Understanding C++ Keywords and Identifiers

C++ is a powerful programming language that supports both structured and object-oriented programming. It is a superset of C, which means that most C constructs are valid in C++, though there are some differences and additions.

Tokens in C++

When a C++ program is compiled, the source code is broken down into tokens. Tokens are the smallest units in a program, and a C++ program is written using these tokens. There are several types of tokens in C++:

• Keywords
• Identifiers
• Constants
• Strings
• Operators

Keywords in C++

Keywords, also known as reserved words, have special meanings in C++ and are always written in lowercase. These words cannot be used as variable names, function names, or any other identifiers. There are 95 reserved keywords in C++. Here are some commonly used keywords:

Keyword	Description
asm	Declares a block of code to be passed to the assembler
auto	Specifies the storage class of a variable
break	Terminates a loop or switch statement
case	Specifies a match in a switch statement
catch	Handles exceptions in a try block
char	Defines a character data type
class	Declares a class
const	Defines a constant variable
continue	Skips the current iteration of a loop
default	Handles unmatched cases in a switch statement
delete	Deallocates memory
do	Starts a do-while loop
double	Defines a double-precision floating-point number
else	Specifies an alternative condition in an if statement
enum	Declares an enumeration type
extern	Specifies external linkage for a variable or function
float	Defines a floating-point number
for	Starts a for loop
friend	Allows access to private members of a class
goto	Transfers control to a labeled statement
if	Starts an if statement
inline	Suggests inlining a function
int	Defines an integer data type
long	Modifies the size of an integer
new	Allocates memory
operator	Overloads operators
private	Specifies private access in a class
protected	Specifies protected access in a class
public	Specifies public access in a class
register	Specifies register storage for a variable
return	Returns a value from a function
short	Modifies the size of an integer
signed	Specifies a signed data type
sizeof	Returns the size of a data type
static	Specifies static storage duration
struct	Declares a structure
switch	Starts a switch statement
template	Declares a template
this	Refers to the current object
throw	Throws an exception
try	Starts a try block for exception handling
typedef	Creates a type alias
union	Declares a union
unsigned	Specifies an unsigned data type
virtual	Declares a virtual function in a class
void	Specifies an empty data type
volatile	Prevents compiler optimization of a variable
while	Starts a while loop

Identifiers in C++

Identifiers are names given to variables, functions, arrays, classes, etc. They are essential in any programming language. Here are the rules for naming identifiers in C++:

• An identifier cannot start with a digit or special character.
• Keywords cannot be used as identifiers.
• Identifiers can only contain alphabetic characters, digits, and underscores.
• Uppercase and lowercase letters are distinct (e.g., A and a are different).

Examples of valid identifiers: GFG, gfg, Learning_of_Cpp.

Examples of Good and Bad Identifiers

Invalid Identifier	Bad Identifier	Good Identifier
Cash prize	C_prize	cashprize
catch	catch_1	catch1
1list	list_1	list1

Example Code

Here's an example of a simple C++ program using identifiers:

#include <iostream>
using namespace std;

int main() {
    int Learning_of_Cpp = 1;
    cout << "Identifier result is: " << Learning_of_Cpp;
    return 0;
}

Output:

Identifier result is: 1

How Keywords Differ from Identifiers

Keywords and identifiers have distinct properties:

Keywords	Identifiers
Predefined or reserved words	Names defined by the programmer
Always lowercase	Can start with uppercase or lowercase
Define the type of entity	Define the name of the entity
Only contain alphabetical characters	Can contain letters, digits, and underscores
Cannot be used as variable names	Can be both uppercase and lowercase

Example Code with Keywords

#include <iostream>
using namespace std;

int main() {
    int n = 2;

    switch (n) {
        case 1:
            cout << "Computer Network" << endl;
            break;
        case 2:
            cout << "C++" << endl;
            break;
        case 3:
            cout << "DBMS" << endl;
            break;
        case 4:
            cout << "Data Structure" << endl;
            break;
        case 5:
            cout << "Operating System" << endl;
            break;
        default:
            cout << "Enter Valid number" << endl;
    }

    return 0;
}

Output:

C++

Tips for Using Keywords and Identifiers

1. Avoid using reserved keywords as variable or function names.
2. Use meaningful identifiers to make your code easier to read and understand.
3. Follow naming conventions like using camelCase or underscores to separate words in identifiers.
4. Keep identifiers short but descriptive to balance readability and conciseness.

By understanding and properly using keywords and identifiers, you can write clear and efficient C++ programs.