Unclear message when mapping an invalid JSON

[harumazzz]

When attempting to decode or map input that is not valid JSON, the library produces an error message such as:

not a T

This message is difficult to interpret for end users or developers, since it doesn’t indicate what went wrong or where the parsing failed.

Example:

#include <jsoncons/json.hpp>
#include <iostream>

struct A;

int main() {
    try {
        std::string invalid = /* an object of struct B*/;
        auto val = jsoncons::decode_json<A>(invalid);
    } catch (const std::exception& e) {
        std::cerr << e.what() << std::endl;
    }
}

Actual output:

not a A

[danielaparker]

Can you provide a complete example? I need to have one when I explore this issue, and I'd rather somebody else create it. Thanks.

[sarahlydia9868]

Code Example

#include <jsoncons/json.hpp>
#include <iostream>

// Target Struct
struct A {
    int value1;
    int value2;
};

// Using the macro to map all members
JSONCONS_ALL_MEMBER_TRAITS(A, value1, value2);

int main() {
    try {
        // This JSON is intentionally wrong:
        // - It has "value1" (correct)
        // - It is missing "value2" (required)
        // - It has "value3" (extraneous)
        std::string invalid_json = R"(
    {
       "value1": 1,
        "value3": 2
    }
)";
        // Attempt to decode
        auto val = jsoncons::decode_json<A>(invalid_json);

    } catch (const std::exception &e) {
        // Catch exception
        std::cerr << e.what() << std::endl;
    }
}

Current Output

When running the code above, the output I receive is:

Not a A: Unable to convert into the provided type

Desired Output

The error message "Unable to convert into the provided type" is too generic. It doesn't tell me why the conversion failed. In this case, it's because value2 was missing from the JSON.

I would like to get a more detailed error message, for example:

Unable to find key "value2" in struct A

Or something similar that specifies which key is causing the problem. This would be extremely helpful for debugging and for returning meaningful error messages to an end-user.

[danielaparker]

Thanks. Error messages were improved when we introduced the non-throwing versions of decode_json, try_decode_json, in version 1.4, see Non-throwing overload. While the throwing versions inherited some of these improvements, that didn't include context. I've changed that so the throwing versions will now display the same messages as the non-throwing versions.

In your example, the message will be

A::value2: Missing required JSON object member at line 2 and column 6

The code is on master.