Re: Weak keyed tables with primitive keys

[Petite Abeille <petite_abeille@...>]

On Oct 21, 2009, at 9:39 PM, Petite Abeille wrote:

> On Oct 21, 2009, at 9:36 PM, Duncan Cross wrote:
>
> > I would put the result into its own table and make the cache
> > weak-valued, not weak-keyed:
> >
> >   cache[ aPath ] = {value=aContent}
>
> Right. But then it would get garbage collected right away as nothing  
> would have any reference to that wrapper table, no?

Something along those lines:

local cache = setmetatable( {}, { __mode = 'v' } )

local function content( aPath )
    local aCache = cache[ aPath ]

    if not aCache then
        print( 'miss', aPath )
        aCache = { io.open( aPath, 'rb' ):read( '*a' ) }
        cache[ aPath ] = aCache
    else
        print( 'hit', aPath )
    end

    return aCache[ 1 ]
end

local aPath = 'TestGC.lua'

content( aPath )
content( aPath )
collectgarbage()
content( aPath )


> miss	TestGC.lua
> hit	TestGC.lua
> miss	TestGC.lua

So the lifespan of the cache would only be the interval between  
collectgarbage, instead of the lifespan of the key, as intended  
originally.