Names for Variables

C++ encourages you to use meaningful names for variables. If a variable represents the cost of a trip, you should call it cost_of_trip or costOfTrip, not just x or cot. You do have to follow a few simple C++ naming rules:

• The only characters you can use in names are alphabetic characters, numeric digits, and the underscore (_) character.

• The first character in a name cannot be a numeric digit.

• Uppercase characters are considered distinct from lowercase characters.

• You can’t use a C++ keyword for a name.

• Names beginning with two underscore characters or with an underscore character followed by an uppercase letter are reserved for use by the implementation—that is, the compiler and the resources it uses. Names beginning with a single underscore character are reserved for use as global identifiers by the implementation.

• C++ places no limits on the length of a name, and all characters in a name are significant. However, some platforms might have their own length limits.

The next-to-last point is a bit different from the preceding points because using a name such as __time_stop or _Donut doesn’t produce a compiler error; instead, it leads to undefined behavior. In other words, there’s no telling what the result will be. The reason there is no compiler error is that the names are not illegal but rather are reserved for the implementation to use. The bit about global names refers to where the names are declared; Chapter 4 touches on that topic.

The final point differentiates C++ from ANSI C (C99), which guarantees only that the first 63 characters in a name are significant. (In ANSI C, two names that have the same first 63 characters are considered identical, even if the 64th characters differ.)

Here are some valid and invalid C++ names:

int poodle;    // valid
int Poodle;    // valid and distinct from poodle
int POODLE;    // valid and even more distinct
Int terrier;   // invalid -- has to be int, not Int
int my_stars3  // valid
int _Mystars3; // valid but reserved -- starts with underscore
int 4ever;     // invalid because starts with a digit
int double;    // invalid -- double is a C++ keyword
int begin;     // valid -- begin is a Pascal keyword
int __fools;   // valid but reserved -- starts with two underscores
int the_very_best_variable_i_can_be_version_112;  // valid
int honky-tonk;        // invalid -- no hyphens allowed

If you want to form a name from two or more words, the usual practice is to separate the words with an underscore character, as in my_onions, or to capitalize the initial character of each word after the first, as in myEyeTooth. (C veterans tend to use the underscore method in the C tradition, whereas those raised in the Pascal tradition prefer the capitalization approach.) Either form makes it easier to see the individual words and to distinguish between, say, carDrip and cardRip, or boat_sport and boats_port.