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Tip of the Day
Language: C++
Expertise: Beginner
May 2, 2000

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Constructors - Derived to Base

Question:
My base class contains this constructor:

class Parent {
  Parent(int a, int b, char* c);
};

My derived class contains this constructor:

class Child : public Parent {
  Child(int a);
};

defined as

Child::Child(int a) : Parent(a,2)
{
  .. some code
}

I need a third class derived from class Child class Grandchild :

public Child {
  Grandchild(int a);
};

but I can't figure out how to define the constructor. I've tried:

Grandchild::Grandchild(int a):Child (a,2)

Grandchild::Grandchild(int a):Parent(a,2)

Grandchild::Grandchild(int a):Child(a):Parent(2)

Grandchild::Grandchild(int a):Child(a),Parent(2)

Answer:
You can't initialize a non-immediate base class in a derived class (virtual base classes are an exception to this rule). What you need is to have class GrandChild initialize Child, and delegate the responsibility of initializing Parent to Child's constructor:

class Grandchild : public Child 
{
 Grandchild(int a):Child (a,2){}//implicit init of Parent
};

Child's constructor automatically initializes Parent with the values a and 2.

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