Library and Table Access Metamethods

The metatable __toString method

• It is common for libraries to define metatable and metamethods

  • The print command looks for the __tostring metamethod
    • The effect of putting the tostring method in the metatable can be seen:
• • • • uncomment the mt.__tostring = Set.tostring line in the example for sets

The index and newindex metamethods

• When an access to an index in a table returns a nil

• The interpreter looks for an __index metamethod

• • If found the metamethod returns the result rather than nil
  • __index can be set to a function, or a table
  • If set to a table a lookup is done in the table for the key passed in
  • If a function the function is called with the table and key as parameters
  • To access a table without invoking __index Use rawget(t,i)

• __newindex is used for table updates

  • if the index absent then the __newindex function is called if it exists
  • rawset(t,i) is used to bypass the check
  • If a table is assigned to __newindex then the assignment is made in this table