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@@ -89,19 +89,21 @@ This differs from `#!cpp std::map`, where the same operations are O(log n).
!!! warning "Quadratic cost of building large objects"
Because every insertion scans all elements inserted so far, building an object of `n` distinct keys costs
**O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key as well as to
parsing one, since the parser inserts each key as it is read.
**O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key (with
`operator[]`, `emplace`, or `insert`) and, for now, to reading one with a binary reader such as
[`from_cbor`](basic_json/from_cbor.md) ([#6104](https://github.com/nlohmann/json/issues/6104)). Parsing JSON text
is not affected: the parser finds the keys of an object with many members in a hash table and takes linear time.
The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows
steeply for machine-generated objects with many thousands of keys. Measured with `-O2 -DNDEBUG` for parsing a flat
object of `n` keys, relative to `#!cpp nlohmann::json` (which uses `#!cpp std::map`):
steeply for machine-generated objects with many thousands of keys. Measured with Apple clang 21, `-O2 -DNDEBUG`,
arm64, for a flat object of `n` keys (best of seven runs):
| `n` | `json` | `ordered_json` | factor |
|--------|--------|----------------|--------|
| 2000 | 0.7 ms | 3.6 ms | 5× |
| 4000 | 0.8 ms | 14.0 ms | 19× |
| 8000 | 1.6 ms | 67.8 ms | 43× |
| 16 000 | 3.3 ms | 181.6 ms | 54× |
| `n` | `json::parse` | `ordered_json::parse` | `ordered_json`, key by key | `ordered_json::from_cbor` |
|--------|---------------|-----------------------|----------------------------|---------------------------|
| 2000 | 0.3 ms | 0.1 ms | 4.5 ms | 4.4 ms |
| 4000 | 0.5 ms | 0.2 ms | 17.1 ms | 16.8 ms |
| 8000 | 0.9 ms | 0.3 ms | 67.0 ms | 66.8 ms |
| 16 000 | 2.0 ms | 0.7 ms | 282.4 ms | 282.4 ms |
If key order matters for objects of that size, consider a container with a lookup index, such as
[`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map)