115 lines
3.4 KiB
Markdown
115 lines
3.4 KiB
Markdown
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# Parsing and Exceptions
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When 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
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```cpp
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json j;
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try
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{
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j = json::parse(my_input);
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}
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catch (json::parse_error& ex)
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{
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std::cerr << "parse error at byte " << ex.byte << std::endl;
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}
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```
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In case exceptions are undesired or not supported by the environment, there are different ways to proceed:
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## Switch off exceptions
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The `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`).
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```cpp
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json j = json::parse(my_input, nullptr, false);
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if (j.is_discarded())
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{
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std::cerr << "parse error" << std::endl;
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}
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```
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Note there is no diagnostic information available in this scenario.
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## Use accept() function
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Alternatively, function `accept()` can be used which does not return a `json` value, but a `#!cpp bool` indicating whether the input is valid JSON.
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```cpp
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if (!json::accept(my_input))
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{
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std::cerr << "parse error" << std::endl;
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}
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```
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Again, there is no diagnostic information available.
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## User-defined SAX interface
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Finally, you can implement the [SAX interface](sax_interface.md) and decide what should happen in case of a parse error.
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This function has the following interface:
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```cpp
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bool parse_error(std::size_t position,
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const std::string& last_token,
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const json::exception& ex);
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```
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The return value indicates whether the parsing should continue, so the function should usually return `#!cpp false`.
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??? example
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```cpp
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#include <iostream>
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#include "json.hpp"
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using json = nlohmann::json;
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class sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json>
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{
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public:
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sax_no_exception(json& j)
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: nlohmann::detail::json_sax_dom_parser<json>(j, false)
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{}
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bool parse_error(std::size_t position,
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const std::string& last_token,
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const json::exception& ex)
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{
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std::cerr << "parse error at input byte " << position << "\n"
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<< ex.what() << "\n"
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<< "last read: \"" << last_token << "\""
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<< std::endl;
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return false;
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}
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};
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int main()
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{
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std::string myinput = "[1,2,3,]";
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json result;
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sax_no_exception sax(result);
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bool parse_result = json::sax_parse(myinput, &sax);
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if (!parse_result)
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{
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std::cerr << "parsing unsuccessful!" << std::endl;
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}
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std::cout << "parsed value: " << result << std::endl;
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}
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```
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Output:
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```
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parse error at input byte 8
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[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
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last read: "3,]"
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parsing unsuccessful!
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parsed value: [1,2,3]
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```
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