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Home » Tip Bank » C++
Language: C++
Expertise: Beginner
Mar 19, 2000

Object Containment and Member Initialization

Question:
I would like to know for containment (having objects as data member), what is meant by "if the inner class object's constructor requires parameters, the initialization syntax must be used in the outer class's constructor"? Please provide me with an example.

Answer:
Suppose you have a class called A, which contains an object of class B as a data member. B has no default constructor. Therefore, A's constructor must initialize the embedded B object using a member initialization list:

class B
{
public:
  B(int n);  // no default ctor
};

class A
{
private: 
  B b;
public:
  A() : b(5) {} // initialize b explicitly
};
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