Beginning Lua Programming

Chapter 8. Extending Lua's Behavior with Metamethods

You use metatables and metamethods mainly to create what can, loosely speaking, be described as user-defined types. For example, if you have defined a table that represents a number-like value, metamethods let you use arithmetical operators with it as though it really were a number. In this chapter, you learn how to do the following:

• Use operators with types that they normally don't work with

• Control what happens when a table is indexed or a table field is assigned to

• Customize how a value is converted to a string

Using Concatenation and Arithmetical Operators on Tables

If you use tables as operands to the concatenation operator, you get the following error:

> A, B = {}, {}
> print(A .. B)
 stdin:1: attempt to concatenate global 'A' (a table value)
 stack traceback:
         stdin:1: in main chunk
         [C]: ?

There's a way to override this, though, so the concatenation operator is handled by a function that you define.

Try It Out: Splicing Two Arrays with the Concatenation Operator

1. Define the function MakeSpliceArr and its upvalue SpliceMeta as follows (notice that SpliceMeta's field __concat starts with two underscores):

   do local SpliceMeta -- SpliceMeta needs to be in -- SpliceMeta.__concat's scope. SpliceMeta = { -- Makes a new array by splicing two arrays together: __concat = function(ArrA, ArrB) assert(type(ArrA) == "table" and type(ArrB) == "table") local Ret = setmetatable({}, SpliceMeta) for I, Elem in ipairs(ArrA) do Ret[I] = Elem end local LenA ...