mirror of https://github.com/nlohmann/json.git synced 2025-05-11 21:53:56 +00:00

* 🔥 consolidate documentation
* ♻️ overwork std specializations
* 🚚 move images files to mkdocs
* ♻️ fix URLs
* 🔧 tweak MkDocs configuration
* 🔧 add namespaces
* 📝 document deprecations
* 📝 document documentation generation
* 🚸 improve search
* 🚸 add examples
* 🚧 start adding documentation for macros
* 📝 add note for https://github.com/nlohmann/json/issues/874#issuecomment-1001699139
* 📝 overwork example handling
* 📝 fix Markdown tables

2021-12-29 13:41:01 +01:00

2.3 KiB

Raw Blame History

nlohmann::basic_json::contains

// (1)
template<typename KeyT>
bool contains(KeyT && key) const;

// (2)
bool contains(const json_pointer& ptr) const;

Check whether an element exists in a JSON object with key equivalent to key. If the element is not found or the JSON value is not an object, #!cpp false is returned.
Check whether the given JSON pointer ptr can be resolved in the current JSON value.

Template parameters

KeyT: A type for an object key other than basic_json::json_pointer.

Parameters

key (in): key value to check its existence.
ptr (in): JSON pointer to check its existence.

Return value

#!cpp true if an element with specified key exists. If no such element with such key is found or the JSON value is not an object, #!cpp false is returned.
#!cpp true if the JSON pointer can be resolved to a stored value, #!cpp false otherwise.

Exception safety

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

Exceptions

The function does not throw exceptions.
The function can throw the following exceptions:
- Throws parse_error.106 if an array index begins with 0.
- Throws parse_error.109 if an array index was not a number.

Complexity

Logarithmic in the size of the JSON object.

Notes

This method always returns #!cpp false when executed on a JSON type that is not an object.
This method can be executed on any JSON value type.

!!! info "Postconditions"

If `#!cpp j.contains(x)` returns `#!c true` for a key or JSON pointer `x`, then it is safe to call `j[x]`.

Examples

??? example "Example (1) check with key"

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

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

Output:

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

??? example "Example (1) check with JSON pointer"

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

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

Output:

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

Version history

Added in version 3.6.0.
Added in version 3.7.0.

2.3 KiB Raw Blame History