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docs: cover fifo_map, gtl, folly::sorted_vector_map, and Qt

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nlohmann::fifo_map works through the adapter that has always been documented
for it, and preserves the insertion order. Restore its mention in the object
order page, which was dropped together with the tsl::ordered_map one: unlike
ordered_map it keeps a lookup index, so it is the insertion-ordered option
without the quadratic cost.

gtl::flat_hash_map and folly::sorted_vector_map work as well, the latter
through an alias that drops the allocator, whose value type it disagrees on.
gtl::btree_map does not, for the same reason as the other btree containers.

None of the Qt containers can be used, each for its own reason: QMap has no
value_type, QHash iterators yield the mapped value rather than a pair, QList
has no max_size(), QByteArray spells empty() as isEmpty(), and QString is
UTF-16.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-28 17:38:57 +00:00
parent a02741fd28
commit 681fb07eb2
2 changed files with 13 additions and 4 deletions
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docs/mkdocs/docs/features/object_order.md
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@@ -51,7 +51,9 @@ If you do want to preserve the **insertion order**, you can use the type [`nlohm
--8<-- "examples/ordered_json.output"
```
Alternatively, you can use a more sophisticated map with a lookup index. `boost::unordered_flat_map`, `absl::flat_hash_map`, and `absl::node_hash_map` all work through a small adapter that restores the template argument order `basic_json` expects; see [Template Parameter Requirements](types/template_parameters.md#objecttype). Note these are *unordered*, not insertion-ordered.
Alternatively, [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) also preserves the insertion order and, unlike [`ordered_map`](../api/ordered_map.md), keeps a lookup index, so it does not have the quadratic cost described below. It is used through a small adapter ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)).
If the order does not matter and you only want faster lookup, `boost::unordered_flat_map`, `absl::flat_hash_map`, `absl::node_hash_map`, and several other hash maps work through an adapter that restores the template argument order `basic_json` expects; see [Template Parameter Requirements](types/template_parameters.md#objecttype). Note these are *unordered*, not insertion-ordered.
[`tsl::ordered_map`](https://github.com/Tessil/ordered-map) cannot be used: its iterators expose the mapped value as `const`, while `basic_json` needs to modify it in place.