Containership or Composition: (Classes within classes) When you define a class object within another class, it is called composition. In the below example class B contains the object   of class A. This is called “has-a” relationship. This is contrary to inheritance which is called “kind-of” relationship.

 

#include <iostream>

using namespace std;

class A

{

public:

    int ai;

    A():ai(10){}

};

class B

{

public:

    int bi;

    A a; /* Embedded object */

    B():bi(20){}

};

class AA

{

public:

    int aai;

    class BB /* Here the entire class itself is declared with the another class */

    {

    public:

        int bbi;

        BB():bbi(50){}

    }temp; /* temp object is created for class BB within class AA. Hence "temp" is an embedded object. For every object you create for class AA, temp will be a data member of that object */

    AA():aai(100){}

};

int main()

{

    B b;

    cout<<"B.A.i "<<b.a.ai<<endl;

    cout<<"B.i "<<b.bi<<endl;

    AA aa;

    cout<<"AA.aai "<<aa.aai<<endl;

    cout<<"aa.temp.bbi "<<aa.temp.bbi<<endl;

    AA::BB bb;

    cout<<"AA::BB.bbi "<<bb.bbi<<endl;

}

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