basic_json::count¶
template<typename KeyT>
size_type count(KeyT&& key) const;
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).
Template parameters¶
KeyT
- A type for an object key.
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.
Example¶
Example
The example shows how count()
is used.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create a JSON object
json j_object = {{"one", 1}, {"two", 2}};
// call count()
auto count_two = j_object.count("two");
auto count_three = j_object.count("three");
// print values
std::cout << "number of elements with key \"two\": " << count_two << '\n';
std::cout << "number of elements with key \"three\": " << count_three << '\n';
}
Output:
number of elements with key "two": 1
number of elements with key "three": 0
Version history¶
- Added in version 1.0.0.