/ Design Patterns / Behavioral patterns / Chain of Responsibility

Chain of Responsibility Design Pattern in C++

Chain of Responsibility design pattern

  1. Put a "next" pointer in the base class
  2. The "chain" method in the base class always delegates to the next object
  3. If the derived classes cannot handle, they delegate to the base class
#include <iostream>
#include <vector>
#include <ctime>
using namespace std;

class Base
{
    Base *next; // 1. "next" pointer in the base class
  public:
    Base()
    {
        next = 0;
    }
    void setNext(Base *n)
    {
        next = n;
    }
    void add(Base *n)
    {
        if (next)
          next->add(n);
        else
          next = n;
    }
    // 2. The "chain" method in the base class always delegates to the next obj
    virtual void handle(int i)
    {
        next->handle(i);
    }
};

class Handler1: public Base
{
  public:
     /*virtual*/void handle(int i)
    {
        if (rand() % 3)
        {
            // 3. Don't handle requests 3 times out of 4
            cout << "H1 passed " << i << "  ";
            Base::handle(i); // 3. Delegate to the base class
        }
        else
          cout << "H1 handled " << i << "  ";
    }
};

class Handler2: public Base
{
  public:
     /*virtual*/void handle(int i)
    {
        if (rand() % 3)
        {
            cout << "H2 passed " << i << "  ";
            Base::handle(i);
        }
        else
          cout << "H2 handled " << i << "  ";
    }
};

class Handler3: public Base
{
  public:
     /*virtual*/void handle(int i)
    {
        if (rand() % 3)
        {
            cout << "H3 passed " << i << "  ";
            Base::handle(i);
        }
        else
          cout << "H3 handled " << i << "  ";
    }
};

int main()
{
  srand(time(0));
  Handler1 root;
  Handler2 two;
  Handler3 thr;
  root.add(&two);
  root.add(&thr);
  thr.setNext(&root);
  for (int i = 1; i < 10; i++)
  {
    root.handle(i);
    cout << '\n';
  }
}

Output

H1 passed 1  H2 passed 1  H3 passed 1  H1 passed 1  H2 handled 1
H1 handled 2
H1 handled 3
H1 passed 4  H2 passed 4  H3 handled 4
H1 passed 5  H2 handled 5
H1 passed 6  H2 passed 6  H3 passed 6  H1 handled 6
H1 passed 7  H2 passed 7  H3 passed 7  H1 passed 7  H2 handled 7
H1 handled 8
H1 passed 9  H2 passed 9  H3 handled 9

Code examples

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