mirror of
https://github.com/nlohmann/json.git
synced 2026-07-09 12:05:10 +00:00
nlohmann
113 lines
3.2 KiB
Markdown
113 lines
3.2 KiB
Markdown
# Parsing and Exceptions
|
|
|
|
When the input is not valid JSON, an exception of type [`parse_error`](https://json.nlohmann.me/home/exceptions/#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](https://json.nlohmann.me/home/exceptions/#parse-errors). 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 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](https://json.nlohmann.me/features/parsing/sax_interface/index.md) 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 <nlohmann/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]
|
|
```
|