json/docs/mkdocs/docs/api/basic_json_view/count.md

nlohmann::basic_json_view::count

size_type count(string_view_t key) const;
size_type count(const char* key) const;
size_type count(const string_t& key) const;

Returns #!cpp 1 if the value is an object with a member with key key, #!cpp 0 otherwise.

Parameters

key (in)

key value of the element to count

Return value

#!cpp 1 if the value is an object and has a member with key key, #!cpp 0 otherwise.

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time on average.

Notes

!!! warning "Duplicate keys: the first member wins"

If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
[`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
`operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
the last value.

This method always returns #!cpp 0 when the value is not an object -- including a discarded view.

Unlike BasicJsonType::count(), whose return value can in principle exceed #!cpp 1 for an ObjectType that allows multiple entries per key, count() here never does: it is exactly contains() as #!cpp 0/#!cpp 1. This holds even if the source text has a duplicate key -- see the Notes on duplicate keys of operator[] -- because a #!cpp count() > 1 result would require counting every member with a matching key (the lookup functions resolve to the first one).

Examples

??? example

The example below validates that every transaction of a batch carries a mandatory `amount` field, using
`count()` before deciding whether to materialize a transaction at all.

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

Output:

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

See also

• find - find a value in an object
• contains - checks whether a key exists
• BasicJsonType::count - the corresponding function of basic_json

Version history

• Added in version 3.13.0.