What's the meaning of F(string) in C++?

Question

Currently I'm writing C++ code on Arduino. In an example, I find a expression

Serial.print(F("Disconnected from central:"));

It's obvious that this statement is used to send string to the serial, but why it uses F(string) instead of using string directly?

I try to google it but with no results. If someone know it, I would be greatly appreciated.

Answer 1

This macro is Arduino-specific, it's not "C++" as such.

It places the string in flash memory, to conserve RAM.

It means the string cannot be modified when the program runs.

One current definition is:

#define F(slit) (reinterpret_cast<const __FlashStringHelper *>(PSTR(slit)))

See the source code for more.