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Anupam (Administrator)
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PostSubject: TUTORIAL 001   Tue Sep 25, 2012 11:33 am

Structure of a program


Probably the best way to start learning a programming language is by writing a program. Therefore, here is our
first program:
Quote :
// my first program in C++

#include <iostream>
using namespace std;

int main ()
{
cout << "Hello World!";
return 0;
}

Quote :
Hello World!


The first panel shows the source code for our first program. The second one shows the result of the program once
compiled and executed. The way to edit and compile a program depends on the compiler you are using. Depending
on whether it has a Development Interface or not and on its version. Consult the compilers section and the manual
or help included with your compiler if you have doubts on how to compile a C++ console program.
The previous program is the typical program that programmer apprentices write for the first time, and its result is
the printing on screen of the "Hello World!" sentence. It is one of the simplest programs that can be written in
C++, but it already contains the fundamental components that every C++ program has. We are going to look line
by line at the code we have just written:
// my first program in C++
This is a comment line. All lines beginning with two slash signs (//) are considered comments and do not
have any effect on the behavior of the program. The programmer can use them to include short
explanations or observations within the source code itself. In this case, the line is a brief description of
what our program is.
#include <iostream>
Lines beginning with a hash sign (#) are directives for the preprocessor. They are not regular code lines
with expressions but indications for the compiler's preprocessor. In this case the directive #include
<iostream> tells the preprocessor to include the iostream standard file. This specific file (iostream)
includes the declarations of the basic standard input-output library in C++, and it is included because its
functionality is going to be used later in the program.
using namespace std;
All the elements of the standard C++ library are declared within what is called a namespace, the
namespace with the name std. So in order to access its functionality we declare with this expression that
we will be using these entities. This line is very frequent in C++ programs that use the standard library,
and in fact it will be included in most of the source codes included in these tutorials.
int main ()
This line corresponds to the beginning of the definition of the main function. The main function is the point
by where all C++ programs start their execution, independently of its location within the source code. It
does not matter whether there are other functions with other names defined before or after it - the
instructions contained within this function's definition will always be the first ones to be executed in any
C++ program. For that same reason, it is essential that all C++ programs have a main function.
The word main is followed in the code by a pair of parentheses (()). That is because it is a function
declaration: In C++, what differentiates a function declaration from other types of expressions are these
parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within
them.
Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is
contained within these braces is what the function does when it is executed.
7




Quote :
cout << "Hello World!";


This line is a C++ statement. A statement is a simple or compound expression that can actually produce
some effect. In fact, this statement performs the only action that generates a visible effect in our first
program.

cout represents the standard output stream in C++, and the meaning of the entire statement is to insert
a sequence of characters (in this case the Hello World sequence of characters) into the standard output
stream (which usually is the screen).
cout is declared in the iostream standard file within the std namespace, so that's why we needed to
include that specific file and to declare that we were going to use this specific namespace earlier in our
code.

Notice that the statement ends with a semicolon character (Wink. This character is used to mark the end of
the statement and in fact it must be included at the end of all expression statements in all C++ programs
(one of the most common syntax errors is indeed to forget to include some semicolon after a statement).
return 0;
The return statement causes the main function to finish. return may be followed by a return code (in our
example is followed by the return code 0). A return code of 0 for the main function is generally interpreted
as the program worked as expected without any errors during its execution. This is the most usual way to
end a C++ console program.
You may have noticed that not all the lines of this program perform actions when the code is executed. There were
lines containing only comments (those beginning by //). There were lines with directives for the compiler's
preprocessor (those beginning by #). Then there were lines that began the declaration of a function (in this case,
the main function) and, finally lines with statements (like the insertion into cout), which were all included within
the block delimited by the braces ({}) of the main function.
The program has been structured in different lines in order to be more readable, but in C++, we do not have strict
rules on how to separate instructions in different lines. For example, instead of
Quote :
int main ()
{
cout << " Hello World!";
return 0;
}
We could have written:
int main () { cout << "Hello World!"; return 0; }


All in just one line and this would have had exactly the same meaning as the previous code.
In C++, the separation between statements is specified with an ending semicolon (Wink at the end of each one, so
the separation in different code lines does not matter at all for this purpose. We can write many statements per
line or write a single statement that takes many code lines. The division of code in different lines serves only to
make it more legible and schematic for the humans that may read it.
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