C++ Coding Standards: 101 Rules, Guidelines, and Best Practices

22. Minimize definitional dependencies. Avoid cyclic dependencies

Summary

Don’t be over-dependent: Don’t #include a definition when a forward declaration will do.

Don’t be co-dependent: Cyclic dependencies occur when two modules depend directly or indirectly on one another. A module is a cohesive unit of release (see page 103); modules that are interdependent are not really individual modules, but super-glued together into what’s really a larger module, a larger unit of release. Thus, cyclic dependencies work against modularity and are a bane of large projects. Avoid them.

Discussion

Prefer forward declarations except where you really need a type’s definition. You need a full definition of a class C in two main cases:

• When you need to know ...