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# Factorio Runtime Docs Version 2.0.7

## Class LuaCustomTable

Lazily evaluated table. For performance reasons, we sometimes return a custom table-like type instead of a native Lua table. This custom type lazily constructs the necessary Lua wrappers of the corresponding C++ objects, therefore preventing their unnecessary construction in some cases.

There are some notable consequences to the usage of a custom table type rather than the native Lua table type: Iterating a custom table is only possible using the `pairs` Lua function; `ipairs` won't work. Another key difference is that custom tables cannot be serialised into a game save file -- if saving the game would require serialisation of a custom table, an error will be displayed and the game will not be saved.

In previous versions of Factorio, this would create a LuaPlayer instance for every player in the game, even though only one such wrapper is needed. In the current version, accessing game.players by itself does not create any LuaPlayer instances; they are created lazily when accessed. Therefore, this example only constructs one LuaPlayer instance, no matter how many elements there are in `game.players`.

    game.players["Oxyd"].character.die()

This statement will execute successfully and `storage.p` will be useable as one might expect. However, as soon as the user tries to save the game, a "LuaCustomTable cannot be serialized" error will be shown. The game will remain unsaveable so long as `storage.p` refers to an instance of a custom table.

    storage.p = game.players  -- This has high potential to make the game unsaveable

The following will produce no output because `ipairs` is not supported with custom tables.

    for _, p in ipairs(game.players) do game.player.print(p.name); end  -- incorrect; use pairs instead

## Example

    -- Custom tables may be iterated using `pairs`.
    for _, p in pairs(game.players) do game.player.print(p.name); end

## Members

valid | :: R boolean | Is this object valid? [...]
Is this object valid? [...]
object_name | :: R string | The class name of this object. [...]
The class name of this object. [...]
[] (index) | :: R Any | Access an element of this custom table.
Access an element of this custom table.
# (length) | :: R uint | Number of elements in this table.
Number of elements in this table.

## Attributes

### valid :: Read boolean

Is this object valid? This Lua object holds a reference to an object within the game engine. It is possible that the game-engine object is removed whilst a mod still holds the corresponding Lua object. If that happens, the object becomes invalid, i.e. this attribute will be `false`. Mods are advised to check for object validity if any change to the game state might have occurred between the creation of the Lua object and its access.

### object_name :: Read string

The class name of this object. Available even when `valid` is false. For LuaStruct objects it may also be suffixed with a dotted path to a member of the struct.

## Operators

### [] (index)

Access an element of this custom table.

### # (length)

Number of elements in this table.