Composition
As the term "composition" implies, when an object contains or possesses another object – in other words, it is composed of another object – we say that there is a composition relationship between them.
As an example, a car has an engine; a car is an object that contains an engine object. Therefore, the car and engine objects have a composition relationship. There is an important condition that a composition relationship must have: the lifetime of the contained object is bound to the lifetime of the container object.
As long as the container object exists, the contained object must exist. But when the container object is about to get destroyed, the contained object must have been destructed first. This condition implies that the contained object is often internal and private to the container.
Some parts of the contained object may be still accessible through the public interface (or behavior functions) of the container class, but the...
