Is it possible to overload operators for strings?

Question

Is it possible to add your own operators and metamethods to strings in Lua?

I wish to do something like this:

local str = "test"

print(str[2]) --> "e"
print(str()) --> "TEST"
print(-str) --> "tset"
print(str + "er") --> "tester"
print(str * 2) --> "testtest"

Answer 1

Yes, using a "secret" metatable for strings in Lua, it is possible to define a number of different operator overloads for strings.

Try the following code:

getmetatable('').__index = function(str,i) return string.sub(str,i,i) end
getmetatable('').__call = function(str,i) return string.upper(str) end
getmetatable('').__unm = function(str,i) return string.reverse(str) end
getmetatable('').__add = function(str,i) return (str .. i) end
getmetatable('').__mul = function(str,i) return string.rep(str, i) end

local str = "test"

print(str[2]) --> "e"
print(str()) --> "TEST"
print(-str) --> "tset"
print(str + "er") --> "tester"
print(str * 2) --> "testtest"

The reason you cannot use setmetatable('',...) is because it can only be used on tables. But with the "hack" above, you can easily insert different methods into strings.

Remember to only use this locally in your own code if you really need it, as there might be conflicts with using this globally in projects.

The proper way to manipulate strings or data however, is to use methods from a module. The metamethods provided in this example is not OOP friendly and affects all strings in the scope in Lua after it has been defined. Lua gives us the power to edit string's metatable, but we should use it with responsibility.

There's nothing that str:sub(), str:upper() and the other methods can't do with examples like these which should be used before changing the meta functionality of strings in the whole program.