What does the ":" operator do in this case in c++?

Question

Arduino

Good afternoon! I recently came across this code for creating a button class for Arduino:

The code

//...
class Button
{
  public: Button (uint8_t pin, bool pullup = false, uint16_t debounceDelay = 50)
    : _pin(pin), _state(LOW), _lastState(LOW),
      _lastMillis(0), _debounceDelay(debounceDelay),
      _lastDebounceTime(0)
  {
    if (pullup == true)
    {
      pinMode(_pin, INPUT_PULLUP);
    }
    else
    {
      pinMode(_pin, INPUT);
    }
}
//...

I was surprised how the operator :(line 5) is used here, and in general the whole construction after it. If anyone knows, please tell me what it is, or better just give a link to a reference resource, I will be very grateful.
Thanks in advance!

Answer 1

This is called the initialization section . In this section, you can independently initialize the fields of the created object even before passing control to the constructor body.
Starting with C++11, you can specify constructor delegation in the initialization section. The selected constructor will also be executed before entering the body of the current constructor.