diff --git a/doc/mkdocs/docs/features/object_order.md b/doc/mkdocs/docs/features/object_order.md index 86bb253ba3..0768f8020e 100644 --- a/doc/mkdocs/docs/features/object_order.md +++ b/doc/mkdocs/docs/features/object_order.md @@ -2,6 +2,8 @@ The [JSON standard](https://tools.ietf.org/html/rfc8259.html) defines objects as "an unordered collection of zero or more name/value pairs". As such, an implementation does not need to preserve any specific order of object keys. +## Default behavior: sort keys + The default type `nlohmann::json` uses a `std::map` to store JSON objects, and thus stores object keys **sorted alphabetically**. ??? example @@ -33,6 +35,8 @@ The default type `nlohmann::json` uses a `std::map` to store JSON objects, and t } ``` +## Alternative behavior: preserve insertion order + If you do want to preserve the **insertion order**, you can try the type [`nlohmann::ordered_json`](https://github.com/nlohmann/json/issues/2179). ??? example @@ -65,3 +69,58 @@ If you do want to preserve the **insertion order**, you can try the type [`nlohm ``` Alternatively, you can use a more sophisticated ordered map like [`tsl::ordered_map`](https://github.com/Tessil/ordered-map) ([integration](https://github.com/nlohmann/json/issues/546#issuecomment-304447518)) or [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map) ([integration](https://github.com/nlohmann/json/issues/485#issuecomment-333652309)). + +### Notes on parsing + +Note that you also need to call the right [`parse`](../api/basic_json/parse.md) function when reading from a file. +Assume file `input.json` contains the JSON object above: + +```json +{ + "one": 1, + "two": 2, + "three": 3 +} +``` + +!!! success "Right way" + + The following code correctly calls the `parse` function from `nlohmann::ordered_json`: + + ```cpp + std::ifstream i("input.json"); + auto j = nlohmann::ordered_json::parse(i); + std::cout << j.dump(2) << std::endl; + ``` + + The output will be: + + ```json + { + "one": 1, + "two": 2, + "three": 3 + } + ``` + +??? failure "Wrong way" + + The following code incorrectly calls the `parse` function from `nlohmann::json` which does not preserve the + insertion order, but sorts object keys. Assigning the result to `nlohmann::ordered_json` compiles, but does not + restore the order from the input file. + + ```cpp + std::ifstream i("input.json"); + nlohmann::ordered_json j = nlohmann::json::parse(i); + std::cout << j.dump(2) << std::endl; + ``` + + The output will be: + + ```json + { + "one": 1, + "three": 3 + "two": 2, + } + ```