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class exception : public std::exception;

This class is an extension of std::exception objects with a member id for exception ids. It is used as the base class for all exceptions thrown by the basic_json class. This class can hence be used as "wildcard" to catch exceptions, see example below.

uml diagram


  • parse_error for exceptions indicating a parse error
  • invalid_iterator for exceptions indicating errors with iterators
  • type_error for exceptions indicating executing a member function with a wrong type
  • out_of_range for exceptions indicating access out of the defined range
  • other_error for exceptions indicating other library errors

Member functions

  • what - returns explanatory string

Member variables

  • id - the id of the exception


To have nothrow-copy-constructible exceptions, we internally use std::runtime_error which can cope with arbitrary-length error messages. Intermediate strings are built with static functions and then passed to the actual constructor.



The following code shows how arbitrary library exceptions can be caught.

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

using json = nlohmann::json;

int main()
        // calling at() for a non-existing key
        json j = {{"foo", "bar"}};
        json k ="non-existing");
    catch (const json::exception& e)
        // output exception information
        std::cout << "message: " << e.what() << '\n'
                  << "exception id: " << << std::endl;


message: [json.exception.out_of_range.403] key 'non-existing' not found
exception id: 403

See also

List of exceptions

Version history

  • Since version 3.0.0.

Last update: November 1, 2023