C++ Basic Syntax

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When we consider a C program it can be defined as a collection of objects that communicate via invoking each others methods. Let us now briefly look into what do 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/ blue print that describe 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.

  • Instant Variables - Each object has its unique set of instant variables. An object's state is created by the values assigned to these instant variables.

C++ Program Structure:

Let us look at a simple code that would print the words Hai Everyone.

Example

#include <iostream>
using namespace std;
// main() is where program execution begins.
int main()
{
   cout << "Hai Everyone"; // prints Hai Everyone
   return 0;
}
                                        

Let us look various parts of the above program:

  1. 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.

  2. The line using namespace std; tells the compiler to use the std namespace. Namespaces are a relatively recent addition to C++.

  3. 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.

  4. The line int main() is the main function where program execution begins.

  5. The next line cout << "This is my first C++ program."; causes the message "This is my first C++ program" to be displayed on the screen.

  6. The next line return 0; terminates main( )function and causes it to return the value 0 to the calling process.

Compile & Execute C++ Program:

Lets look at how to save the file, compile and run the program. Please follow the steps given below:

  1. Open a text editor and add the code as above.

  2. Save the file as : NewFile.cpp

  3. Open a command prompt and go to the directory where you saved the file.

  4. Type 'g++ NewFile.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.

  5. Now type ' a.out' to run your program.

  6. You will be able to see ' Hai Everyone ' printed on the window.

Example

$ g++ NewFile.cpp
$ ./a.out
Hai Everyone
                                        

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

Semicolons & 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:

Example

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:

Example

{
   cout << "Hai Everyone"; // prints Hai Everyone
   return 0;
}
                                        

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

Example

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

is the same as

Example

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:

Example

mohd       Sophia    abc   move_name  a_123
myname50   _temp   j     a23b9      retVal
                                        

C++ Keywords:

The Below 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. Therefore, in the statement,

Example

int age;
                                        

There must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them. On the other hand, in the statement

Example

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

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.


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