Metatable is not indexed, even though setmetatable is used

Question

According to the Lua manual, setmetatable still works the same as in Lua 5.0. Yet for some reason, when I try this code in Lua 5.1.5 and 5.3.1, it appears that the metatable is not accessed:

ClassTable = {}
ClassTable.getString = function(self) 
  return self.x .. ""
end

inst = {}
setmetatable(inst, ClassTable)
inst.x = 7

--doens't work
assert(getmetatable(inst) == ClassTable)
print(inst:getString())

The first case works, however in the second case the I get the error which suggests that the metatable is not being used:

./lua: /test.lua:12: attempt to call method 'getString' (a nil value)
stack traceback:
    test.lua:12: in main chunk
    [C]: ?

This also has nothing to do with the method call operator ":" as even getting the value of the method doesn't go to the metatable.

print(inst.getString)
nil

Rochet2 Rochet2 · Accepted Answer · 2015-12-13T22:23:51

To make the table inst access the metatable you need to use the metamethod __index.

So you can correct the code by adding this line at the top below ClassTable.getString definition:

ClassTable.__index = ClassTable

Despite the name, the __index metamethod does not need to be a function: It can be a table, instead. When it is a function, Lua calls it with the table and the absent key as its arguments. When it is a table, Lua redoes the access in that table.

http://www.lua.org/pil/13.4.1.html

Metatable is not indexed, even though setmetatable is used

1 Answers