Identifiers and Keywords in C++

In a C++ program (i.e., a program written in C++ language), every word is either classified as an identifier or a keyword.

Identifiers, as the name suggests, are used to identify or name various program elements such as variables, constants, functions etc.

On the other hand, Keywords are a kind of reserved word which have standard, predefined meanings in C++.

Now, let us discuss these concepts in details:

Identifiers:

As stated before, identifiers are names that are given to various program elements such as variables, constants, functions, arrays etc. There are certain rules regarding identifier names in C++, as stated below:

1. Identifiers must consist of letters and digits, in any order, except that the first character must be a letter
2. Both upper-case and lower-case letters are permitted, though common usage favours the use of lower-case letters for most types of identifiers. Upper-case and lower-case letters are not interchangeable i.e., an upper-case letter is not equivalent to the corresponding lower-case letter. Thus the names rate, Rate and RATE denote different identifiers. It is a general practice to use lower-case or mixed case for variable and function names and upper-case for constants
3. The underscore character (_) can also be included, and is considered to be a letter. An underscore is often used in the middle of an identifier. An identifier may also begin with an underscore, though this is rarely done in practice
4. An identifier should contain enough characters so that its meaning is readily apparent. On the other hand, an excessive number of characters should be avoided
5. There are certain reserved words, called keywords, that have standard, predefined meanings in C++. These keywords can be used only for their intended purpose. They cannot be used as a programmer-defined identifiers.
6. It is required to note here that the keywords are all lower-case. Since upper-case and lower-case characters are not equivalent, it is possible to utilize an upper-case keyword as an identifier. Normally, however, this is not done, as it is considered a poor programming practice

Examples of valid identifiers:
z
x11
sum_1
_temperature
names
area

Examples of Invalid identifiers:
7th
“a”
order-no
error flag

Keywords:

As stated before, keywords are the words whose meaning has already been explained to the C++ compiler (or in a broad sense to the computer). The keywords cannot be used as identifier names because, if we do so, we are trying to assign a new meaning to the keyword, which is not allowed by the computer. The keywords are also called “Reserved words”.

Examples of Keywords in C++:
and
or
register
char
int
float
class
function
public
private
private
switch