A: Architectural ideas

A.1: Separate stable code from less stable code

• Isolating less stable code facilitates its unit testing, interface improvement, refactoring, and eventual deprecation.

A.2: Express potentially reusable parts as a library

• A library is a collection of declarations and definitions maintained, documented, and shipped together.
• A library could be a set of headers (a "header-only library") or a set of headers plus a set of object files.
• You can statically or dynamically link a library into a program, or you can #include a header-only library.

A.4: There should be no cycles among libraries

• A cycle complicates the build process.
• Cycles are hard to understand and might introduce indeterminism (unspecified behavior).
• Note: A library can contain cyclic references in the definition of its components.
  • However, a library should not depend on another that depends on it.