Composite (Design pattern)

That structural design pattern allows to create trees of objects and to interact with them as they are just a single object.

#include <iostream>
#include <vector>

// Component
class IBird
{
public:
  virtual void fly() = 0;
  virtual ~IBird() {}
};

// Leave
class Bird : public IBird
{
public:
  void fly() { std::cout << "flap flap" << std::endl; }
};

// Composite
class BirdFlock : public IBird
{
public:
  void fly() { for(auto bird : m_Flock){ if(bird){bird->fly();} } }
  void add(IBird *bird) { m_Flock.push_back(bird); }
private:
  std::vector<IBird*> m_Flock;
};

int main()
{
  Bird bird1, bird2, bird3, bird4, bird5;

  BirdFlock birdFlockPart;
  birdFlockPart.add(&bird1);
  birdFlockPart.add(&bird2);
  birdFlockPart.add(&bird3);

  BirdFlock birdFlockAll;
  birdFlockAll.add(&birdFlockPart);
  birdFlockAll.add(&bird4);
  birdFlockAll.add(&bird5);

  birdFlockAll.fly();
}

birdFlockAll
   |
   |---> birdFlockPart
   |        |
   |        |---> bird1
   |        |---> bird2
   |        |---> bird3
   |
   |---> bird4
   |---> bird5