Niels Lohmann
52b72b5684
- Make the iterator range illustrations legible in dark mode (prefers-color-scheme rules inside the SVGs). - Add a "Byte layout" section with a color-coded byte table to each binary format page (BJData, BON8, BSON, CBOR, MessagePack, UBJSON), and highlight the smallest size per file in the comparison table. - Add "See also" sections to the overview pages that lacked one. - Split the release notes into one page per release, grouped by minor version on the overview page, with an "API changes" section per release generated from the API pages' version histories (hooks/api_changes.py). - Fact-check the release notes and version histories against the release tags and correct wrong or misleading entries (e.g., json_serializer 2.1.0, merge_patch 3.1.0, items, count, operator[], to_bjdata, std::string_view key support). Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.4 KiB
nlohmann::basic_json::emplace
template<class... Args>
std::pair<iterator, bool> emplace(Args&& ... args);
Inserts a new element into a JSON object constructed in-place with the given args if there is no element with the key
in the container. If the function is called on a JSON null value, an empty object is created before appending the value
created from args.
Template parameters
Args- compatible types to create a
basic_jsonobject
Iterator invalidation
For ordered_json, adding a value to an object can yield a reallocation, in which case all
iterators (including the end() iterator) and all references to the elements are invalidated.
Parameters
args(in)- arguments to forward to a constructor of
basic_json
Return value
a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and
a #!cpp bool denoting whether the insertion took place.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a #!json null
value is converted to an empty object before the element is added and keeps that type if adding the element throws.
Exceptions
Throws type_error.311 when called on a type other than JSON
object or #!json null; example: "cannot use emplace() with number"
Complexity
Logarithmic in the size of the container, O(log(size())).
Examples
??? example
The example shows how `emplace()` can be used to add elements to a JSON object. Note how the `#!json null` value was
silently converted to a JSON object. Further note how no value is added if there was already one value stored with
the same key.
```cpp
--8<-- "examples/emplace.cpp"
```
Output:
```json
--8<-- "examples/emplace.output"
```
See also
- emplace_back add a value to an array
- insert add values to an array/object
- Modifying values - the article on modifying values
Version history
- Added in version 2.0.8.
- Fixed in version 3.13.0: for
ordered_json, the value could previously only be passed as an rvalue; it can now also be passed as an lvalue or a#!cpp constlvalue, matching the behavior ofjson.