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POLua weak references
 

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    <p>I'm working on a project in Lua where I will be creating tables and storing them in a master table, to be erased at a later time. I will pass around references to these tables to other sibling tables.</p> <pre><code>master = {} table.insert(master, {name = 'hello'}) table.insert(master, {name = 'world', pre = master[1]}) </code></pre> <p>The problem that occurs is that when I wish to erase the reference from the master table, the reference still remains in master[2] here. Obviously my first solution was to make the tables have weak values. (through .__mode on a metatable, not shown here)</p> <p>This worked, and would work, so long as I would never store a singly-referenced table within these tables.</p> <pre><code>table.insert(master, {name = 'goodbye', pre = master[2], some_table = {123}}) </code></pre> <p>The third element, <code>some_table</code> would eventually be collected, because the tables have weak values, and this table (<code>some_table</code>) is not referenced anywhere else. This is undesired behavior. My latest solution involves creating "weak reference objects" to the tables within the master table. A naive implementation follows:</p> <pre><code>function WeakRef(t) r = {__mode = 'v', __index = t, __newindex = t} setmetatable(r, r) return r end </code></pre> <p>These weak reference objects act similarly to <code>boost::weak_ptr</code>s and accomplish my goal, but I am uncertain if they are the best solution to my problem.</p> <p>Is there a better way; a more elegant solution? Is my design, which requires this master table, perhaps flawed?</p>
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    1. USRyan Stein
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    1. CO"The problem that occurs is that when I wish to erase the reference from the master table, the reference still remains in master[2] here." I'm curious as to why this is a problem. Does other code _truly_ need to be notified as to the removal of the object from the master table? Because the GC doesn't guarantee a timeframe for when these things get collected. What exactly is the design intended to do that requires such tight memory control in Lua?
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      1. USNicol Bolas
    2. COThey do not necessarily need to be notified right away, per se. The intended design is a master array indicating physical objects within a simulation, and when the object is no longer necessary, it may be removed. It is similar to having a set of strong pointers in one array, then spreading around weak pointers to the objects. My problem is that I am giving objects references to other objects and this can hold up collection if I wish to immediately remove an object from the simulation. Does this make sense?
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      1. USRyan Stein
    3. COI understand what you're doing. I'm not sure _why_ you're doing it. Since weak references are not guaranteed to immediately disappear, what does it matter if they do or not? That's the part I don't understand: why do the other references need to be weak? Weak references will not prevent accidental use of a dead object, since the GC may not have collected it yet. The only reasons for them to be weak is either to prevent circular dependencies (which Lua doesn't have a problem with) or to manage memory more efficiently. Are you allocating enough data that memory management is an issue?
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