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nlohmann
120 lines
3.1 KiB
Markdown
120 lines
3.1 KiB
Markdown
# <small>nlohmann::basic_json::</small>contains
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```cpp
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// (1)
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bool contains(const typename object_t::key_type& key) const;
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// (2)
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template<typename KeyType>
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bool contains(KeyType&& key) const;
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// (3)
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bool contains(const json_pointer& ptr) const;
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```
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1. Check whether an element exists 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, `#!cpp false` 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. Check whether the given JSON pointer `ptr` can be resolved in the current JSON value.
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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 to check its existence.
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`ptr` (in)
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: JSON pointer to check its existence.
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## Return value
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1. `#!cpp true` if an element with specified `key` exists. If no such element with such a key is found or the JSON value
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is not an object, `#!cpp false` is returned.
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2. See 1.
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3. `#!cpp true` if the JSON pointer can be resolved to a stored value, `#!cpp false` otherwise.
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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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## Exceptions
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1. The function does not throw exceptions.
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2. The function does not throw exceptions.
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3. The function does not throw exceptions.
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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 `#!cpp false` when executed on a JSON type that is not an object.
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- This method can be executed on any JSON value type.
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!!! info "Postconditions"
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If `#!cpp j.contains(x)` returns `#!c true` for a key or JSON pointer `x`, then it is safe to call `j[x]`.
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## Examples
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??? example "Example: (1) check with key"
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The example shows how `contains()` is used.
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```cpp
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--8<-- "examples/contains__object_t_key_type.cpp"
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```
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Output:
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```json
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--8<-- "examples/contains__object_t_key_type.output"
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```
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??? example "Example: (2) check with key using string_view"
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The example shows how `contains()` is used.
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```cpp
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--8<-- "examples/contains__keytype.c++17.cpp"
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```
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Output:
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```json
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--8<-- "examples/contains__keytype.c++17.output"
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```
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??? example "Example: (3) check with JSON pointer"
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The example shows how `contains()` is used.
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```cpp
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--8<-- "examples/contains__json_pointer.cpp"
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```
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Output:
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```json
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--8<-- "examples/contains__json_pointer.output"
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```
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## See also
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- [find](find.md) find a value in an object
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- [count](count.md) returns the number of occurrences of a key
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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 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
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3. Added in version 3.7.0.
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