Author: saqibkhan

C++ Comments

Program comments are explanatory statements that you can include in the C++ code. These comments help anyone reading the source code. All programming languages allow for some form of comments.

Types of C++ Comments

C++ supports two types of comments: single-line comments and multi-line comments. All characters available inside any comment are ignored by the C++ compiler.

The types of C++ comments are explained in detail in the next sections:

1. C++ Single-line Comments

A single-line comment starts with //, extending to the end of the line. These comments can last only till the end of the line, and the next line leads to a new comment.

Syntax

The following syntax shows how to use a single-line comment in C++:

// Text to be commented

Example

In the following example, we are creating single-line comments −

Open Compiler

#include <iostream>usingnamespace std;intmain(){// this is a single line comment
  cout <<"Hello world!"<< endl;// for a new line, we have to use new comment sections
  cout <<"This is second line.";return0;}

Output

Hello world!
This is second line.

2. C++ Multi-line Comments

Multi-line comments start with /* and end with */. Any text in between these symbols is treated as a comment only.

Syntax

The following syntax shows how to use a multi-line comment in C++:

/* This is a comment *//* 
  C++ comments can also
  span multiple lines
*/

Example

In the following example, we are creating multi-line comments −

Open Compiler

#include <iostream>usingnamespace std;intmain(){/* Printing hello world!*/
  cout <<"Hello World!"<< endl;/*
  This is a multi-line comment
  Printing another message
  Using cout
  */
  cout <<"Tutorials Point";return0;}

Output

Hello World!
Tutorials Point

Comments within Statements

We can also comment-out specific statements within a code block inside a C++ program. This is done using both types of comments.

Example

The following example explains the usage of multi-line comments within statements −

Open Compiler

#include <iostream>usingnamespace std;intmain(){
  cout <<"This line"/*what is this*/<<" contains a comment"<< endl;return0;}

Output

This line contains a comment

Example

The following example explains the usage of single-line comments within statements −

Open Compiler

#include <iostream>usingnamespace std;intmain(){
  cout <<"This line"// what is this<<" contains a comment"<< endl;return0;}

Output

This line contains a comment

Nesting Comments

Within a /* and */ comment, // characters have no special meaning. Within a // comment, /* and */ have no special meaning. Thus, you can “nest” one kind of comment within the other kind.

Example

The following example explains the usage of comments within comments using nesting −

Open Compiler

#include <iostream>usingnamespace std;intmain(){/* Comment out printing of Hello World:

cout << "Hello World"; // prints Hello World

*/
  cout <<"New, Hello World!";return0;}

Output

New, Hello World!

Single-line or Multi-line Comments – When to Use?

Single-line comments are generally used for short lines of comments in general. This is seen in cases where we have to mention a small hint for the algorithm in the code.

Multi-line comments are generally used for longer lines of comments, where the visibility of the whole comment line is necessary. The longer the length of the comment, the more number of statements are needed by the multi-line comments.

Purpose of Comments

Comments are used for various purposes in C++. Some of the main areas of application of comments are given as follows:

• To represent a short and concise step in the program for users to understand better.
• To explain a step in a detailed way that is not expressed explicitly in the code.
• To leave different hints for users to grab in the code itself.
• To leave comments for fun or recreation.
• To temporarily disable part of the code for debugging purposes.
• To add metadata to the code for future purposes.
• To create documentation for the code, for example, in Github pages.

When we consider a C++ program, it can be defined as a collection of objects that communicate via invoking each other’s methods. Let us now briefly look into what a class, object, methods, and instant variables mean.

• Object − Objects have states and behaviors. Example: A dog has states – color, name, breed as well as behaviors – wagging, barking, eating. An object is an instance of a class.
• Class − A class can be defined as a template/blueprint that describes the behaviors/states that object of its type support.
• Methods − A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
• Instance Variables − Each object has its unique set of instance variables. An object’s state is created by the values assigned to these instance variables.

C++ Program Structure

The basic structure of a C++ program consists of the following parts:

• Header file inclusion section: This is the section where we include all required header files whose functions we are going to use in the program.
• Namespace section: This is the section where we use the namespace.
• The main() section: In this section, we write our main code. The main() function is an entry point of any C++ programming code from where the program’s execution starts.

To learn more about it, read: C++ Hello, World Program.

Example

Let us look at a simple code that would print the words Hello World.

Open Compiler

#include <iostream>usingnamespace std;// main() is where program execution begins.intmain(){
   cout <<"Hello World";// prints Hello Worldreturn0;}

Example Explanation

Let us look at the various parts of the above program −

• The C++ language defines several headers, which contain information that is either necessary or useful to your program. For this program, the header <iostream> is needed.
• The line using namespace std; tells the compiler to use the std namespace. Namespaces are a relatively recent addition to C++.
• The next line ‘// main() is where program execution begins.‘ is a single-line comment available in C++. Single-line comments begin with // and stop at the end of the line.
• The line int main() is the main function where program execution begins.
• The next line cout << “Hello World”; causes the message “Hello World” to be displayed on the screen.
• The next line return 0; terminates main() function and causes it to return the value 0 to the calling process.

