loading from a stream and exceptions

[user1095108]

There are 2 possible scenarios when loading from a stream:

• there might be an error while loading from the stream (bad sector),
• the stream might contain nothing (zero size file),
• the json the stream contains might be invalid.

I suppose the library will throw meaningful exceptions if those are enabled, but if disabled, it will abort. There should exist an alternative way to report an error (such as returning an empty json object, for example), without crashing an application.

[nlohmann]

The status quo is like this:

#include "json.hpp"
#include <fstream>

using json = nlohmann::json;

int main()
{
    std::fstream f_non("nonexisting.json");
    std::fstream f_empty("empty.json");
    std::fstream f_error("error.json");
    
    try {
        json::parse(f_non);
    } catch (json::parse_error &e)
    {
        std::cerr << e.what() << std::endl;
    }

    try {
        json::parse(f_empty);
    } catch (json::parse_error &e)
    {
        std::cerr << e.what() << std::endl;
    }

    try {
        json::parse(f_error);
    } catch (json::parse_error &e)
    {
        std::cerr << e.what() << std::endl;
    }
}

Output:

[json.exception.parse_error.111] parse error: bad input stream
[json.exception.parse_error.101] parse error at 1: syntax error - unexpected end of input
[json.exception.parse_error.101] parse error at 1: syntax error - invalid literal; last read '@'

That is, in all three cases (nonexisting file, empty file, file with syntax error), a json::parse_error exception is thrown. In case exceptions are switched off, all three cases call std::abort().

[user1095108]

Yes, but the application programmer has no way to notify the user of an error, if you abort. This is something I'd like to be able to do in my app.

[user1095108]

You could return an empty object or even use std::optional.

[user1095108]

Or std::pair containing an object and bool. The bool indicating an error.

[nlohmann]

A validity check will be implemented with #458. This would avoid the last 2 exceptions. The first exception is thrown in the input_adapter class - I have to check whether it makes sense to remove it there. This could avoid refactoring the parser methods.

Note that std::optional is not part of C++11, and stepping forward to C++17 would exclude too many users.

[user1095108]

You still have std::pair then

[user1095108]

Also, hacking your own std::optional is not too difficult

[nlohmann]

I'm not convinced yet whether changing the signature of the parse function is worth it in the first place.

[user1095108]

Of course, you should introduce another function. But really, what bothers me most is, if I "pipe" a stream into a Json object your library might abort or throw. This is completely wrong. You should check if exceptions are enabled in the stream before throwing or aborting. You can set flags in the stream, you don't need to throw or abort.

[nlohmann]

I understand.

[JakeShirley]

I too would appreciate being able to parse without using exceptions. 👍

[nlohmann]

Opened #623 as place to discuss the structure of the parsing functions.