c++ - What is the member variables list after the colon in a constructor good for?

Question

I'm reading this C++ open source code and I came to a constructor but I don't get it ( basically because I don't know C++ :P )

I understand C and Java very well.

 TransparentObject::TransparentObject( int w, int x, int y, int z ) : 
     _someMethod( 0 ),
     _someOtherMethod( 0 ),
     _someOtherOtherMethod( 0 ),
     _someMethodX( 0 ) 
  {
       int bla;
       int bla;
  }

As far I can "deduce" The first line only declares the construtor name, the "::" sounds like "belongs to" to me. And the code between {} is the constructor body it self.

I "think" what's after the paremeters and the first "{" are like methods default parameters or something, but I don't find a reasonable explanation on the web. Most of the C++ constructors that I found in the examples are almost identical to those in Java.

I'm I right in my assumptions? "::" is like belongs to, and the list after params and body are like "default args" or something?

UPDATE: Thanks for the answers. May those be called methods? ( I guess no ) and what is the difference of call them within the constructor body

Answer 1

The most common case is this:

class foo{
private:
    int x;
    int y;
public:
    foo(int _x, int _y) : x(_x), y(_y) {}
}

This will set x and y to the values that are given in _x and _y in the constructor parameters. This is often the best way to construct any objects that are declared as data members.

It is also possible that you were looking at constructor chaining:

class foo : public bar{
    foo(int x, int y) : bar(x, y) {}
};

In this instance, the class's constructor will call the constructor of its base class and pass the values x and y.

To dissect the function even further:

TransparentObject::TransparentObject( int w, int x, int y, int z ) : 
   _someMethod( 0 ),
   _someOtherMethod( 0 ),
   _someOtherOtherMethod( 0 ),
   _someMethodX( 0 ) 
{
     int bla;
     int bla;
}

The ::-operator is called the scope resolution operator. It basically just indicates that TransparentObject is a member of TransparentObject. Secondly, you are correct in assuming that the body of the constructor occurs in the curly braces.

UPDATE: Thanks for the answers. May those be called methods? ( I guess no ) and what is the difference of call them within the constructor body

There is much more information on this subject than I could possibly ever give you here. The most common area where you have to use initializer lists is when you're initializing a reference or a const as these variables must be given a value immediately upon creation.