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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;
    try
    {
        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 as last parameter a `bool` variable `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`.

Example

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

    using json = nlohmann::json;

    class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
    {
      public:
        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;
    }

Output:

    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: May 1, 2022
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