How to make sure a string or string literal is a valid JSON?

I'm gonna be sending strings to the parser that might or might not be valid JSONs and I need a way to check they are. I'm programming for a Nintendo 3DS, but using try to catch an exception does not work and the application hangs anyway if it finds an error while parsing. What can I do?

[jmazzola]

it'd be nice to have a method perhaps a boolean to "validate" json. Similar to how this website works:

http://codebeautify.org/jsonviewer#

[gregmarr]

There is a library that just does validation and supports this parser:
https://github.com/tristanpenman/valijson

That doesn't do what I need, I asked the author. If I provide a non-valid JSON it'll still hang :/

[gregmarr]

Really, a hang? That's very surprising. Can you share a sample of the json that causes a hang?

I doubt it's got something to do with the JSON, it's because there is no good way that I know of handling exceptions when programming for a Nintendo 3DS so the program will just freeze.

[gregmarr]

JSON for Modern C++ Version 2.1.0
@nlohmann nlohmann released this 19 days ago · 56 commits to develop since this release

Release date: 2017-01-28

✨ Exceptions can now be switched off. This can be done by defining the preprocessor symbol JSON_NOEXCEPTION or by passing -fno-exceptions to your compiler. In case the code would usually thrown an exception, abort() is now called.

[gregmarr]

Oh, that probably doesn't help much either, as it calls abort.

[gregmarr]

So what you really need is for the library to return an error code instead of throwing or aborting when calling parse().

Yes, pretty much.

[nlohmann]

I understand the issue. Inside parse_internal() all exceptions are thrown by expect()/unexpect(). (https://github.com/nlohmann/json/blob/develop/src/json.hpp#L11455) We may use a different replacement for JSON_THROW there, but still would have the problem of leaving the function immediately in case of an error.

Would it be possible to return an empty json object instead?

[nlohmann]

If replacing the throw call is possible, returning a discarded object would be better. You would check it with is_discarded().

Hey, commenting the JSON_THROW part on except() and unexcept() and then using an "if (json::parse(string) != NULL)" seems to work!

[nlohmann]

To properly provide a solution for this issue, I think it would be best to have a dedicated check function that returns a boolean and does not create any basic_json values at all.