amorfo77/json

0 Fork 0

mirror of https://github.com/nlohmann/json.git synced 2026-10-19 04:28:12 +00:00

Files

T

Niels Lohmann 7b755737b4 Address the review of json_view element access

From gregmarr's review of #5622:

- count() counts every member with the key, so count(key) > 1 detects
  duplicate keys; it was contains() as 0/1 before.
- get_to() passes types that get<T>() does not convert directly to
  from_json with the existing value, as basic_json::get_to() does, so that
  a from_json which sets only the fields present keeps the others.
- array_index() checks the empty token and "-" first; the results are
  unchanged.
- Docs: no "if an exception is thrown" where a view never changes
  anything; the borrowed source must also stay unmodified; items() on a
  temporary view is safe; back()/front() notes shortened; the basic_json
  see-also entries say how the view's results can differ.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-18 10:05:51 +02:00

3.6 KiB

Raw Blame History

nlohmann::basic_json::find

// (1)
iterator find(const typename object_t::key_type& key);
const_iterator find(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
iterator find(KeyType&& key);
template<typename KeyType>
const_iterator find(KeyType&& key) const;

// (3)
template<typename T>
iterator find(T) = delete;
template<typename T>
const_iterator find(T) const = delete;

Finds an element in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, end() is returned.
See 1. This overload is only available if KeyType is comparable with #!cpp typename object_t::key_type and #!cpp typename object_comparator_t::is_transparent denotes a type.
Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, #!cpp j.find(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null #!cpp const char* and, from there, to the key type, causing undefined behavior at runtime.

Template parameters

KeyType: A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17).

Parameters

key (in): key value of the element to search for.

Return value

Iterator to an element with a key equivalent to key. If no such element is found or the JSON value is not an object, a past-the-end iterator (see end()) is returned.

Exception safety

Strong exception safety: if an exception occurs, the original value stays intact.

Complexity

Logarithmic in the size of the JSON object.

Notes

This method always returns end() when executed on a JSON type that is not an object.
Calling this function with an integer argument (for example, #!cpp find(0)) does not compile: such an argument would otherwise implicitly convert to a null #!cpp const char* and, from there, cause undefined behavior when constructing a #!cpp std::string for the object key. To access an array element instead, use at, operator[], or compare against size.

Examples

??? example "Example: (1) find object element by key"

The example shows how `find()` is used.

```cpp
--8<-- "examples/find__object_t_key_type.cpp"
```

Output:

```json
--8<-- "examples/find__object_t_key_type.output"
```

??? example "Example: (2) find object element by key using string_view"

The example shows how `find()` is used.

```cpp
--8<-- "examples/find__keytype.c++17.cpp"
```

Output:

```json
--8<-- "examples/find__keytype.c++17.output"
```

Version history

Added in version 3.11.0.
Added in version 1.0.0. Changed to support comparable types in version 3.11.0. Fixed in version 3.13.0 to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of causing undefined behavior at runtime.

3.6 KiB Raw Blame History