Adding a collection of functions to an object at runtime

Question

What id like to do is implement the roles programming technique within my code. I am using C++. C++11 is fine.

What i need is to be able to define a collection of functions. This collection can not have state.

Some of the functions that this collection has will be deferred/delegated.

e.g. (Illustration only:)

class ACCOUNT {
  int balance = 100;
  void withdraw(int amount) { balance -= amount; }
}

ACCOUNT savings_account;

class SOURCEACCOUNT {
  void withdraw(int amount); // Deferred.
  void deposit_wages() { this->withdraw(10); }
  void change_pin() { this->deposit_wages(); }
}

SOURCEACCOUNT *s;
s = savings_account; 

// s is actually the savings_account obj,
// But i can call SOURCEACCOUNT methods.
s->withdraw(...);
s->deposit();
s->change_pin();

I dont want to include SOURCEACCOUNT as a Base Class of ACCOUNT and do a cast, as i want to simulate runtime inheritance. (ACCOUNT doesnt know about SOURCEACCOUNT)

Im open to any suggestions; Can i extern or similar the function inside the SOURCEACCOUNT class? C++11 unions? C++11 call forwarding? changing the 'this' pointer?

Thankyou

Answer 1

It sounds like you want to create a SOURCEACCOUNT (or a variety of other classes) which takes a reference to an ACCOUNT and have some methods of the enclosing class delegate to ACCOUNT:

class SOURCEACCOUNT{
  ACCOUNT& account;
public:
  explicit SOURCEACCOUNT(ACCOUNT& a):account(a){}
  void withdraw(int amount){ account.withdraw(amount); }
  // other methods which can either call methods of this class
  // or delegate to account
};