问题描述

我一直在提升我在 C++ 中嵌入 Lua 脚本的经验，我可以用一只手，在这里.

I've been furthering my experience in embedding Lua scripting in C++, and I could use a hand, here.

考虑以下两个类:

// Person.hpp
#pragma once
#include <string>

class Person {
    private:
        std::string p_Name;
        int p_Age;

    public:
        Person(const std::string & strName, const int & intAge)
            : p_Name(strName), p_Age(intAge) { }

        Person() : p_Name(""), p_Age(0) { }

        std::string getName() const { return p_Name; }
        int getAge() const { return p_Age; }

        void setName(const std::string & strName) { p_Name = strName; }
        void setAge(const int & intAge) { p_Age = intAge; }
};

...和...

// PersonManager.hpp
#pragma once
#include "Person.hpp"
#include <vector>

class PersonManager {
    // Assume that this class is a singleton, and therefore
    // has no public constructor, but a static function that returns the
    // singleton instance.
    private:
        std::vector<Person *> pm_People;

    public:
        bool personExists(const std::string & strName) { /* ... */ }
        bool addPerson(const std::string & strName, const int & intAge) { /* ... */ }
        Person * getPerson(const std::string & strName) { /* ... */ }
        void removePerson(const std::string & strName) { /* ... */ }
        void removeAllPeople() { /* ... */ }
};

... 其中 getPerson 使用 personExists 检查 pm_People 向量以查看具有指定名称的人是否存在.

... where getPerson checks the pm_People vector to see if the person with the specified name exists, using personExists.

现在，考虑以下从 Lua 获取 Person 对象并返回其年龄的函数.

Now, consider the following function that gets a Person object from Lua and returns its age.

// Lua_Person.cpp
#include "Lua_Person.hpp"       // "Lua_Person.hpp" declares the function called to expose the "Person" functions to Lua.
#include "PersonManager.hpp"
#include "Person.hpp"

int lua_GetPersonAge(lua_State * LS) {
    // Validate the userdata.
    luaL_checktype(LS, 1, LUA_TUSERDATA);

    // Get the "Person" userdata.
    Person * luaPerson = reinterpret_cast<Person *>(lua_touserdata(LS, 1));

    // Check to see if the Person pointer is not null.
    if(luaPerson == nullptr)
        luaL_error(LS, "lua_GetPersonAge: You gave me a null pointer!");

    // Push the person's age onto the Lua stack.
    lua_pushnumber(LS, luaPerson->getAge());

    // Return that age integer.
    return 1;
}

我想要做的是从 PersonManager 单例中获取一个已经实例化和现有的 Person 对象，使用 getPerson，并公开那个反对 Lua 的对象，所以我可以做这样的事情:

What I want to do is to get an already-instantiated and existing Person object from the PersonManager singleton, using getPerson, and expose that object to Lua, so I can do something like this:

local testPerson = People.get("Stack Overflower")
print(testPerson:getAge())

我尝试了类似下面的代码块，但无济于事:

I tried something like the code block below, to no avail:

int lua_GetPerson(lua_State * LS) {
    // Validate the argument passed in.
    luaL_checktype(LS, 1, LUA_TSTRING);

    // Get the string.
    std::string personName = lua_tostring(LS, 1);

    // Verify that the person exists.
    if(PersonManager::getInstance().personExists(personName) == false)
        luaL_error(LS, "lua_GetPerson: No one exists with this ID: %s", personName.c_str());

    // Put a new userdata into a double pointer, and assign it to the already existing "Person" object requested.
    Person ** p = static_cast<Person **>(lua_newuserdata(LS, sizeof(Person *)));    // <Userdata>
    *p = PersonManager::getInstance().getPerson(personName);

    // Put that person object into the "Meta_Person" metatable.
    // Assume that metatable is created during the registration of the Person/Person Manager functions with Lua.
    luaL_getmetatable(LS, "Meta_Person");   // <Metatable>, <Userdata>
    lua_setmetatable(LS, -2);               // <Metatable>

    // Return that metatable.
    return 1;
}

有人可以在这里伸出援手，或者至少为我指明正确的方向吗?我没有使用任何 lua 包装库，只是直接使用 Lua.

Can anybody lend a helping hand here, or at least point me in the right direction? I am not using any lua wrapper libraries, just straight Lua.

谢谢.

