Pass parameters as reference from Lua to C++ -

i have lua function registered expects metatable parameter , want modify contents of table in c++ runtime:

int operate(lua_state *l) {     std::vector<int> values{};      if (auto length = get_length_at_index(l, 1))     {         result.reserve(length);          lua_pushnil(l);         while (lua_next(l, 1))         {             result.push_back(lua_tointeger(l, -1));             lua_pop(l, 1);         }     }      (auto &v : values)     {         v *= 2;     }      return 0; }  int main() {     auto l = lual_newstate();      lual_dofile(l, "script.lua");     lua_register(l, "operate", operate);      return 0; } 

the code above reads "lua.script" looks this:

values = {1, 2, 3, 4, 5, 6} operate(values)  index = 1, #values     print(values[index]) end 

the expected output values 1 6 multiplied 2, output unmodified values. obvious happens because i'm copying metatable std::vector , modify copy.

is there way operate c++ runtime on lua objects in way reflect changes applied c++?

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ps: know can return table function return value, want know if there's other options.

edit: didn't see knew why. explanation has been moved bottom future readers.

after first loop, table remains on top of stack, simplifying following solution. replace second loop with:

int count = int(result.size()); (int = 0; < count; ++i) {     lua_pushinteger(l, result[i] * 2);     lua_rawseti(l, -2, i);  } 

-2 refers lua value below top of stack(similarly, -1 refers top). when rawseti reached lua stack looks like:

[-1]: copy of result[i]
[-2]: values table

rawseti pops value off top of stack , stores in specified table , index, making table top of stack again.

note can in fact within first loop, should avoid making habit of because lua operations cause undefined behavior while iterating, , further complicates code. see this post clarification.

why happen?

in summary, code writes c++ memory, not lua table.

the first loop iterates, reading each value , adding result container.

lua_pushnil(l); while (lua_next(l, 1)) {     result.push_back(lua_tointeger(l, -1));     lua_pop(l, 1); } 

the second loop doubles value of each element within c++ container.

for (auto &v : values) {     v *= 2; }