A basic structure of a C++
program can be better understood with the help of a sample program code. Let’s
see the following example and corresponding illustration.
//First program in C++
#include<iostream.h>
int main( )
{
cout<<”Welcome to the
world of C++”;
return 0;
}
Comments:
In
the above program at first we have this line - // First program in C++
In
C++ it is a comment line. All line beginning with two consecutive front slashes
(//) are considered as comment lines and is ignored by the compiler during the
compilation of the program. C++ also supports old C style comment writing i.e.
using of /*…..*/ this is actually called as BLOCK COMMENT or MULTIPLE LINE COMMENT.
Thus in C++ we have two styles of writing comments
i.e. Single line comment by using (//) or Multiple line comment or Block
comment using (/* … */).
Preprocessor directive:
The
line beginning with a hash sign (#) is called preprocessor directive. Before
compilation of the program starts it tells the compiler to include the
specified header file(s) (iosrteam.h) with the associated source code.
#include tells the compiler to include the source file into
your program.
Header files / Standard library header files:
Header
files contain prototype, variable, constants declarations and function
definition for the associated library functions. In the above sample program
“iostream.h” is a header file. It contains necessary definition for basic
standard input/output operations.
The header file iostream.h should be included at the beginning of all programs that
uses standard input/output statements (cin
/ cout). In the context of naming conventions of these header files you may
notice some variations. Some implementation may use iostream.h or iostream.hpp
some can use iostream.hxx. We should
appropriate header files depending on the contents of the program and
implementation.
Here is a list of very commonly used
C++ standard library header files.
|
Header
file
|
Purpose
|
|
<assert.h>
|
Contains
macros and information for adding diagnostics that aid the program debugging.
|
|
<ctyp.h>
|
Contains
function prototypes for functions that test characters for certain properties
and function prototypes for functions that can convert lowercase to uppercase
letters and vice versa.
Example:
toupper(), tolower, isalpha(), isalnum(), isdigit()
|
|
<float.h>
|
Contains
the floating – point size limits of the system.
|
|
<limits.h>
|
Contains
the integral size limits of the system.
|
|
<math.h>
|
Contains
function prototypes for mathematical functions.
|
|
<stdio.h>
|
Contains
function prototypes for standard input/output library functions. Example –
gets(), puts()
|
|
<stdlib.h>
|
Contains
function prototypes for conversion of numbers to text, text to numbers, memory
allocation, random numbers and various other functions.
|
|
<string.h>
|
Contains
function prototypes for string processing function.
|
|
<time.h>
|
Contains
function prototypes and types for manipulation of time and date.
|
|
<iostream.h>
|
Contains
function, prototypes for the standard input and standard output functions
|
|
<iomanip.h>
|
Contains
function, prototypes for the data stream manipulation that enable formatting
of streams of data.
|
|
<fstream.h>
|
Contains
function, prototypes for function that perform input from disk file and
output to disk file.
|
The main( ) function:
In
C++ the main( ) function is truly a main function by nature. A C/C++ program
must have a main( ) function. This is the function which executes first and all
other library or user-defined functions are called from main( ) function. This
is the point from where all C / C++ program start their execution. When the
main( ) function terminates the program terminates. The body of the main( ) is
enclosed in pair of curly braces { }. The braces contain the C / C++ statements
that are compiled and executed by C / C++ compiler. In C++ the function main( ) can have
following forms:
- <return-type> main( ) //Form 1
Example: int main( )
- main( ) //Form 2
In
Form 1 the type of data to be returned by function main( ) is mentioned (int)
In
Form 2 the type of data to be returned by function is not explicitly declared.
However in C++ by default every function returns a data of type int by default.
EMore on C++ data types and how they are associated
with function return types will be discussed in the subsequent chapters.
cout
In C++ identifier cout represents the standard output
stream. It inserts a sequence of characters into the standard output stream
(usually screen). It is declared in iostream.h standard header file.
Action of out is
reversed by cin; represents the
standard input stream. It reads data from keyboard and assigns to variable.
Use of Semicolon(;):
In C / C++ a semicolon (;)
marks the end of the statement. It must be included at the end of each C/ C++
statement.
‘return' – statement:
The return statement causes the main (practically any
C/C++ function) function end. Usually a return
statement
is followed by return code. Here in this program it is 0 (Zero). A return code
0 means that the program worked as expected without any error during the
execution. This is the most usual way to
end a C++ program. However if return type of main( ) function is declared as
void. It is not necessary to write a return
statement with a return code or simple return
can be written without a return code.
Example 1:
//First program in C++
#include<iostream.h>
void main( )
{
cout<<”Welcome to the
world of C++”;
return; //return statement
without any return code
}
Example 2:
//First program in C++
#include<iostream.h>
void main( )
{
cout<<”Welcome to the
world of C++”;
// here no return statement
is present
}
