initializing a object in an another class object .(after doing some operations on that constructor.)

Question

I want to initialize a class member which is also another class object.The problem is that, I have to initialize the member with variables that I figure out after doing some operations on my constructor. Let me show the sample code.

class Child_class
{
    private:
        int var1, var2, var3;

        public:
        DateTime(int var1 = 1970, int var2 = 1, int var3 = 1);

};

second class:

class Owner_class
{
    private:

        Child_class foo;

    public:
        // I have to make some string split operations on string_var variable
        // and get some new variables.After that, I need to initialize child_class with  new variables               
        Owner_class( string string_var, int test);
}

One way to do that, I know I could write :

Owner_class::Owner_class():
   Child_class(new_var1,new_var2,new_var3) {
    // but since I'll find new_var1,new_var2 and new_var3 here.I couldnt use this method.
    // Am I right ?  
}

Is there anyone to help me ? thanks in advance!

Answer 1

You can write a function to do the calculations for you and return a Child_class object, and use this to initialize your instance in the Owner_class constructor initialization list:

Child_class make_child_object(string string_var, int test)
{
  // do stuff, instantiate Child_class object, return it
}

Then

Owner_class(string s, int n) : foo(make_child_object(s, n) {}

If this method is unsuitable, then an alternative is to give Child_class a default constructor, and assign it a value in the Owner_class constructor body:

Owner_class(string s, int n) 
{
  // foo has been default constructed by the time you gethere.
  // Do your stuff to calculate arguments of Child_class constructor.
  ....
  // construct a Child_class instance and assign it to foo
  foo = Child_class(a, b, c);
}