What does the colon mean in a constructor? [duplicate]

Question

Possible Duplicates:
C++ weird constructor syntax
Variables After the Colon in a Constructor
What does a colon ( : ) following a C++ constructor name do?

For the C++ function below:

cross(vector<int> &L_, vector<bool> &backref_, vector< vector<int> > &res_) : 

    L(L_), c(L.size(), 0), res(res_), backref(backref_) {

    run(0); 

}

What does the colon (":") tell the relationships between its left and right part? And possibly, what can be said from this piece of code?

Answer 1

This is a way to initialize class member fields before the c'tor of the class is actually called.

Suppose you have:

class A {

  private:
        B b;
  public:
        A() {
          //Using b here means that B has to have default c'tor
          //and default c'tor of B being called
       }
}

So now by writting:

class A {

  private:
        B b;
  public:
        A( B _b): b(_b) {
          // Now copy c'tor of B is called, hence you initialize you
          // private field by copy of parameter _b
       }
}