但是，我在堆栈上找不到参数。设置它的正确方法是什么？还是不可能？

Answer 1

绝对可以在 Lua 中为 userdata 类型添加方法，如官方网站的 Programming in Lua 指南中所述。

当你键入以下 Lua 代码时：

myUserdata:someMethod(arg1,arg2,arg3)

假设myUserdata 是一个“userdata”对象，解释器将执行以下操作。

1. 调用getmetatable(myUserdata).__index(myUserdata,"someMethod")获取someMethod的值。
2. 致电someMethod(myUserdata,arg1,arg2,arg3)。 someMethod 可以是任何可以从 Lua 调用的东西。示例：Lua 或 C 函数，或带有 __call 元方法的表/用户数据。

您的__index 元方法应该只返回一个实现该方法的函数（或另一个可从 Lua 调用的对象）。像这样的：

// IMO, quite a misleading name for the __index metamethod (there is a __call metamethod)
int LuaGameObject::LuaCallFunction( lua_State *l)
{
    // todo: error checking
    OGameObject* pGameObject = static_cast<OGameObject*>(lua_touserdata( luaState, 1 ));
    std::string memberName = lua_tostring( luaState, 2 );

    int result = 1;
    if (memberName == "method1") {
        lua_pushcfunction(l,LuaGameObject::luaMethod1);
    } else if (memberName == "method2") {
        lua_pushcfunction(l,LuaGameObject::luaMethod2);
    } else {
        result = 0;
    }

    return result;
}

__index 元方法返回的函数的基本框架：

int LuaGameObject::luaMethod1(lua_State* l) {
    // todo: error checking.
    OGameObject* pGameObject = static_cast<OGameObject*>(lua_touserdata(l, 1));
    float arg1 = lua_tonumber(l, 2);
    // get other args
    pGameObject->method1(arg1 /*, more args if any.*/);
    // optionally push return values on the stack.
    return 0; // <-- number of return values.
}

Answer 2

好的，经过更多研究，我现在相信我不能使用 __index 元函数来调用带有参数的 c 仿函数。它只将表名和键传递给回调。

但是，对于任何感兴趣的人，它可以用于类似表格的对象，但不能用于函数（因为参数不会被压入堆栈）。我将它用于我的“财产”对象。它们没有参数，可以在lua中使用如下...

local king = EmpireManager:king
king:name = "Arthur"
local name = king:name

这些正确链接到并调用适当的 C++ objects.functions

Actor::SetName(std::string name)
std::string Actor::GetName()

Answer 3

我在从我的对象中调用方法时遇到了同样的问题，并使用这篇文章来开发解决方案。

希望下面的例子对你有用。

#include <iostream>
#include <string>
#include <map>
#include <functional>

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

//template<class UserdataType>          // if will be work with lua garbage collector, use a function like that to delete the this_ptr (1st param)
//int DeletePtr(lua_State *lua_state) { // It's necessary register the metatable.__gc and to trust in gc (create just pointer of LuaObjects
//  UserdataType** this_ptr = reinterpret_cast<UserdataType**>(lua_touserdata(lua_state, 1));
//  delete (*this_ptr);
//  return 0;
//}

template<class UserdataType>
int Closure(lua_State *lua_state) {
  UserdataType** ptr = reinterpret_cast<UserdataType**>(lua_touserdata(lua_state, 1)); // This closure is being called by call operator ()
  return (*ptr)->CallFunction(lua_state);                                              // To access the function name called use lua stack index with lua_upvalueindex(-1)
}                                                                                      // Call the object method to resolve this called there

template<class UserdataType>
int ReturnClosure(lua_State *lua_state) {                // This function is called as a lookup of metatable.__index
  lua_pushcclosure(lua_state, Closure<UserdataType>, 1); // then we will return a closure to be called through call operator ()
  return 1;                                              // The 1st param (the only one) is the action name of function
}                                                        // Then a closure will grant access to ReturnClosure params as upvalues (lua_upvalueindex)

