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 the number of members with key key if the value is an object, 0 otherwise. Members with a repeated key are all counted.

Parameters¶

key (in): key value of the element to count

Return value¶

the number of members of the object with key key (duplicates included), or 0 if the value is not an object or has no such member.

Exception safety¶

No-throw guarantee: this function never throws exceptions.

Complexity¶

Linear in the number of members: unlike the lookup functions, count() does not stop at the first match, because it has to count every member with the key. Members are compared one after another, in document order. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. The hash index of the keys of objects with 128 or more members does not help here, since every member is visited.

Notes¶

This method always returns 0 when the value is not an object -- including a discarded view.

If the source text repeats a key, count() returns more than 1 for it, even though the materialized value (like the result of parse()) would have only one member with that key. Hence count(key) > 1 detects duplicate keys. This is unlike the lookup functions (operator[], at, find, contains), which resolve to the first member; see the Notes on duplicate keys of operator[].

Examples¶

Example

The example below validates that every transaction of a batch carries exactly one amount field, using count() before deciding whether to materialize a transaction at all. It reports a transaction with no amount field and one with a duplicated amount field.

#include <iostream>
#include <nlohmann/json_view.hpp>

using json_document = nlohmann::json_document;

int main()
{
    // validate that every transaction of a batch carries exactly one
    // "amount" field before materializing any of them into a nlohmann::json
    // value -- count() counts every member with the key
    json_document batch = json_document::parse(R"(
      [
        {"id": 1, "amount": 9.99},
        {"id": 2},
        {"id": 3, "amount": 5.00, "amount": 500.00}
      ]
    )");

    const auto transactions = batch.root();
    for (std::size_t i = 0; i < transactions.size(); ++i)
    {
        const auto transaction = transactions[i];
        const auto amounts = transaction.count("amount");
        if (amounts != 1)
        {
            std::cout << "transaction " << i << " has " << amounts << " \"amount\" fields\n";
            continue;
        }
        std::cout << "transaction " << i << ": " << transaction["amount"].materialize().dump() << '\n';
    }
}

Output:

transaction 0: 9.99
transaction 1 has 0 "amount" fields
transaction 2 has 2 "amount" fields

Version history¶

Added in version 3.13.0 unreleased.