What are these "::" double colons

Projects Programming
1

Hey guys,

I started to program the Arduino. But now I am at a point I don't know what this means:

MFRC522::MF_KEY_SIZE

I try to understand these double colons. An other programmer used it and I saw it and I don't know what these are doing. I try to use this for the MFRC522 Library with the RFID Board.

I hope you can help me thanks.

The whole Example is this:

MFRC522::MIFARE_Key key; // access key

void setup() {
  Serial.begin(9600); // start serial connection
  SPI.begin(); // start SPI bus
  mfrc522.PCD_Init(); // init RFID module

  // initialize with default key value
  for (byte i = 0; i < MFRC522::MF_KEY_SIZE; i++) {
    key.keyByte[i] = 0xFF;
  }
}
2

Scope resolution operator.

3

loosely, the part before the "::" specifies the class or module that the function name after should come from. That way, both the MFRC522 library and the MultiFrob library can define a constand MF_KEY_SIZE without danger of them colliding.