Question

Possible Duplicate:
What is this weird colon-member syntax in the constructor?

I am looking at this. What does the part after the colon mean ?

explicit Box(double l, double w, double h) : Rectangle(l, w), height(h) {}

I am used to initializing the values in the function body {}.

Answer 1

This is the actual way of initializing the data members. Normally constructors have two phases, namely initialization and computation.

/* Initialization */
explicit Box(double l, double w, double h): Rectangle(l, w), height(h)

And whatever you do inside { }, that comes under computation phase.

Even if you don’t write a initialize list in your code the compiler puts hidden code to initialize your data members. So if you write

explicit Box(double l, double w, double h):Rectangle(l, w)  
{  
  height = h; /* re-assigning value */ 
}

It means you are just assigning values to which are all ready been initialized.

explicit Box(double l, double w, double h): Rectangle(l, w), height(h)

This type of initialization is mandatory when you have constant or reference data members in your class. Because you can’t assign values to them inside the constructor body i.e. inside { }