c++ - 如何在与LUA脚本一起使用的Lambda中引用 'this'

Question

我试图将LUA API添加到我的C++程序中，并且试图允许该脚本绘制到我的GUI中。到目前为止，我的lambda函数具有以下功能:

auto addToDrawList = [](lua_State* L) -> int
{
    int DrawType = (int)lua_tonumber(L, -2);
    std::string Label = (std::string)lua_tostring(L, -1);

    bool found = false;
    for (int i = 0; i <= DrawList.size(); i++)
    {
        if (DrawList[i].Active == false && !found)
        {
            switch (DrawType)
            {
            case(0):
                break;
            case(1):
                DrawList[i].Active = true;
                DrawList[i].DrawType = Type::TextBox;
                DrawList[i].Label = Label;
                break;
            }
            found = true;
        }
    }

    return 0;
};

这是我正在运行的LUA脚本:

const char* LUA_FILE = R"(
    addToDrawList(1, "Test")
)";

这就是我将函数推送到LUA堆栈的方式:

lua_State* L = luaL_newstate();

lua_newtable(L);
int uiTableInd = lua_gettop(L);
lua_pushvalue(L, uiTableInd);
lua_setglobal(L, "Ui");

lua_pushcfunction(L, addToDrawList);
lua_setfield(L, -2, "addToDrawList");

问题出在我的第一个脚本中，因为它无法到达this内部的“DrawList”数组。

因此，为解决此问题，我尝试通过执行以下操作将this添加到lambda的捕获列表中:

auto addToDrawList = [this](lua_State* L) -> int

似乎可以解决该错误，但是最后一个脚本出现了问题:

lua_pushcfunction(L, addToDrawList);

我一直在Internet上搜索修复程序，但是找不到。

Answer 1

lua_pushcfunction()采用C样式的函数指针。无需捕获的lambda可以转换为这样的函数指针，但是不能捕获。

请改用 lua_pushcclosure() 1。它将允许您将用户定义的值(称为upvalues)与C函数相关联，例如this指针或仅指向DrawList的指针等。

When a C function is created, it is possible to associate some values with it, thus creating a C closure (see §3.4); these values are then accessible to the function whenever it is called. To associate values with a C function, first these values should be pushed onto the stack (when there are multiple values, the first value is pushed first). Then lua_pushcclosure is called to create and push the C function onto the stack, with the argument n telling how many values should be associated with the function. lua_pushcclosure also pops these values from the stack.

1:lua_pushcfunction()只是lua_pushcclosure()的包装，定义了0个upvalue。

例如:

auto addToDrawList = [](lua_State* L) -> int
{
    const MyClassType *pThis = (const MyClassType*) lua_topointer(L, lua_upvalueindex(1));

    // use pThis->DrawList as needed...

    return 0;
};

...

lua_State* L = luaL_newstate();
...
//lua_pushcfunction(L, addToDrawList);
lua_pushlightuserdata(L, this);
lua_pushcclosure(L, addToDrawList, 1);
...