std::string vs String ?

[robertoinzerillo]

I have a minor technical question that experienced coders (which I'm definitely not) would surely answer with specific clarity.

Both String and std::string classes are being used here. As I understand String is the Arduino standard class and std::string is the one provided by the esp32 core. Is that right?

[robertoinzerillo]

After reading/learning a bit more I guess I have to correct myself. Both String and std::string used in this code seem to be part of the ESP32 core itself.
String is declared in \esp32\3.1.1\cores\esp32\WString.h
string is aliased in \esp32\tools\esp-x32\2405\xtensa-esp-elf\include\c++\13.2.0\bits\stringfwd.h
Am I getting it right?

The question arose from my willing to finally being able to read and understand this source code, and I'm slowly getting a grip with it.
The use of both String and string class objects made me wonder why (opportunity, usefulness, habit?), and so I fell down this rabbit hole, which hopefully will teach me something.

[sjmtlewy]

Note the two are not interchangeable in this Code, as I discovered to my peril earlier. std::string is used here as a poor mans byte buffer to build the advertising packet. This works with the std string as the bytes are naively appended to the underlying char array. If you change it to the arduino String, it will stop working because when appending anything other than a char, that class will convert the value to a string representation. So e.g. when you try to append a 0x6 from a byte array, you end up adding the UTF-16 encoding of the number 6 instead (arduino bytes are unsigned chars notnregularl chars).

[robertoinzerillo]

right ... I appreciate the detailed explanation.
I'm moving forward on my c++ learning path. Studying this code with the intention of customizing it and (possibly) expanding it is giving me a practical reason to delve deeper into c++ and how the Arduino community is using it.

I understand e.g. that using an Arduino defined String() object (and related methods: e.g. substring())inside the .ino code is natural (not only useful) because that's what a generic user is going to do; and later on converting it to a c_style null-terminated string inside the .cpp (that's going to act as a c++ library) makes even more sense, in order to process the data further with more common/generic std:: functions.
See: bind_key[i] = strtol(key.substring(i * 2, i * 2 + 2).c_str(), NULL, BIND_KEY_LEN);

I found it odd at first to see a mixed use of those two "string" types. It makes more sense to me now that I've spent a few days reading, learning and getting a bit more acquainted with the source and the language.