How to properly handle the Arduino String

Question

I've read that the Arduino String is bad as it causes memory fragmentation, so I try to avoid them mostly. But I still have some occurences where I'm not quite sure if they are ok after all.

The first is in print statements:
I have a function which prints a success or error message to the serial port, like so: Serial.println("Success");. Is this ok or should I (for example) pre define the String using #define SUCCESS "SUCCESS"?

Which leads me to the next case, defines:
I'm using MQTT and have the topics defined like above, e.g. #define TOPIC_COLOR "light/color". Is this fine?

A third one is with ArduinoJSON:
I'm using Strings as keys, like so lights["red"] = redVal;. This call is made at multiple places so I assume the string would be created multiple times? A workaround/improvement I've thought of is using an enum defining the properties and using that as key.

Thanks for all comments, tips and explanations!

Answer 1

+1 to @majenko's comments. I tend to avoid the String class, as I like to know what is actually going on, and I stick to char * and char [] in general (and the stdlib functions like sprintf, strlen, etc).

Couple of notes on your question:

1. "foo/bar" is fine. Some characters have special meanings in the C language itself, so you need to escape them with the \. For example if " was part of your literal string, you'd write "foo\"bar\""

2. I tend to use ArduinoJSON to parse. If my app is just sending JSON payloads, I just build it with sprintf or print which results in smaller programs. But, I guess it is a matter of choice. It is a great library.