What is the meaning this external member functions definition of a Class?

I'm working on the project while going through some libraries to custom-build for my project. I found below part of the code that I could not understand. Please help me.

BLECharacteristic::BLECharacteristic(void)
  : uuid()
{
  _init();
}

BLECharacteristic::BLECharacteristic(BLEUuid bleuuid)
  : uuid(bleuuid)
{
  _init();
}

BLECharacteristic::BLECharacteristic(BLEUuid bleuuid, uint8_t properties)
  : uuid(bleuuid)
{
  _init();
  setProperties(properties);
}

I understand the external definition of a function from a class like :

BLECharacteristic::BLECharacteristic(void)
    {
      _init();
    }

I also know while calling a function with different arguments, it will call the respective function with the same parameters.

But what I don't get is the extra part : uuid()

Never saw this kind of function definition, also I'm confused with is below part is the definition of BLECharacteristics(void) or uuid()

As I couldn't find anywhere the function uuid()

BLECharacteristic::BLECharacteristic(void)
      : uuid()
    {
      _init();
    }

Here complete Git Source Code