Use diagnostic positions in exceptions

[nlohmann]

We recently merged #4517 which added a macro JSON_DIAGNOSTIC_POSITIONS that, when defined, adds two std::size_t members to JSON values indicating the position of the first and last byte of this element in the input text. We should use this information to provide better diagnostic messages in exceptions.

[hnampally]

@nlohmann I have been looking into this, I was wondering if Json_Diagnostic_Positions and Json_Diagnostics options are mutually exclusive, also please provide some hints for this task.

[nlohmann]

No, the two options are not mutually exclusive:

• JSON_DIAGNOSTICS allows to add a JSON Pointer to the exception messages that can help to locate the part of the JSON that lead to the error. See https://json.nlohmann.me/api/macros/json_diagnostics/ for more information.
• JSON_DIAGNOSTIC_POSITIONS allows to query for each parse JSON value the byte positions in the input (see https://json.nlohmann.me/api/macros/json_diagnostic_positions/). The goal of this issue is to add this to the exception messages.

So an example for the default behavior would be:

[json.exception.type_error.302] type must be number, but is string

for JSON_DIAGNOSTICS, this can be:

[json.exception.type_error.302] (/address/housenumber) type must be number, but is string

Now for JSON_DIAGNOSTIC_POSITIONS we can query the position in the input and could do something like this:

[json.exception.type_error.302] (byte 34-39) type must be number, but is string

And if both options are set, we could have both:

[json.exception.type_error.302] (/address/housenumber, byte 34-39) type must be number, but is string

[hnampally]

Thanks for the clarification, I think I got it now, I will raise a PR shortly.

[hnampally]

@nlohmann Please let me know if the changes I made 2cc096a makes sense, for now I just modified the existing examples to test the code, I wonder if I need to add a new example for this in docs and/or add/modify existing unit tests.