How to reference 'this' in a lambda used with a LUA script - c++

Question

I'm trying to add the LUA API to my C++ program, and I'm attempting to allow the script to draw to my GUI. So far, I have this for my lambda function:
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;
};
This as my LUA script being run:
const char* LUA_FILE = R"(
addToDrawList(1, "Test")
)";
This is how I'm pushing my function to the LUA stack:
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");
The problem is within my first script, as it can't get to the 'DrawList' array as its inside of this.
So, to resolve it, I tried to add this to the lambda's capture list by doing this:
auto addToDrawList = [this](lua_State* L) -> int
Which appeared to work and resolve the error, but then I had an issue with the last script:
lua_pushcfunction(L, addToDrawList);
I've been searching the Internet for a fix, but I can't find any.

Answer 1

lua_pushcfunction() takes a C-style function pointer. A capture-less lambda can be converted to such a function pointer, but a capturing lambda cannot.
Use lua_pushcclosure()1 instead. It will allow you to associate user-defined values (known as upvalues) with the C function, such as your this pointer, or just a pointer to DrawList, etc.
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() is just a wrapper for lua_pushcclosure() with 0 upvalues defined.
For example:
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);
...