Standard way of Serializing utf-8 characters in a JSON String - c++

Question

What's the standard way of serializing a utf-8 string in JSON? Should it be with u escaped sequence or should it be the hex code.
I want to serialize some sensor readings with units in a JSON Format.
For example I have temperature readings with units °C. Should it be serialized as
{
"units": "\u00b0"
}
´´´
or should it be something like
´´´
{
"units":"c2b0"
}
Or could both of these supported by the standard.

Answer 1

If JSON is used to exchange data, it must use UTF-8 encoding (see RFC8259). UTF-16 and UTF-32 encodings are no longer allowed. So it is not necessary to escape the degree character. And I strongly recommend against escaping unnecessarily.
Correct and recommended
{
"units": "°C"
}
Of course, you must apply a proper UTF-8 encoding.
If JSON is used in a closed ecosystem, you can use other text encodings (though I would recommend against it unless you have a very good reason). If you need to escape the degree character in your non-UTF-8 encoding, the correct escaping sequence is \u00b0.
Possible but not recommended
{
"units": "\u00b0C"
}
Your second approach is incorrect under all circumstances.
Incorrect
{
"units":"c2b0"
}
It is also incorrect to use something like "\xc2\xb0". This is the escaping used in C/C++ source code. It also used by debugger to display strings. In JSON, it always invalid.
Incorrect as well
{
"units":"\xc2\xb0"
}

Answer 2

JSON uses unicode to be encoded, but it is specified that you can use \uxxxx escape codes to represent characters that don't map into your computer native environment, so it's perfectly valid to include such escape sequences and use only plain ascii encoding to transfer JSON serialized data.