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95 lines
2.3 KiB
Markdown
95 lines
2.3 KiB
Markdown
# <small>nlohmann::basic_json::</small>contains
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```cpp
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// (1)
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template<typename KeyT>
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bool contains(KeyT && key) const;
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// (2)
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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 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. 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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`KeyT`
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: A type for an object key other than `basic_json::json_pointer`.
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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 key is found or the JSON value
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is not an object, `#!cpp false` is returned.
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2. `#!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 can throw the following exceptions:
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- Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index begins with
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`0`.
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- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index was not a
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number.
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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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1. This method always returns `#!cpp false` when executed on a JSON type that is not an object.
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2. 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.cpp"
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```
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Output:
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```json
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--8<-- "examples/contains.output"
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```
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??? example "Example (1) 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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## Version history
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1. Added in version 3.6.0.
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2. Added in version 3.7.0.
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