Niels Lohmann
b730946432
Since #4958, a key type implicitly convertible to std::string_view was accepted by is_usable_as_basic_json_key_type without checking that the object's comparator can actually compare object_t::key_type with that key type. The key was then forwarded unchanged to the underlying map, so const operator[], at, find, count, contains, erase and value failed to compile (a hard error inside <map>) for a key convertible only to std::string_view, and value() rejected such keys outright. For keys convertible to both std::string and std::string_view, the KeyType&& templates now won overload resolution over the object_t::key_type overloads and then failed the same way, a regression from 3.12.0. Only the non-const operator[] worked, because it uses emplace(), which constructs a std::string from the key explicitly. ordered_json was not affected, since ordered_map checks comparability itself. Add a trait, is_string_view_convertible_key_type, that recognizes a key type that is convertible to std::string_view but not directly comparable with the object's key type, provided std::string_view itself is comparable with it. at(), operator[], find(), count(), contains(), erase() and value() now route such keys through a new lookup_key() helper that converts them to std::string_view before they reach the object, matching how the object's transparent comparator already supports std::string_view lookups. Fixes #5663. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
92 lines
2.8 KiB
Markdown
92 lines
2.8 KiB
Markdown
# <small>nlohmann::basic_json::</small>count
|
|
|
|
```cpp
|
|
// (1)
|
|
size_type count(const typename object_t::key_type& key) const;
|
|
|
|
// (2)
|
|
template<typename KeyType>
|
|
size_type count(KeyType&& key) const;
|
|
```
|
|
|
|
1. Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will
|
|
always be `0` (`key` was not found) or `1` (`key` was found).
|
|
2. 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.
|
|
|
|
## Template parameters
|
|
|
|
`KeyType`
|
|
: A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
|
|
[`string_t`](string_t.md) using [`object_comparator_t`](object_comparator_t.md).
|
|
This can also be a string view (C++17).
|
|
|
|
## Parameters
|
|
|
|
`key` (in)
|
|
: key value of the element to count.
|
|
|
|
## Return value
|
|
|
|
Number of elements with key `key`. If the JSON value is not an object, the return value will be `0`.
|
|
|
|
## Exception safety
|
|
|
|
Strong exception safety: if an exception occurs, the original value stays intact.
|
|
|
|
## Complexity
|
|
|
|
Logarithmic in the size of the JSON object.
|
|
|
|
## Notes
|
|
|
|
- This method always returns `0` when executed on a JSON type that is not an object.
|
|
- Calling this function with an integer argument (for example, `#!cpp count(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`](at.md),
|
|
[`operator[]`](operator%5B%5D.md), or compare against [`size`](size.md).
|
|
|
|
## Examples
|
|
|
|
??? example "Example: (1) count number of elements"
|
|
|
|
The example shows how `count()` is used.
|
|
|
|
```cpp
|
|
--8<-- "examples/count__object_t_key_type.cpp"
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
--8<-- "examples/count__object_t_key_type.output"
|
|
```
|
|
|
|
??? example "Example: (2) count number of elements using string_view"
|
|
|
|
The example shows how `count()` is used.
|
|
|
|
```cpp
|
|
--8<-- "examples/count__keytype.c++17.cpp"
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
--8<-- "examples/count__keytype.c++17.output"
|
|
```
|
|
|
|
## See also
|
|
|
|
- [find](find.md) find a value in an object
|
|
- [contains](contains.md) checks whether a key exists
|
|
|
|
## Version history
|
|
|
|
1. Added in version 3.11.0.
|
|
2. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0. Fixed in version 3.13.0 to
|
|
consistently accept `std::string_view`-convertible keys, as already supported by [`operator[]`](operator[].md),
|
|
[`at`](at.md), [`value`](value.md), and other lookup functions.
|
|
3. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time instead of
|
|
causing undefined behavior at runtime.
|