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.