class LuaObject {
public:
  LuaObject() :  userdata_name("userdata1") {
  } 

  void CreateNewUserData(lua_State* lua_ptr, const std::string& global_name) {
    RegisterUserData(lua_ptr);
    LuaObject** this_ptr = reinterpret_cast<LuaObject**>(lua_newuserdata(lua_ptr, sizeof(LuaObject*)));
    *this_ptr = this;
    luaL_getmetatable(lua_ptr, userdata_name.c_str()); 
    lua_setmetatable(lua_ptr, -2);                        // setmetatable(this_ptr, userdata_name)
    lua_setglobal(lua_ptr, global_name.c_str());          // store to global scope
  }

  int CallFunction(lua_State* lua_state) const {
    std::string name = lua_tostring(lua_state, lua_upvalueindex(1)); // userdata:<function>(param2, param3)
    auto it = functions.find(name);                                  // <function> lua_tostring(lua_state, lua_upvalueindex(1))
    if (it != functions.end()) {                                     // <implicit this> lua_touserdata(l, 1)
      return it->second(lua_state);                                  // <param #1> lua_touserdata(l, 2)
    }                                                                // <param #2> lua_touserdata(l, 3)
    return 0;                                                        // <param #n> lua_touserdata(l, n+1)
  }

  void NewFunction(const std::string& name, std::function<int(lua_State*)> func) {
    functions[name] = func;
  }
private:
  void RegisterUserData(lua_State* lua_ptr) {
    luaL_getmetatable(lua_ptr, userdata_name.c_str());
    if (lua_type(lua_ptr, -1) == LUA_TNIL) {
      /* create metatable for userdata_name */
      luaL_newmetatable(lua_ptr, userdata_name.c_str());
      lua_pushvalue(lua_ptr, -1); /* push metatable */
 
      /* metatable.__gc = DeletePtr<LuaObject> */
      //lua_pushcfunction(lua_ptr, DeletePtr<LuaObject>);
      //lua_setfield(lua_ptr, -2, "__gc");

      /* metatable.__index = ReturnClosure<LuaObject> */
      lua_pushcfunction(lua_ptr, ReturnClosure<LuaObject>);
      lua_setfield(lua_ptr, -2, "__index");
    }
  }
  std::map<std::string, std::function<int(lua_State*)>> functions;
  std::string userdata_name;
};

int main(int argc, char* argv[]) {
  lua_State* lua_state = luaL_newstate();
  luaL_openlibs(lua_state);
    
  LuaObject luaobj;
  luaobj.CreateNewUserData(lua_state, "test_obj");
  luaobj.NewFunction("action", [](lua_State* l)->int {
    std::string result = "action has been executed";
    LuaObject** ptr = reinterpret_cast<LuaObject**>(lua_touserdata(l, 1));
    result += "\n #1 param is user_data (self == this) value = " + std::to_string(reinterpret_cast<size_t>(*ptr));     
    for (int i = 2; i <= lua_gettop(l); ++i) {
      result += "\n #" + std::to_string(i)+ " = " + lua_tostring(l, i);
    }
    result += "\n #n param is passed on call operator () #n = " + std::to_string(lua_gettop(l));
    lua_pushfstring(l, result.c_str());
    return 1;
  });

  std::string lua_code;    
  lua_code += "print(test_obj:unknown_function()) \n";  
  lua_code += "print(test_obj:action())           \n";
  lua_code += "print(test_obj:action(1))          \n";
  lua_code += "print(test_obj:action(1, 2))       \n";
  lua_code += "print(test_obj:action(1, 2, 'abc'))\n";

  if (!(luaL_loadbuffer(lua_state, lua_code.c_str(), lua_code.length(), NULL) == 0 && lua_pcall(lua_state, 0, LUA_MULTRET, 0) == 0)) {
      std::cerr << "Lua Code Fail: " << lua_tostring(lua_state, -1) << std::endl;        
  }  
  lua_close(lua_state);
  return 0;
}

输出：

action has been executed
 #1 param is user_data (self == this) value = 13629232
 #n param is passed on call operator () #n = 1
action has been executed
 #1 param is user_data (self == this) value = 13629232
 #2 = 1
 #n param is passed on call operator () #n = 2
action has been executed
 #1 param is user_data (self == this) value = 13629232
 #2 = 1
 #3 = 2
 #n param is passed on call operator () #n = 3
action has been executed
 #1 param is user_data (self == this) value = 13629232
 #2 = 1
 #3 = 2
 #4 = abc
 #n param is passed on call operator () #n = 4