我用来公开我的 Person 和 PersonManager 函数的函数如下:

The functions that I use to expose my Person and PersonManager functions are as follows:

void exposePerson(lua_State * LS) {
    static const luaL_reg person_functions[] = {
        { "getAge", lua_getPersonAge },
        { nullptr, nullptr }
    };

    luaL_newmetatable(LS, "Meta_Person");
    lua_pushstring(LS, "__index");
    lua_pushvalue(LS, -2);
    lua_settable(LS, -3);

    luaL_openlib(LS, nullptr, person_functions, 0);
}

void exposePersonManager(lua_State * LS) {
    static const luaL_reg pman_functions[] = {
        { "get", lua_getPerson },
        { nullptr, nullptr }
    };

    luaL_openlib(LS, "People", pman_functions, 0);

    lua_pop(LS, 1);
}

推荐答案

让我们从头开始，也就是在 Lua 中注册 PersonManager.由于它是单例，我们将其注册为全局.

Let's start off the top, that is by registering PersonManager in Lua. Since it's a singleton, we'll register it as a global.

void registerPersonManager(lua_State *lua)
{
    //First, we create a userdata instance, that will hold pointer to our singleton
    PersonManager **pmPtr = (PersonManager**)lua_newuserdata(
        lua, sizeof(PersonManager*));
    *pmPtr = PersonManager::getInstance();  //Assuming that's the function that 
                                            //returns our singleton instance

    //Now we create metatable for that object
    luaL_newmetatable(lua, "PersonManagerMetaTable");
    //You should normally check, if the table is newly created or not, but 
    //since it's a singleton, I won't bother.

    //The table is now on the top of the stack.
    //Since we want Lua to look for methods of PersonManager within the metatable, 
    //we must pass reference to it as "__index" metamethod

    lua_pushvalue(lua, -1);
    lua_setfield(lua, -2, "__index");
    //lua_setfield pops the value off the top of the stack and assigns it to our 
    //field. Hence lua_pushvalue, which simply copies our table again on top of the stack.
    //When we invoke lua_setfield, Lua pops our first reference to the table and 
    //stores it as "__index" field in our table, which is also on the second 
    //topmost position of the stack.
    //This part is crucial, as without the "__index" field, Lua won't know where 
    //to look for methods of PersonManager

    luaL_Reg personManagerFunctions[] = {
         'get', lua_PersonManager_getPerson,
          nullptr, nullptr
    };

    luaL_register(lua, 0, personManagerFunctions);

    lua_setmetatable(lua, -2);

    lua_setglobal(lua, "PersonManager");
}

现在我们处理PersonManager的get方法:

int lua_PersonManager_getPerson(lua_State *lua)
{
    //Remember that first arbument should be userdata with your PersonManager 
    //instance, as in Lua you would call PersonManager:getPerson("Stack Overflower");
    //Normally I would first check, if first parameter is userdata with metatable 
    //called PersonManagerMetaTable, for safety reasons

    PersonManager **pmPtr = (PersonManager**)luaL_checkudata(
        lua, 1, "PersonManagerMetaTable");
    std::string personName = luaL_checkstring(lua, 2);

    Person *person = (*pmPtr)->getPerson(personName);
    if (person)
        registerPerson(lua, person);    //Function that registers person. After 
                //the function is called, the newly created instance of Person 
                //object is on top of the stack
    else
        lua_pushnil(lua);

    return 1;
}

void registerPerson(lua_State *lua, Person *person)
{
    //We assume that the person is a valid pointer
    Person **pptr = (Person**)lua_newuserdata(lua, sizeof(Person*));
    *pptr = person; //Store the pointer in userdata. You must take care to ensure 
                    //the pointer is valid entire time Lua has access to it.

    if (luaL_newmetatable(lua, "PersonMetaTable")) //This is important. Since you 
        //may invoke it many times, you should check, whether the table is newly 
        //created or it already exists
    {
        //The table is newly created, so we register its functions
        lua_pushvalue(lua, -1);
        lua_setfield(lua, -2, "__index");

        luaL_Reg personFunctions[] = {
            "getAge", lua_Person_getAge,
            nullptr, nullptr
        };
        luaL_register(lua, 0, personFunctions);
    }

    lua_setmetatable(lua, -2);
}

最后处理Person的getAge.

int lua_Person_getAge(lua_State *lua)
{
    Person **pptr = (Person**)lua_checkudata(lua, 1, "PersonMetaTable");

    lua_pushnumber(lua, (*pptr)->getAge());
    return 1;
}

您现在应该在执行 Lua 代码之前调用 registerPersonManager，最好在您创建新的 Lua 状态并打开所需的库之后调用.

You should now call registerPersonManager before executing your Lua code, best just after you create new Lua state and open needed libraries.

现在在 Lua 中，您应该能够做到这一点:

Now within Lua, you should be able to do that:

local person = PersonManager:getPerson("Stack Overflower");
print(person:getAge());

我目前无法访问 Lua 或 C++ 来测试它，但这应该可以帮助您入门.请注意您授予 Lua 访问权限的 Person 指针的生命周期.

I don't have access to either Lua or C++ at the moment to test it, but that should get you started. Please be careful with lifetime of the Person pointer you give Lua access to.

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