Delegating C++ object memory allocation to Lua

So far, we have been creating C++ objects in C++ and making Lua store its pointer in userdata. This was done in LuaModuleExporter::luaNew, as follows:

static int luaNew(lua_State *L)
{
    ...
    T **userdata = reinterpret_cast<T **>(
        lua_newuserdatauv(L, sizeof(T *), 0));
    T *obj = luaModuleDef.createInstance(L, nullptr);
    *userdata = obj;
    ...
}

In this case, the Lua userdata only stores a pointer. As you may recall, Lua userdata can represent a much larger piece of memory, so you might be wondering if we can store the whole C++ object in userdata, instead of just the pointer. Yes, we can. Let’s learn how to do it.

Using C++ placement new

In C++, the most common way to create an object is to call new T(). This does two things:

• It creates a piece...