Re: string immutability

[Enrico Colombini <erix@...>]

Thijs, here is an example:

---------------------------------
function GCAndShow(prompt)
    -- (I don't remember how to force a full collection,
    -- this hack is good enough for this example)
    collectgarbage()
    collectgarbage()
    collectgarbage()
    local kb = collectgarbage('count')
    io.write(string.format('%s: %.1f KB\n', prompt, kb))
end

GCAndShow('initial state')

s1 = string.rep('x', 999999)
GCAndShow('allocated a large string')

s2 = s1
GCAndShow('assigned the same string')

s3 = string.rep('y', 999999)
GCAndShow('allocated another, different, large string')

s4 = string.rep('x', 999998) .. 'x'
assert(s4 == s1)
GCAndShow('constructed a large string identical to an existing string')

s1 = nil; s2 = nil; s3=nil; s4 = nil;
GCAndShow('deleted all strings')

---------------------------------

Lua 5.1 prints:

initial state: 20.6 KB
allocated a large string: 1012.4 KB
assigned the same string: 997.4 KB
allocated another, different, large string: 1989.0 KB
constructed a large string identical to an existing string: 1989.0 KB
deleted all strings: 20.8 KB

---------------------------------

Lua 5.2 prints:

initial state: 15.0 KB
allocated a large string: 991.6 KB
assigned the same string: 991.6 KB
allocated another, different, large string: 1968.2 KB
constructed a large string identical to an existing string: 1968.2 KB
deleted all strings: 15.0 KB

---------------------------------

As you can see, in both cases a string built by concatenation does not 
take up extra memory if it is identical to an existing one. Change the 
final 'x' of the s4 expression to an 'y' (also comment out the assert) 
and you will see the difference.

--
  Enrico