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 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. However, most often, associations are implemented using pointers, where the object points at the associated object.

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 reflexive 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 one 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

84 comments to 10.4 — Association

  • So linked list is implemented using reflexive association relation...

  • Hi Alex,

    I see that for functions taking in std::string as an argument, you sometimes pass by value and at other times pass by reference (even though it isn't being modified.) I'd really like to know what the best practice is.

    If the modification is to be seen in the calling function, then yes, pass by reference is the way to go. If not, what's the best practice? Pass by value, or a (const) reference?


    • Hi Nitin!

      If the argument isn't being modified, pass by const reference, unless it's a built-in type (int, float, double, ...).
      If the argument is being modified, pass by pointer if the caller is supposed to see the change, otherwise pass by value.
      Only pass by non-const reference, when passing by pointer is not possible or too complicated (eg. operators).

  • David

    Hi Alex! I wanted to implement the Patient/Doctor classes with each class' member functions defined in separate cpp files. Is the following code the best way to do this:





    • Hi David!

      * @Patient::getName and @Doctor::getName should be defined in source files and should return const references.
      * @Patient::Patient and @Doctor::Doctor should be defined in source files.
      * @Patient doesn't ever edit it's doctors, you could store const doctors.
      * @patient.hpp:20, @patient.cpp:10,18, @doctor.hpp:18, @doctor.cpp:12,20: Use uniform initizalization.
      * The name printing loops could be for-each loops, because you don't need to index.
      * @std::vector::size returns a @std::size_t, use that instead of unsigned long.
      * Both operator<<s: Use @std::vector::empty.

      • David

        Hi nascardriver! Thanks for all the great feedback. One question - if I define @Patient::Patient and @Doctor::Doctor in source files, how will the definitions "see" one another? Would I need to #include the source files?

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