10.4 — Association

In the previous two lessons, we’ve looked at two types of object composition, composition and aggregation. Object composition is used to model relationships where a complex object is built from one or more simpler objects (parts).

In this lesson, we’ll take a look at a weaker type of relationship between two otherwise unrelated objects, called an association. Unlike object composition relationships, in an association, there is no implied whole/part relationship.


To qualify as an association, an object and another object must have the following relationship:

  • The associated object (member) is otherwise unrelated to the object (class)
  • The associated object (member) can belong to more than one object (class) at a time
  • The associated object (member) does not have its existence managed by the object (class)
  • The associated object (member) may or may not know about the existence of the object (class)

Unlike a composition or aggregation, where the part is a part of the whole object, in an association, the associated object is otherwise unrelated to the object. Just like an aggregation, the associated object can belong to multiple objects simultaneously, and isn’t managed by those objects. However, unlike an aggregation, where the relationship is always unidirectional, in an association, the relationship may be unidirectional or bidirectional (where the two objects are aware of each other).

The relationship between a doctors and patients is a great example of an association. The doctor clearly has a relationship with his patients, but conceptually it’s not a part/whole (object composition) relationship. A doctor can see many patients in a day, and a patient can see many doctors (perhaps they want a second opinion, or they are visiting different types of doctors). Neither of the object’s lifespans are tied to the other.

We can say that association models as “uses-a” relationship. The doctor “uses” the patient (to earn income). The patient uses the doctor (for whatever health purposes they need).

Implementing associations

Because associations are a broad type of relationship, they can be implemented in many different ways.

In this example, we’ll implement a bi-directional Doctor/Patient relationship, since it makes sense for the Doctors to know who their Patients are, and vice-versa.

This prints:

James is seeing patients: Dave
Scott is seeing patients: Dave Betsy
Dave is seeing doctors: James Scott
Frank has no doctors right now
Betsy is seeing doctors: Scott

In general, you should avoid bidirectional associations if a unidirectional one will do, as they add complexity and tend to be harder to write without making errors.

Reflexive association

Sometimes objects may have a relationship with other objects of the same type. This is called a reflective association. A good example of a reflexive association is the relationship between a university course and its prerequisites (which are also university courses).

Consider the simplified case where a Course can only have prerequisite. We can do something like this:

This can lead to a chain of associations (a course has a prerequisite, which has a prerequisite, etc…)

Associations can be indirect

In all of the above cases, we’ve used a pointer to directly link objects together. However, in an association, this is not strictly required. Any kind of data that allows you to link two objects together suffices. In the following example, we show how a Driver class can have a unidirectional association with a Car without actually including a Car pointer member:

In the above example, we have a CarLot holding our cars. The Driver, who needs a car, doesn’t have a pointer to his Car -- instead, he has the ID of the car, which we can use to get the Car from the CarLot when we need it.

In this particular example, doing things this way is kind of silly, since getting the Car out of the CarLot requires an inefficient lookup (a pointer connecting the two is much faster). However, there are advantages to referencing things by a unique ID instead of a pointer. For example, you can reference things that are not currently in memory (maybe they’re in a file, or in a database, and can be loaded on demand). Also, pointers can take 4 or 8 bytes -- if space is at a premium and the number of unique objects is fairly low, referencing them by an 8-bit or 16-bit integer can save lots of memory.

Composition vs aggregation vs association summary

Here’s a summary table to help you remember the difference between composition, aggregation, and association:

Property Composition Aggregation Association
Relationship type Whole/part Whole/part Otherwise unrelated
Members can belong to multiple classes No Yes Yes
Members existence managed by class Yes No No
Directionality Unidirectional Unidirectional Unidirectional or bidirectional
Relationship verb Part-of Has-a Uses-a
10.5 -- Dependencies
10.3 -- Aggregation

8 comments to 10.4 — Association

  • Ahmed

    Great explanation , thank you :))

  • Matt

    Typo at the begininng, after the "Association" sub-heading, first sentence… I think "and" should be "an".

    Also, your overloaded operator<< functions for both Doctor and Patient use "std:cout" instead of "out".

  • Matt

    Typo at the beginning: "quality" should be "qualify" I think.

  • Bing

    Hello Alex

    First of all, thank you for your generosity on sharing your tremendous knowledge of c++. There is one thing, regarding the DOCTOR and PATIENT program, I am not sure about. I copied and pasted part that I don’t know

    void addPatient(Patient *pat)
            // Our doctor will add this patient
            // and the patient will also add this doctor

    how does that pointer-to-member operator, -> , work here? because later you write like following in your program



    Can you explain the mechanism behind there with this pointer-to-member operator? Thank you!

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