141 lines
4.3 KiB
Markdown
141 lines
4.3 KiB
Markdown
# <small>nlohmann::basic_json::</small>count
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```cpp
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// (1)
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size_type count(const typename object_t::key_type& key) const;
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// (2)
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template<typename KeyType>
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size_type count(KeyType&& key) const;
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// (3)
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template<typename T>
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size_type count(T) const = delete;
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```
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1. Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will
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always be `0` (`key` was not found) or `1` (`key` was found).
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2. See 1. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
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`#!cpp typename object_comparator_t::is_transparent` denotes a type.
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3. Deleted: this overload is only available if `T` is an integral type and is declared as deleted, so that a call with
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an integer `key` (for example, `#!cpp j.count(0)`) fails to compile. Otherwise, the integer literal `0` would convert to
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a null `#!cpp const char*` and, from there, to the key type, causing undefined behavior at runtime.
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## Template parameters
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`KeyType`
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: A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
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[`string_t`](string_t.md) using [`object_comparator_t`](object_comparator_t.md).
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This can also be a string view (C++17).
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## Parameters
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`key` (in)
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: key value of the element to count.
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## Return value
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Number of elements with key `key`. If the JSON value is not an object, the return value will be `0`.
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## Exception safety
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Strong exception safety: if an exception occurs, the original value stays intact.
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## Complexity
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Logarithmic in the size of the JSON object.
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## Notes
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- This method always returns `0` when executed on a JSON type that is not an object.
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- Calling this function with an integer argument (for example, `#!cpp count(0)`) does not compile: such an argument
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would otherwise implicitly convert to a null `#!cpp const char*` and, from there, cause undefined behavior when
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constructing a `#!cpp std::string` for the object key. To check for an array element instead, use [`at`](at.md),
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[`operator[]`](operator%5B%5D.md), or compare against [`size`](size.md).
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## Examples
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??? example "Example: (1) count number of elements"
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The example shows how `count()` is used.
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```cpp
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create a JSON object
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json j_object = {{"one", 1}, {"two", 2}};
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// call count()
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auto count_two = j_object.count("two");
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auto count_three = j_object.count("three");
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// print values
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std::cout << "number of elements with key \"two\": " << count_two << '\n';
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std::cout << "number of elements with key \"three\": " << count_three << '\n';
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}
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```
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Output:
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```json
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number of elements with key "two": 1
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number of elements with key "three": 0
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```
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??? example "Example: (2) count number of elements using string_view"
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The example shows how `count()` is used.
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```cpp
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#include <iostream>
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#include <string_view>
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#include <nlohmann/json.hpp>
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using namespace std::string_view_literals;
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using json = nlohmann::json;
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int main()
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{
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// create a JSON object
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json j_object = {{"one", 1}, {"two", 2}};
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// call count()
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auto count_two = j_object.count("two"sv);
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auto count_three = j_object.count("three"sv);
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// print values
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std::cout << "number of elements with key \"two\": " << count_two << '\n';
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std::cout << "number of elements with key \"three\": " << count_three << '\n';
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}
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```
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Output:
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```json
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number of elements with key "two": 1
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number of elements with key "three": 0
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```
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## See also
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- [find](find.md) find a value in an object
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- [contains](contains.md) checks whether a key exists
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- [basic_json_view::count](../basic_json_view/count.md) - the same check on a zero-copy view
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## Version history
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1. Added in version 3.11.0.
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2. Added in version 1.0.0. Changed to support comparable key types in version 3.11.0. Support for keys convertible to
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`std::string_view` added in version 3.13.0, consistently across the lookup functions.
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3. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time instead of
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causing undefined behavior at runtime.
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