Metamethods

Almost every value in Lua can have a metatable. This metatable is an ordinary Lua table that defines the behavior of the original value under certain events. You can change several aspects of the behavior of a value by setting specific fields in its metatable. For instance, when a non-numeric value is the operand of an addition, Lua checks for a function in the field __add of the value’s metatable. If it finds one, Lua calls this function to perform the addition.

Tables have individual metatables, although multiple tables can share their metatables. Values of all other types share one single metatable per type; that is, there is one single metatable for all numbers, one for all strings, etc. By default, a value has no metatable, but the string library sets a metatable for the string type

API

• getmetatable : query the metatable of a value.
• setmetatable : replace the metatable of a table. You cannot change the metatable of other types.

Binary Operations

If either operand for an operation is not the datatype needed, Lua will try to call a metamethod. The operation is as follows

• Checking the first operand even if it is the expected datatype
• If that operand does not define a metamethod for the operation, then Lua will check the second operand.
• With a found metamethod, it is called with the two operands as arguments, and the result of the call is the result of the operation.
• If no metamethod is found, Lua raises an error.

Index operations

__index happens when value is not a table or when key is not present in table. The metavalue is looked up in the metatable of table. The metavalue for this event can be either a function, a table, or any value with an __index metavalue. __newindex Like the index event, happens when indexed value is not a table or when key is not present in table. The metavalue is looked up in the metatable of table. Like with indexing, the metavalue for this event can be either a function, a table, or any value with an __newindex metavalue.

Length Operation

If the object is not a string, Lua will try its metamethod. If there is a metamethod, Lua calls it with the object as argument, and the result of the call is the result of the operation. If there is no metamethod but the object is a table, then Lua uses the table length operation

Call Operation

This event happens when Lua tries to call a non-function value. The metamethod is looked up in func. If present, the metamethod is called with the value as its first argument, followed by the arguments of the original call (args). All results of the call are the results of the operation. This is the only metamethod that allows multiple results.