C++ Keywords

In this article, we will discuss keywords in C++ with their several features and functions.

What are Keywords in C++?

In C++, a keyword is a reserved word that has a predefined meaning in the language and is not suitable for use as an identifier (such as class names, variable names, or function names). The syntax and organization of C++ programs depend heavily on keywords because they define control statements, data types, and other language features.

Keywords are explicitly reserved identifier of C++ language features. It cannot be used as user-define program elements or other name for the program variables.

Reserved Identifiers:

In C++, keywords are reserved, defined concepts with certain meanings. The compiler recognizes them and utilizes them to interpret the structure and behavior of a program. Because they are reserved, they cannot be used to name classes, variables, functions, or other user-defined elements.

For example, int, while, return, and class are all reserved keywords in C++.

C++ Language Features:

Every keyword in C++ stands for a fundamental aspect of the language. Here are several instances:

1. Data types: int, char, float, double, bool.
2. Control structures: if, else, switch, while, for, do.
3. Memory management: new, delete.
4. Object-oriented programming: class, public, private, protected.
5. Exception handling: try, catch, throw.

Keywords cannot be used as identifiers:

Reserved terms cannot be used as variable, function, or class names. A compilation error will appear if we attempt to do this.

Incorrect Example Syntax:

int class = 10;  // Error: 'class' is a reserved keyword

Correct Usage:

int myClass = 10;  // Allowed, since 'myClass' is not a keyword         

C++ programming language has several reserved keywords used in the program. ANSI C++ also uses the keywords available in ANSI C programming language.

ANSI C:

Numerous C compilers had their own versions of the language prior to its standardization. When programs were transferred between systems, this resulted in portability problems. A standard version of C, ANSI C (also referred to as C89 or C90), was developed by ANSI in 1989 to solve this problem.

ANSI C is the standard version of the C programming language developed by the American National Standards Institute. This kind of C was developed to address issues that arose from various C programs that were written and compiled across a wide variety of systems.

Versions of ANSI C:

ANSI C (C89/C90) was the first standardized version of C (1989/1990).

• C99: In C99, new features including inline functions, flexible array members, and new data types (long long, _Bool) were added.
• C11: Multi-threading capabilities, improved Unicode handling, and safer standard library operations were introduced.
• C17: A small update with a few bug fixes and small improvements.
• C23: The latest iteration, with enhanced usability and safety.

Key Features of ANSI C (C89):

Several key features of ANSI C are as follows:

• Standard Data Types: int, char, float, double, void.
• Control Statements: if, else, switch, for, while, do.
• Functions & Libraries: printf(), scanf(), math.h, stdlib.h.
• Pointers & Memory Management: malloc(), free().
• File Handling: fopen(), fclose(), fread(), fwrite().

Example code for ANSI C (C89) Standard Code:

#include <stdio.h>


// Function prototype (ANSI C Standard)

void greet();

int main() {

    int num = 10;  // Standard integer declaration

    printf("Number: %d
", num);  // ANSI C standard function


    greet();  // Calling the function


    return 0;  // ANSI C requires return type in main()

}

// Function definition

void greet() {

    printf("Hello, this is an ANSI C program!
");

}

Output:

Number: 10

Hello, this is an ANSI C program!

Explanation:

• #include <stdio.h>: Standard input/output header.
• printf(): Standard ANSI C function.
• Function prototype (void greet();): ANSI C style.
• return 0; in main(): Required in ANSI C.

Example Code for C++ Version:

#include <iostream>  // C++ I/O library

using namespace std;

// C++ class (Not available in ANSI C)

class Greeter {

public:

    void greet() {

        cout << "Hello, this is a C++ program!" << endl;

    }

};

int main() {

    int num = 10;  // Same as ANSI C

    cout << "Number: " << num << endl;  // C++ uses cout instead of printf

    Greeter obj;  // Object-oriented feature

    obj.greet();  // Calling class method

    return 0;

}

Output:

Number: 10

Hello, this is a C++ program!

Differences with the above program from ANSI C:

• Uses <iostream> instead of <stdio.h>.
• Uses cout instead of printf().
• Introduces OOP (class Greeter), which ANSI C does not support.
• Uses public access modifier inside a class.

ANSI C programming language keywords:

Auto	Double	Int	struct
break	Else	Long	switch
Case	Enum	Register	typedef
Char	Extern	Return	union
const	Float	Short	Unsigned
continue	For	Signed	Void
default	Goto	Sizeof	Volatile
Do	If	Static	While

Keywords added by ANSI C++ programming language:

Asm	Private	bool	reinterpret_cast
catch	Protected	const_cast	static_cast
Class	Public	dynamic_cast	True
delete	Template	explicit	Typeid
friend	This	export	Typename
inline	Throw	false	Using
New	Try	mutable	wchar_t
operator	Virtual	namespace

Conclusion:

In conclusion, the C++ keywords are essential parts of the C++ language. Because these reserved words are already assigned meanings, they cannot be used as variable, function, or class names. C++ has all the keywords of ANSI C and additional ones like class, public, private, new, and delete, which are necessary for object-oriented programming, memory management, exception handling, and other features which forward and backward compatibility. It is because these keywords class, public, private, new, and delete, give the encapsulation and dynamic memory management, which is the hallmark of any powerful C++ software. In addition, these keywords make it easy to produce efficient and defect-free C++ applications because the keywords define how the compiler will interpret and implement the code.