17. 嵌入Lua | 游戏引擎浅入浅出

## 17. 嵌入Lua

```text 
「游戏引擎 浅入浅出」是一本开源电子书，PDF/随书代码/资源下载： https://github.com/ThisisGame/cpp-game-engine-book 
```

引擎支持Lua的目的，主要就是为了热更新。

C++与Lua交互，手写代码会很麻烦，下面是C++向Lua注册类的流程。

![](md/cpp-game-engine-book/imgs/integrate_lua/integrate_lua/lua_call_class.jpg)

### 1. 手写代码注册C++到Lua

```bash
CLion项目文件位于 samples\integrate_lua\integrate_lua\lua_call_cpp_wrap
```

下面用一个简单例子，将`Player`注册到lua中，来看看到底多复杂。

```c++
class Player
{
public:
    int AddHp(int add)
    {
        hp_+=add;
        return hp_;
    }

int hp_=0;
};
```

执行的lua文件如下：

```lua
local player = Player()
print(player)
print("player.hp_:" .. player.hp_)
print("player.AddHp(4):" .. player:AddHp(4))
print("player.hp_:" .. player.hp_)
```

下面编写注册代码。
由于Lua只能与C交互，所以C++的类函数，都要写一个对应的C函数。<a id="antiCollectorAdTxt" href="https://github.com/ThisisGame/cpp-game-engine-book">「游戏引擎 浅入浅出」是一本开源电子书，PDF/随书代码/资源下载： https://github.com/ThisisGame/cpp-game-engine-book</a>

```c++
static int CreatePlayer(lua_State* L)
{
    Player** pData = (Player**)lua_newuserdata(L, sizeof(Player*));
    *pData = new Player();
    luaL_getmetatable(L, "Player");
    lua_setmetatable(L, -2);
    return 1;
}

static int DestroyPlayer(lua_State* L)
{
    delete *(Player**)lua_topointer(L, 1);
    return 0;
}

static int CallAddHp(lua_State* L)
{
    Player* pPlayer = *(Player**)lua_topointer(L, 1);
    lua_pushnumber(L, pPlayer->AddHp(lua_tonumber(L, 2)));
    return 1;
}

static int lua_index(lua_State* L)
{
    Player* pPlayer = *(Player**)lua_topointer(L, 1);

if (strcmp(lua_tostring(L, 2), "hp_") == 0)
    {
        lua_pushnumber(L, pPlayer->hp_);
    }
    else if (strcmp(lua_tostring(L, 2), "AddHp") == 0)
    {
        lua_pushcfunction(L, CallAddHp);
    }
    else
    {
        return 0;
    }
    return 1;
}

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

lua_pushcfunction(L, CreatePlayer);
    lua_setglobal(L, "Player");

luaL_newmetatable(L, "Player");

lua_pushstring(L, "__gc");
    lua_pushcfunction(L, DestroyPlayer);
    lua_settable(L, -3);

lua_pushstring(L, "__index");
    lua_pushcfunction(L, lua_index);
    lua_settable(L, -3);

lua_pop(L, 1);

luaL_dofile(L, "a.lua");

lua_close(L);

return 0;
}
```

需要写60行代码,才能注册一个 10 行的类到Lua，实际上这个例子都是我复制的，因为我根本不会用Lua的API。

还好Github上有很多绑定库可以用，这里选择`Sol2`。

### 2. 使用Sol2注册C++类到Lua

```bash
CLion项目文件位于 samples\integrate_lua\integrate_lua\lua_call_cpp_sol2
```

引入`Sol2`库之后，只需要6行代码，就可以将Player注册到Lua中。

```c++
int main(int argc, char * argv[])
{
    sol::state sol_state;
    sol_state.open_libraries(sol::lib::base);
    //注册构造函数到lua
    sol::usertype<Player> usertype_player=sol_state.new_usertype<Player>("Player",sol::constructors<Player()>());
    //注册成员函数和变量到lua
    usertype_player["AddHp"]=&Player::AddHp;
    usertype_player["hp_"]=&Player::hp_;

sol_state.script_file("../a.lua");

return 0;
}
```

![](md/cpp-game-engine-book/imgs/integrate_lua/integrate_lua/use_sol2.jpg)