Class creation and basic handling

A luaT class is basically either a Lua table or userdata with an appropriate metatable. This appropriate metatable, that we call in this section root metatable is created with luaT_newmetatable.

The root metatable of a luaT object has itself a metatable that we call metaclass. The metaclass is the actual metatable containing all the methods of the class. If the class inherit from another class, then the metaclass will itself have a metatable corresponding to the parent metaclass: the metaclasses are cascaded according to the class inheritance. Multiple inheritance is not supported.

The root metatable of a luaT object contains Lua operators like __index, __newindex, __tostring, __add (etc...). These operators will respectively look for __index__, __newindex__, __tostring__, __add__ (etc...) in the metaclass. If found, the corresponding function or value will be returned, else a Lua error will be raised.

If one wants to provide __index__ or __newindex__ in the metaclass, these operators must follow a particular scheme:

__index__ must either return a value and true or return false only. In the first case, it means __index__ was able to handle the given argument (for e.g., the type was correct). The second case means it was not able to do anything, so __index in the root metatable can then try to see if the metaclass contains the required value.

__newindex__ must either return true or false. As for __index__, true means it could handle the argument and false not. If not, the root metatable __newindex will then raise an error if the object was a userdata, or apply a rawset if the object was a Lua table.

Other metaclass operators like __tostring__, __add__, etc... do not have any particular constraint.

LUAT_API const void* luaT_newmetatable(lua_State *L, const char *tname, const char *parenttname, lua_CFunction constructor, lua_CFunction destructor, lua_CFunction factory);

LUAT_API void luaT_pushmetatable(lua_State *L, const void *id); LUAT_API void luaT_pushmetaclass(lua_State *L, const void *id); LUAT_API int luaT_getmetaclass(lua_State *L, int index);