what is containership or nesting in C++?

what i understood is: Containership: a class contains another class objects as member data.

please explain wit开发者_运维百科h an example.

Thanks.

Class nesting is simply to have a class defined in another class, like that :

class A
{
public:
   class B 
   {
   public:
       class C{};
   };

};

Then you can access a nested class using the scope operator, like you would do with namespaces :

A a;
A::B b;
A::B::C c;

Now when a class contain another class's object, it's an aggregation :

class D
{
public:

   A myA;

   void do_something();

private:
   A::B myB;

};

Then you can access the member like that if it's public :

D d;
process( d.myA ); // access to myA

If it's not accessible, then you can provide it via a function. Anyway, inside the class functions, you can directly access the member :

void D::do_something()
{
    doit( myB );
    // or
    doit( this->myB );
}

Containership simply means an object can be accessed inside another.

For instance :

class Contained
{
    int foo;
};

class Container
{
    Contained bar;
};

You can access foo like this :

Container c;
c.bar.foo = 42;

Class nesting is different. It means you are declaring a class from inside another :

class AngryMammoth
{
    class CrazyVulture
    {
        int legCount;
    };

    int numberOfPeopleKilledSoFar;
};

class In {};
class Out {
  In object;
};

Similar to inheritance, an object of class A is embedded in the object of class B:

class A {
  int x;
};
class B {
  int y;
  A a1;
  A a2;
};

If you would instantiate B b, you would get something like this on your memory:

int y; // b.y
int x; // b.a1.x
int x; // b.a2.x

Contrary to a pointer attribute, a nested object actually sits inside the outer object. Also you do not have to worry about creating/deleting A as it is automatically done when B is created/deleted.