mean of the last : in members declaration

what is the meaning of the last :class() declaration on the the folowing member function definition.?

Code:

---

CLASSX::CONSTRUCTORX (params, ..) :CLASSY(param2){
...
}
  I know that :
class CLASSX: public CLASSY
{
...
CONSTRUCTORX(param,..)
}

---

and
the class CLASSY have a constructor with one param.
First I think that it is for call the CLASEY construsctor but, if CLASSY<--CLASSX i suppose that the constructor CLASSY cosntructor is calling by default.
or not?
help please!

The : indicates an initializer list. This is where base class constructors are called and member variables are initialized. If you leave it out, the default constructor for CLASSY will be called, but sometimes you want to or have to call a different constructor than the default constructor. You do that by using the initializer list.

note that in the same way you can also initialize the member vars

Thanks you, boys