GotW #7a: Minimizing Compile-Time Dependencies, Part 1



Managing dependencies well is an essential part of writing solid code. C++ supports two powerful methods of abstraction: object-oriented programming and generic programming. Both of these are fundamentally tools to help manage dependencies, and therefore manage complexity. It’s telling that all of the common OO/generic buzzwords—including encapsulation, polymorphism, and type independence—along with the lion’s share of design patterns, are really about describing ways to manage complexity within a software system by managing the code’s interdependencies.



When we talk about dependencies, we usually think of run-time dependencies like class interactions. In this Item, we will focus instead on how to analyze and manage compile-time dependencies. As a first step, try to identify (and root out) unnecessary headers.

Problem



JG Question



1. For a function or a class, what is the difference between a forward declaration and a definition?

Guru Question



2. Many programmers habitually #include many more headers than necessary. Unfortunately, doing so can seriously degrade build times, especially when a popular header file includes too many other headers.

In the following header file, what #include directives could be immediately removed without ill effect? You may not make any changes other than removing or rewriting #include directives. Note that the comments are important.