34.5. Focus Your Attention with the Help of Conventions

A set of conventions is one of the intellectual tools used to manage complexity. Earlier chapters talk about specific conventions. This section lays out the benefits of conventions with many examples.

Cross-Reference

For an analysis of the value of conventions as they apply to program layout, see "How Much Is Good Layout Worth?" and "Objectives of Good Layout" in Section 31.1.

Many of the details of programming are somewhat arbitrary. How many spaces do you indent a loop? How do you format a comment? How should you order class routines? Most of the questions like these have several correct answers. The specific way in which such a question is answered is less important than that it be answered consistently each time. Conventions save programmers the trouble of answering the same questions making the same arbitrary decisions again and again. On projects with many programmers, using conventions prevents the confusion that results when different programmers make the arbitrary decisions differently.

A convention conveys important information concisely. In naming conventions, a single character can differentiate among local, class, and global variables; capitalization can concisely differentiate among types, named constants, and variables. Indentation conventions can concisely show the logical structure of a program. Alignment conventions can indicate concisely that statements are related.

Conventions protect against known hazards. You can establish conventions to eliminate the use of dangerous practices, to restrict such practices to cases in which they're needed, or to compensate for their known hazards. You could eliminate a dangerous practice, for example, by prohibiting global variables or prohibiting multiple statements on a line. You could compensate for a hazardous practice by requiring parentheses around complicated expressions or requiring pointers to be set to NULL immediately after they're deleted to help prevent dangling pointers.

Conventions add predictability to low-level tasks. Having conventional ways of handling memory requests, error processing, input/output, and class interfaces adds a meaningful structure to your code and makes it easier for another programmer to figure out as long as the programmer knows your conventions. As mentioned in an earlier chapter, one of the biggest benefits of eliminating global data is that you eliminate potential interactions among different classes and subsystems. A reader knows roughly what to expect from local and class data. But it's hard to tell when changing global data will break some bit of code four subsystems away. Global data increases the reader's uncertainty. With good conventions, you and your readers can take more for granted. The amount of detail that has to be assimilated will be reduced, and that in turn will improve program comprehension.

Conventions can compensate for language weaknesses. In languages that don't support named constants (such as Python, Perl, UNIX shell script, and so on), a convention can differentiate between variables intended to be both read and written and those that are intended to emulate read-only constants. Conventions for the disciplined use of global data and pointers are other examples of compensating for language weaknesses with conventions.

Programmers on large projects sometimes go overboard with conventions. They establish so many standards and guidelines that remembering them becomes a fulltime job. But programmers on small projects tend to go "underboard," not realizing the full benefits of intelligently conceived conventions. You should understand their real value and take advantage of them; you should use them to provide structure in areas in which structure is needed.