Thread: Trouble understanding constructor

I'm new to c++.
Here's the constructor I'm trying to understand:

Code:

RealNum::RealNum(float initR) : r(initR)  {}

I don't understand the part that's : r(initR)
What does it mean?
r is a float that holds a real number.
If it was java code I would think r is a method name and initR is it's arguments.

Here's the class interface

Code:

class RealNum {
  protected: 
    float r;
  public:                
    RealNum(float initR);
    float realPart();
    virtual void square(); 
    virtual void print();  
};

Thanks, this was a great help: Initialization Lists in C++ - Cprogramming.com

Originally Posted by Elysia

It's called the initializer list. It initialises the variables specified in the list instead of assigning them. This can be important since initialising a variable calls its constructor, but assigning calls it assignment operator. For built-in types such as float, there is no difference between the two.