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Niels Lohmann
08dcdaf9b8
- API pages for operator[], at, front, back, find, contains, count, begin, end, cbegin, cend, items, and type_name of basic_json_view, linked both ways with the basic_json pages - the feature page and size() describe document order and duplicate keys - the examples show when the view helps: reading a few fields of a large text, probing optional members, and members in source order Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.9 KiB
2.9 KiB
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;
- 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
KeyTypeis comparable with#!cpp typename object_t::key_typeand#!cpp typename object_comparator_t::is_transparentdenotes a type.
Template parameters
KeyType- A type for an object key other than
json_pointerthat is comparable withstring_tusingobject_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::stringfor the object key. To access an array element instead, useat,operator[], or compare againstsize.
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"
```
See also
- count returns the number of occurrences of a key
- contains checks whether a key exists
- basic_json_view::find - the same lookup on a zero-copy view
Version history
- Added in version 3.11.0.
- Added in version 1.0.0. Changed to support comparable types in version 3.11.0.
- 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.