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std::hash<nlohmann::basic_json>

namespace std {
    struct hash<nlohmann::basic_json>;
}

Return a hash value for a JSON object. The hash function tries to rely on std::hash where possible. Furthermore, the type of the JSON value is taken into account to have different hash values for null, 0, 0U, and false, etc.

Examples

Example

The example shows how to calculate hash values for different JSON values.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    std::cout << "hash(null) = " << std::hash<json> {}(json(nullptr)) << '\n'
              << "hash(false) = " << std::hash<json> {}(json(false)) << '\n'
              << "hash(0) = " << std::hash<json> {}(json(0)) << '\n'
              << "hash(0U) = " << std::hash<json> {}(json(0U)) << '\n'
              << "hash(\"\") = " << std::hash<json> {}(json("")) << '\n'
              << "hash({}) = " << std::hash<json> {}(json::object()) << '\n'
              << "hash([]) = " << std::hash<json> {}(json::array()) << '\n'
              << "hash({\"hello\": \"world\"}) = " << std::hash<json> {}("{\"hello\": \"world\"}"_json)
              << std::endl;
}

Output:

hash(null) = 2654435769
hash(false) = 2654436030
hash(0) = 2654436095
hash(0U) = 2654436156
hash("") = 6142509191626859748
hash({}) = 2654435832
hash([]) = 2654435899
hash({"hello": "world"}) = 4469488738203676328

Note the output is platform-dependent.

Version history

  • Added in version 1.0.0.
  • Extended for arbitrary basic_json types in version 3.10.5.

Last update: September 18, 2022