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Parsing and Exceptions

When the input is not valid JSON, an exception of type parse_error is thrown. This exception contains the position in the input where the error occurred, together with a diagnostic message and the last read input token. The exceptions page contains a list of examples for parse error exceptions. In case you process untrusted input, always enclose your code with a try/catch block, like

json j;
    j = json::parse(my_input);
catch (json::parse_error& ex)
    std::cerr << "parse error at byte " << ex.byte << std::endl;

In case exceptions are undesired or not supported by the environment, there are different ways to proceed:

Switch off exceptions

The parse() function accepts a bool parameter allow_exceptions which controls whether an exception is thrown when a parse error occurs (true, default) or whether a discarded value should be returned (false).

json j = json::parse(my_input, nullptr, false);
if (j.is_discarded())
    std::cerr << "parse error" << std::endl;

Note there is no diagnostic information available in this scenario.

Use accept() function

Alternatively, function accept() can be used which does not return a json value, but a bool indicating whether the input is valid JSON.

if (!json::accept(my_input))
    std::cerr << "parse error" << std::endl;

Again, there is no diagnostic information available.

User-defined SAX interface

Finally, you can implement the SAX interface and decide what should happen in case of a parse error.

This function has the following interface:

bool parse_error(std::size_t position,
                 const std::string& last_token,
                 const json::exception& ex);

The return value indicates whether the parsing should continue, so the function should usually return false.

#include <iostream>
#include "json.hpp"

using json = nlohmann::json;

class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
    sax_no_exception(json& j)
      : nlohmann::detail::json_sax_dom_parser<json>(j, false)

    bool parse_error(std::size_t position,
                     const std::string& last_token,
                     const json::exception& ex)
        std::cerr << "parse error at input byte " << position << "\n"
                  << ex.what() << "\n"
                  << "last read: \"" << last_token << "\""
                  << std::endl;
        return false;

int main()
    std::string myinput = "[1,2,3,]";

    json result;
    sax_no_exception sax(result);

    bool parse_result = json::sax_parse(myinput, &sax);
    if (!parse_result)
        std::cerr << "parsing unsuccessful!" << std::endl;

    std::cout << "parsed value: " << result << std::endl;


parse error at input byte 8
[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
last read: "3,]"
parsing unsuccessful!
parsed value: [1,2,3]

Last update: August 5, 2022