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Niels Lohmann 77acd4563c Add element access, iteration, values, and JSON pointers to json_view

Give basic_json_view the read-only access functions of basic_json:
operator[] and at() with keys and indices, front()/back(), find(),
contains(), count(), begin()/end() and cbegin()/cend(), items()
with structured bindings from C++17 on, and type_name().

Exceptions have the ids and messages of the const functions of
basic_json. Where basic_json has undefined behavior the view
answers safely: operator[] with a missing key or an out-of-range
index returns a discarded view, and front()/back() of an empty
container throw invalid_iterator.214. Objects are iterated in
document order, and all members are visited; duplicate-key lookups
find the first member (as yyjson and simdjson do), while parse(),
materialize(), and the map conversions keep the last value, as
parse() does. Keys of up to 16 bytes are compared with two
overlapping loads.

Add value conversions: get<T>()/get_to() for arithmetic types,
bool, nullptr_t, strings (std::basic_string copied,
string_view_t without a copy), BasicJsonType, views, std::vector,
and maps with string keys; get_string() for the string without a
copy; number_token() for the number exactly as written in the
source; value() with keys and JSON pointers; and operator[]/at()/
contains() with JSON pointers. Everything else, including types
with from_json(), goes through materialize() of that subtree.
get<T>() of arithmetic types is inlined down to the conversion, so
reading an integer needs no call.

detail::json_pointer_access exposes a pointer's reference tokens
to code outside basic_json.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-07 16:42:26 +02:00

4.6 KiB

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nlohmann::basic_json::contains

// (1)
bool contains(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
bool contains(KeyType&& key) const;

// (3)
bool contains(const json_pointer& ptr) const;

Check whether an element exists in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, #!cpp false is returned.
See 1. This overload is only available if KeyType is comparable with #!cpp typename object_t::key_type and #!cpp typename object_comparator_t::is_transparent denotes a type.
Check whether the given JSON pointer ptr can be resolved in the current JSON value.

Template parameters

KeyType: A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17).

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 a key is found or the JSON value is not an object, #!cpp false is returned.
See 1.
#!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 does not throw exceptions.
The function does not throw exceptions.

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.
Calling this function with an integer argument (for example, #!cpp contains(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::string for the object key. To check for an array element instead, use at, operator[], or compare against size.

!!! 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]`.

!!! warning "Deprecation"

Overload (3) also accepts a [`json_pointer`](../json_pointer/index.md) whose template argument is a `basic_json`
specialization (e.g., `nlohmann::json_pointer<nlohmann::json>`) instead of a string type. This is deprecated since
version 3.11.0 and will be removed in a future major version; use `basic_json::json_pointer` (for `json`,
`nlohmann::json_pointer<std::string>`) instead.

You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
function.

See the [migration guide](../../integration/migration_guide.md#json-pointers) for how to update existing code.

Examples

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

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

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

Output:

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

??? example "Example: (2) check with key using string_view"

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

```cpp
--8<-- "examples/contains__keytype.c++17.cpp"
```

Output:

```json
--8<-- "examples/contains__keytype.c++17.output"
```

??? example "Example: (3) 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 3.11.0.
Added in version 3.6.0. Extended template KeyType to support comparable types in version 3.11.0. Fixed in version 3.13.0 to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
Added in version 3.7.0.
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.

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