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Definition
Composition is the relationship between types that arises when objects of one type contain objects of another type. Composition is also known as layering, containment, aggrega- tion, and embedding.
class Flight {
public:
...
private:
int id;
Model model;
Factory factory;
}
Composition means “has-a” relationship
Application Domain: objects in your programs correspond to things in the world you are modeling, e.g., people, vehicles, video frames, etc.
When objects occurred between objects in Application Domain, it express a “has-a” relationship. The Flight class above is an example of “has-a” relationship
Composition means “is-implemented-in-terms- of” relationship
Implementation Domain: objects are purely implementation artifacts, e.g., buffers, mutexes, search trees, etc
When objects occurred between objects in Implementation Domain, it express a “is-implemented-in-terms- of” relationship.
Example: Implement set in terms of list.
Set is implemented in std as a balanced search tree. This design is reasonable when speed is the primary factor of consideration but it is not a good design when we consider space as a more important factor. We can reimplement set by list, however, set is not a list since list allow duplicate item but set doesn’t. Thus, we can implement set in terms of list.
template<class T>
class Set {
public:
bool member(const T& item) const;
void insert(const T& item); void remove(const T& item);
std::size_t size() const;
private:
std::list<T> rep;
};
Set’s member functions can lean heavily on functionality already offered by list.
template<typename T>
bool Set<T>::member(const T& item) const {
return std::find(rep.begin(), rep.end(), item) != rep.end();
}
template<typename T>
void Set<T>::insert(const T& item) {
if (!member(item)) rep.push_back(item);
}
template<typename T>
void Set<T>::remove(const T& item) {
typename std::list<T>::iterator it = std::find(rep.begin(), rep.end(), item);
if (it != rep.end()) rep.erase(it);
}
template<typename T>
std::size_t Set<T>::size() const {
return rep.size();
}
Things to Remember
- Composition has meanings completely different from that of public inheritance.
- In the application domain, composition means has-a. In the imple- mentation domain, it means is-implemented-in-terms-of.