1.4c __ Keywords and naming identifiers

Keywords

C++ reserves a set of 84 words (as of C++14) for its own use. These words are called keywords (or reserved words), and each of these keywords has a special meaning within the C++ language.

Here is a list of all the C++ keywords (through C++14):

alignas ** alignof ** and and_eq asm auto bitand bitor bool * break case catch

char char16_t ** char32_t ** class compl const constexpr ** const_cast * continue decltype default delete

do double dynamic_cast * else enum explicit * export * extern false * float for friend

goto if inline int long mutable * namespace * new noexcept ** not not_eq nullptr **

operator or or_eq private protected public register reinterpret_cast * return short signed sizeof

static static_assert ** static_cast * struct switch template this thread_local ** throw true * try typedef

typeid * typename * union unsigned using * virtual void volatile wchar_t * while xor xor_eq

* These 15 keywords were added in C++98. Some older reference books or material may omit these.
** These 9 keywords were added in C++11. If your compiler is not C++11 compliant, these keywords may not be functional.

C++11 also adds two special identifiers: override and final. These have a specific meaning when used in certain contexts.

You have already run across some of these keywords, including int, void, and return. Along with a set of operators, these keywords and special identifiers define the entire language of C++ (preprocessor commands excluded). Because these keywords and special identifiers have special meaning, your IDEs will change the text color of these words (usually to blue) to make them more visible.

By the time you are done with this tutorial series, you will understand what almost all of these words do!

Identifier naming rules

The name of a variable, function, type, or other kind of object in C++ is called an identifier. C++ gives you a lot of flexibility to name identifiers as you wish. However, there are a few rules that must be followed when naming identifiers:

• The identifier can not be a keyword. Keywords are reserved.
• The identifier can only be composed of letters (lower or upper case), numbers, and the underscore character. That means the name can not contain symbols (except the underscore) nor whitespace (spaces or tabs).
• The identifier must begin with a letter (lower or upper case) or an underscore. It can not start with a number.
• C++ distinguishes between lower and upper case letters. nvalue is different than nValue is different than NVALUE.

Now that you know how you can name a variable, let’s talk about how you should name a variable or function.

First, it is a convention in C++ that variable names should begin with a lowercase letter. If the variable name is one word, the whole thing should be written in lowercase letters.

1 2 3 4 5

int value; // correct   int Value; // incorrect (should start with lower case letter) int VALUE; // incorrect (should start with lower case letter) int VaLuE; // incorrect (see your psychiatrist) ;)

Generally, functions are also started with a lowercase letter (though there’s some disagreement on this point). We’ll start ours with lower case letters for consistency, since main() (which all programs must have) starts with a lowercase letter, as do all of the functions in the C++ standard library.

Identifier names that start with a capital letter are typically used for structs, classes, and enumerations (all of which we will cover later).

If the variable or function name is multi-word, there are two common conventions: separated by underscores, or intercapped (sometimes called CamelCase, since the capital letters stick up like the humps on a camel).

1 2 3 4 5 6 7 8 9 10 11

int my_variable_name; // correct (separated by underscores) void my_function_name() // correct (separated by underscores)   int myVariableName; // correct (intercapped/CamelCase) void myFunctionName(); // correct (intercapped/CamelCase)   int my variable name; // invalid (whitespace not allowed) void my function name(); // invalid (whitespace not allowed)   int MyVariableName; // valid but incorrect (should start with lower case letter) void MyFunctionName(); // valid but not best practice

In this tutorial, we will typically use the intercapped approach because it’s easier to read (it’s easy to mistake an underscore for a space in dense blocks of code). But it’s common to see either -- the C++ standard library uses the underscore method for both variables and functions. Sometimes you’ll see a mix of the two: underscores used for variables and intercaps used for functions.

It’s worth noting that if you’re working in someone else’s code, it’s generally considered better to match the style of the code you are working in than to rigidly follow the naming conventions laid out above.

Second, you should avoid naming your identifiers starting with an underscore, as these names are typically reserved for OS, library, and/or compiler use.

Third, and this is perhaps the most important rule of all, give your identifiers names that actually describe what they are. It is typical for inexperienced programmers to make variable names as short as possible, either to save on typing or because they figure the meaning is obvious. This is almost always a mistake. Ideally, variables should be named in a way that would help someone who has no idea what your code does be able to figure it out as quickly as possible. In 3 months, when you look at your program again, you’ll have forgotten how it works, and you’ll thank yourself for picking variable names that make sense. The more complex the code the variable is being used in, the better name it should have.

int ccount	Bad	Nobody knows what a ccount is
int customerCount	Good	Clear what we’re counting
int i	Bad*	Generally bad unless use is trivial or temporary, such as loop variables
int index	Either	Okay if obvious what we’re indexing
int totalScore	Good	Descriptive
int _count	Bad	Do not start variable names with underscore
int count	Either	Okay if obvious what we’re counting
int data	Bad	What kind of data?
int value1, value2	Either	Can be hard to differentiate between the two
int numberOfApples	Good	Descriptive
int monstersKilled	Good	Descriptive
int x, y	Bad*	Generally bad unless use is trivial, such as in trivial mathematical functions

* Note: it is okay to use trivial variable names for variables that have a trivial use, such as loop variables, or trivial mathematical functions.

Fourth, a clarifying comment can go a long way. For example, say we’ve declared a variable named numberOfChars that is supposed to store the number of characters in a piece of text. Does the text “Hello World!” have 10, 11, or 12 characters? It depends on whether we’re including whitespace or punctuation. Rather than naming the variable numberOfCharsIncludingWhitespaceAndPunctuation, which is rather lengthy, a well placed comment on the declaration line should help the user figure it out:

1 2	// holds number of chars in a piece of text -- including whitespace and punctuation! int numberOfChars;

Quiz

Pick which variables are improperly named according to standard conventions (ie. how you should name a variable) and indicate why.

1) int sum;
2) int _apples;
3) int VALUE;
4) int my variable name;
5) int TotalCustomers;
6) int void;
7) int numFruit;
8) int 3some;
9) int meters_of_pipe;

Hide Solution

2) Variable names should not start with an underscore.
3) Variable names should start with a lower case letter.
4) Variable names can not contain spaces.
5) Variable names should start with a lower case letter.
6) void is a keyword.
8) Variable names can not start with a number.

Numbers 1, 7, and 9 follow standard conventions.