How to call functions inside package.preload in C++ - c++

Question

I'm trying to call A:update(x) and get a returned value x + 3 in C++.
Here's my code:
#include <lua.hpp>
void main()
{
lua_State *L = luaL_newstate();
luaL_openlibs(L);
lua_settop(L, 0);
luaL_dostring(L, "package.preload['A'] = function () local A = {}\n"
"function A:update(x) return x + 3 end \n"
"return A end");
//call function
lua_getglobal(L, "require");
lua_pushstring(L, "A");
if (lua_pcall(L, 1, LUA_MULTRET, 0) != 0) {
std::cerr << "lua:" << lua_tostring(L, 1) << '\n';
lua_pop(L, 1);
}
int top = lua_gettop(L);
lua_getfield(L, -1, "update");
if (!lua_isfunction(L, -1))
{
std::cerr << "lua:" << lua_tostring(L, 1) << '\n';
lua_pop(L, 1);
}
lua_pushnumber(L, 5); //pass the argument 5
if (lua_pcall(L, 1, LUA_MULTRET, 0))
{
std::cerr << "lua:" << lua_tostring(L, 1) << '\n';
lua_pop(L, 1);
}
if (lua_gettop(L) - top)
{
if (lua_isnumber(L, -1))
{
std::cout << "RETURNED : " << lua_tonumber(L, -1) << std::endl;
}
}
lua_pop(L, 1); // pop 'update'
lua_pop(L, 1); // pop 'A'
lua_close(L);
}
I expect it to print RETURNED : 8 but I get the following error:
Thread 1:EXC_BAD_ACCESS (code=1, address=0x0)
How should I correct my code to work?
EDITED: It worked as soon as I change A:update(x) to A.update(x). I thought they work identically except I can use self in a function that uses :. Could someone please explain to me why this happens?

Answer 1

The notation A:update(x) is syntactic sugar for A.update(A,x). That means you have to call the function update with two parameters. You are lacking the first of the two parameters.
The first parameter A is already on the stack but is located “below” the update function. Using lua_pushvalue we can push a copy of the table onto the stack.
Thus you have to call the function like this (omitting the error handling bits)
lua_getfield(L, -1, "update");
lua_pushvalue(L, -2); // push a copy of "A" onto the stack
lua_pushnumber(L, 5); //pass the argument 5
lua_pcall(L, 2, LUA_MULTRET, 0);