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

jsonj;
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).

jsonj=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:

boolparse_error(std::size_tposition,
conststd::string&last_token,
constjson::exception&ex);

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

#include<iostream>
#include<nlohmann/json.hpp>
usingjson=nlohmann::json;
classsax_no_exception:publicnlohmann::detail::json_sax_dom_parser<json>
{
public:
sax_no_exception(json&j)
:nlohmann::detail::json_sax_dom_parser<json>(j,false)
{}
boolparse_error(std::size_tposition,
conststd::string&last_token,
constjson::exception&ex)
{
std::cerr<<"parse error at input byte "<<position<<"\n"
<<ex.what()<<"\n"
<<"last read: \""<<last_token<<"\""
<<std::endl;
returnfalse;
}
};
intmain()
{
std::stringmyinput="[1,2,3,]";
jsonresult;
sax_no_exceptionsax(result);
boolparse_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]