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Constant box.NULL
There are some major problems with using Lua **nil** values in tables. For example: you can't correctly assess the length of a table that is not a sequence. (Learn more about data types in `Lua <https://www.lua.org/manual/5.1/manual.html#2.2>`__ and `LuaJIT <http://luajit.org/ext_ffi_semantics.html>`__.)
**Example:**
tarantool> t = {0, nil, 1, 2, nil}
---
...

tarantool> t
---
- - 0
- null
- 1
- 2
...

tarantool> #t
---
- 4
...
The console output of ``t`` processes **nil** values in the middle and at the end of the table differently. This is due to undefined behavior.
Trying to find the length for sparse arrays in LuaJIT leads to another scenario of `undefined behavior <https://www.lua.org/manual/5.1/manual.html#2.5.5>`_.
To avoid this problem, use Tarantool's ``box.NULL`` constant instead of **nil**. ``box.NULL`` is a placeholder for a **nil** value in tables to preserve a key without a value.
Using box.NULL
``box.NULL`` is a value of the `cdata <http://luajit.org/ext_ffi_semantics.html>`__ type representing a NULL pointer. It is similar to ``msgpack.NULL``, ``json.NULL`` and ``yaml.NULL``. So it is some not **nil** value, even if it is a pointer to NULL.
Use ``box.NULL`` only with capitalized NULL (``box.null`` is incorrect).
Technically speaking, ``box.NULL`` equals to ``ffi.cast('void *', 0)``.
tarantool> t = {0, box.NULL, 1, 2, box.NULL}
---
...

tarantool> t
---
- - 0
- null # cdata
- 1
- 2
- null # cdata
...

tarantool> #t
---
- 5
...
Notice that ``t[2]`` shows the same ``null`` output in both examples. However in this example ``t[2]`` and ``t[5]`` are of the cdata type, while in the previous example their type was **nil**.
Avoid using implicit comparisons with nullable values when using ``box.NULL``. Due to `Lua behavior <https://www.lua.org/manual/5.1/manual.html#2.4.4>`_, returning anything except **false** or **nil** from a condition expression is considered as **true**. And, as it was mentioned earlier, ``box.NULL`` is a pointer by design.
That is why the expression ``box.NULL`` will always be considered **true** in case it is used as a condition in a comparison. This means that the code
``if box.NULL then func() end``
will always execute the function ``func()`` (because the condition ``box.NULL`` will always be neither **false** nor **nil**).
Distinction of nil and box.NULL
Use the expression ``if x == nil`` to check if the ``x`` is either a **nil** or a ``box.NULL``.

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testing_project / Documentation: reference/reference_lua/box_null — English

tarantool> t = {0, nil, 1, 2, nil}
---
...

tarantool> t
---
- - 0
- null
- 1
- 2
...

tarantool> #t
---
- 4
...
2 years ago
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