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1# Parsing and exceptions
2
3When the input is not valid JSON, an exception of type [`parse_error`](../../home/exceptions.md#parse-errors) 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](../../home/exceptions.md#parse-errors). In case you process untrusted input, always enclose your code with a `#!cpp try`/`#!cpp catch` block, like
4
5```cpp
6json j;
7try
8{
9    j = json::parse(my_input);
10}
11catch (json::exception::parse_error& ex)
12{
13    std::cerr << "parse error at byte " << ex.byte << std::endl;
14}
15```
16
17In case exceptions are undesired or not supported by the environment, there are different ways to proceed:
18
19
20## Switch off exceptions
21
22The `parse()` function accepts as last parameter a `#!cpp bool` variable `allow_exceptions` which controls whether an exception is thrown when a parse error occurs (`#!cpp true`, default) or whether a discarded value should be returned (`#!cpp false`).
23
24```cpp
25json j = json::parse(my_input, nullptr, false);
26if (j.is_discarded())
27{
28    std::cerr << "parse error" << std::endl;
29}
30```
31
32Note there is no diagnostic information available in this scenario.
33
34## Use accept() function
35
36Alternatively, function `accept()` can be used which does not return a `json` value, but a `#!cpp bool` indicating whether the input is valid JSON.
37
38```cpp
39if (!json::accept(my_input))
40{
41    std::cerr << "parse error" << std::endl;
42}
43```
44
45Again, there is no diagnostic information available.
46
47
48## User-defined SAX interface
49
50Finally, you can implement the [SAX interface](sax_interface.md) and decide what should happen in case of a parse error.
51
52This function has the following interface:
53
54```cpp
55bool parse_error(std::size_t position,
56                 const std::string& last_token,
57                 const json::exception& ex);
58```
59
60The return value indicates whether the parsing should continue, so the function should usually return `#!cpp false`.
61
62??? example
63
64    ```cpp
65    #include <iostream>
66    #include "json.hpp"
67
68    using json = nlohmann::json;
69
70    class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
71    {
72      public:
73        sax_no_exception(json& j)
74          : nlohmann::detail::json_sax_dom_parser<json>(j, false)
75        {}
76
77        bool parse_error(std::size_t position,
78                         const std::string& last_token,
79                         const json::exception& ex)
80        {
81            std::cerr << "parse error at input byte " << position << "\n"
82                      << ex.what() << "\n"
83                      << "last read: \"" << last_token << "\""
84                      << std::endl;
85            return false;
86        }
87    };
88
89    int main()
90    {
91        std::string myinput = "[1,2,3,]";
92
93        json result;
94        sax_no_exception sax(result);
95
96        bool parse_result = json::sax_parse(myinput, &sax);
97        if (!parse_result)
98        {
99            std::cerr << "parsing unsuccessful!" << std::endl;
100        }
101
102        std::cout << "parsed value: " << result << std::endl;
103    }
104    ```
105
106    Output:
107
108    ```
109    parse error at input byte 8
110    [json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
111    last read: "3,]"
112    parsing unsuccessful!
113    parsed value: [1,2,3]
114    ```
115