Common Reuse Principle

Principle

Don’t force users of a component to depend on things they don’t need.

Classes and modules that tend to be reused together belong in the same component.

Background

Reusable classes collaborate with other classes that are part of the reusable abstraction. CRP states that those belong together. In such a component, we would expect to see classes that have lots of dependencies on each other.

A simple example might be a container class and its associated iterators.

When one component uses another, a dependency is created between the components. Perhaps the using component uses only one class within the used component, but that still doesn’t weaken the dependency. Because of that dependency, every time the used component is changed, the using component will likely need corresponding changes. This is true even if the using component doesn’t care about the changes made in the used component.

Thus, when we depend on a component, we want to make sure we depend on every class in that component. Put another way, we want to make sure that the classes that we put into a component are inseparable (that it is impossible to dpeend on some and not on the others).

Therefore the CRP tells us more about which classes shouldn’t be together than about which classes should be together.

The CRP says that classes that are not rightly bound to each other should not be in the same component.

Relation to ISP

The CRP is generic version of the ISP. The ISP advises us not to depend on classes that have methods we don’t use. The CRP advises us not to depend on components that have classes we don’t use.

Don’t depend on things you don’t need.