Niels Lohmann
071a0dfbee
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>
2.1 KiB
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.
Notes
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, not just finding 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 ofbasic_json
Version history
- Added in version 3.13.0.