154 lines
4.8 KiB
Markdown
154 lines
4.8 KiB
Markdown
# <small>nlohmann::basic_json::</small>find
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```cpp
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// (1)
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iterator find(const typename object_t::key_type& key);
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const_iterator find(const typename object_t::key_type& key) const;
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// (2)
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template<typename KeyType>
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iterator find(KeyType&& key);
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template<typename KeyType>
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const_iterator find(KeyType&& key) const;
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// (3)
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template<typename T>
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iterator find(T) = delete;
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template<typename T>
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const_iterator find(T) const = delete;
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```
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1. Finds an element in a JSON object with a key equivalent to `key`. If the element is not found or the
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JSON value is not an object, `end()` is returned.
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2. See 1. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
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`#!cpp typename object_comparator_t::is_transparent` denotes a type.
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3. Deleted: this overload is only available if `T` is an integral type and is declared as deleted, so that a call with
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an integer `key` (for example, `#!cpp j.find(0)`) fails to compile. Otherwise, the integer literal `0` would convert to
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a null `#!cpp const char*` and, from there, to the key type, causing undefined behavior at runtime.
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## Template parameters
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`KeyType`
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: A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
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[`string_t`](string_t.md) using [`object_comparator_t`](object_comparator_t.md).
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This can also be a string view (C++17).
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## Parameters
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`key` (in)
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: key value of the element to search for.
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## Return value
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Iterator to an element with a key equivalent to `key`. If no such element is found or the JSON value is not an object,
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a past-the-end iterator (see `end()`) is returned.
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## Exception safety
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Strong exception safety: if an exception occurs, the original value stays intact.
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## Complexity
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Logarithmic in the size of the JSON object.
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## Notes
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- This method always returns `end()` when executed on a JSON type that is not an object.
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- Calling this function with an integer argument (for example, `#!cpp find(0)`) does not compile: such an argument
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would otherwise implicitly convert to a null `#!cpp const char*` and, from there, cause undefined behavior when
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constructing a `#!cpp std::string` for the object key. To access an array element instead, use [`at`](at.md),
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[`operator[]`](operator%5B%5D.md), or compare against [`size`](size.md).
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## Examples
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??? example "Example: (1) find object element by key"
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The example shows how `find()` is used.
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```cpp
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create a JSON object
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json j_object = {{"one", 1}, {"two", 2}};
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// call find
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auto it_two = j_object.find("two");
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auto it_three = j_object.find("three");
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// print values
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std::cout << std::boolalpha;
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std::cout << "\"two\" was found: " << (it_two != j_object.end()) << '\n';
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std::cout << "value at key \"two\": " << *it_two << '\n';
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std::cout << "\"three\" was found: " << (it_three != j_object.end()) << '\n';
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}
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```
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Output:
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```json
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"two" was found: true
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value at key "two": 2
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"three" was found: false
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```
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??? example "Example: (2) find object element by key using string_view"
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The example shows how `find()` is used.
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```cpp
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#include <iostream>
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#include <string_view>
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#include <nlohmann/json.hpp>
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using namespace std::string_view_literals;
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using json = nlohmann::json;
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int main()
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{
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// create a JSON object
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json j_object = {{"one", 1}, {"two", 2}};
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// call find
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auto it_two = j_object.find("two"sv);
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auto it_three = j_object.find("three"sv);
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// print values
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std::cout << std::boolalpha;
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std::cout << "\"two\" was found: " << (it_two != j_object.end()) << '\n';
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std::cout << "value at key \"two\": " << *it_two << '\n';
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std::cout << "\"three\" was found: " << (it_three != j_object.end()) << '\n';
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}
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```
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Output:
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```json
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"two" was found: true
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value at key "two": 2
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"three" was found: false
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```
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## See also
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- [count](count.md) returns the number of occurrences of a key
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- [contains](contains.md) checks whether a key exists
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- [basic_json_view::find](../basic_json_view/find.md) - the same lookup on a zero-copy view (a duplicate key resolves
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to the first member, whereas `parse()` keeps the last)
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## Version history
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1. Added in version 3.11.0.
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2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0. Support for keys convertible to
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`std::string_view` added in version 3.13.0, consistently across the lookup functions.
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3. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of
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causing undefined behavior at runtime.
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