Compile and Execute C++ Program

Let’s look at how to save the file, compile and run the program. Please follow the steps given below −

• Open a text editor and add the code as above.
• Save the file as: hello.cpp
• Open a command prompt and go to the directory where you saved the file.
• Type ‘g++ hello.cpp’ and press enter to compile your code. If there are no errors in your code the command prompt will take you to the next line and would generate a.out executable file.
• Now, type ‘a.out’ to run your program.
• You will be able to see ‘ Hello World ‘ printed on the window.

$ g++ hello.cpp
$ ./a.out
Hello World

Make sure that g++ is in your path and that you are running it in the directory containing file hello.cpp.

You can compile C/C++ programs using makefile. For more details, you can check our ‘Makefile Tutorial’.

Semicolons and Blocks in C++

In C++, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity.

For example, following are three different statements −

x = y;
y = y +1;add(x, y);

A block is a set of logically connected statements that are surrounded by opening and closing braces. For example −

{
   cout <<"Hello World";// prints Hello Worldreturn0;}

C++ does not recognize the end of the line as a terminator. For this reason, it does not matter where you put a statement in a line. For example −

x = y;
y = y +1;add(x, y);

is the same as

x = y; y = y +1;add(x, y);

C++ Identifiers

A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores, and digits (0 to 9).

C++ does not allow punctuation characters such as @, $, and % within identifiers. C++ is a case-sensitive programming language. Thus, Manpower and manpower are two different identifiers in C++.

Here are some examples of acceptable identifiers −

mohd       zara    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal

C++ Keywords

The following list shows the reserved words in C++. These reserved words may not be used as constant or variable or any other identifier names.

asm	else	new	this
auto	enum	operator	throw
bool	explicit	private	true
break	export	protected	try
case	extern	public	typedef
catch	false	register	typeid
char	float	reinterpret_cast	typename
class	for	return	union
const	friend	short	unsigned
const_cast	goto	signed	using
continue	if	sizeof	virtual
default	inline	static	void
delete	int	static_cast	volatile
do	long	struct	wchar_t
double	mutable	switch	while
dynamic_cast	namespace	template

Trigraphs

A few characters have an alternative representation, called a trigraph sequence. A trigraph is a three-character sequence that represents a single character and the sequence always starts with two question marks.

Trigraphs are expanded anywhere they appear, including within string literals and character literals, in comments, and in preprocessor directives.

Following are most frequently used trigraph sequences −

Trigraph	Replacement
??=	#
??/	\
??’	^
??(	[
??)	]
??!	|
??<	{
??>	}
??-	~

All the compilers do not support trigraphs and they are not advised to be used because of their confusing nature.

Whitespace in C++

A line containing only whitespace, possibly with a comment, is known as a blank line, and C++ compiler totally ignores it.

Whitespace is the term used in C++ to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins.

Statement 1

int age;

In the above statement there must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them.

Statement 2

fruit = apples + oranges;// Get the total fruit

In the above statement 2, no whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish for readability purpose.

C++ Program Structure with Object-oriented Approach

C++ also supports the object-oriented programming approach along with the procedural programming approach.

Example

This example demonstrates the C++ program based on an object-oriented approach.

Open Compiler

#include <iostream>usingnamespace std;classNumbers{private:int a;int b;public:// Function to set valuesvoidsetValues(int x,int y){
a = x;
b = y;}// Function to add these numbersdoubleaddition(){return a + b;}// Function to display valuesvoiddisplay(){ cout &lt;&lt;"a: "&lt;&lt; a &lt;&lt;", b: "&lt;&lt; b &lt;&lt; endl;}};intmain(){// Create an object of Numbers class
  Numbers num;// Set values
  num.setValues(10,20);// Display the values
  num.display();// Find the additionint sum = num.addition();
  cout <<"Sum of numbers: "<< sum << endl;return0;}

Parts of C++ Program Structure with Object-oriented Approach

The different parts of the C++ program structure with an object-oriented approach are as follows:

1. Class Declaration

A class is a template for an object, or we can say a class is a factory to produce an object. It is a kind of custom data type, where you construct a structure for an object.

A class declaration has the following parts:

• Access modifiers: C++ supports three types of access modifiers: private, public, and protected. Accessibilities of the data members and member functions are defined by the access modifiers.
• Data members and member functions: The variables used in the class declaration are known as data members, and the member functions are those functions that work on the data members.

Example

As per the above example, the following part of the declaration of a class –

classNumbers{private:int a;int b;public:// Function to set valuesvoidsetValues(int x,int y){
a = x;
b = y;}// Function to add these numbersdoubleaddition(){return a + b;}// Function to display valuesvoiddisplay(){ cout &lt;&lt;"a: "&lt;&lt; a &lt;&lt;", b: "&lt;&lt; b &lt;&lt; endl;}};</code></pre>


The following are the data members which are defined under the private access modifier i.e., these data members can be used by the member functions within the class –



private:int a;int b;



The following are the member functions used in the class –



voidsetValues(int x,int y);doubleaddition();voiddisplay();



2. Object Creation



In the above example, the following statement is the object creation statement −



Numbers num;