Probably the best way to start learning a programming language is by writing a program. If you wish to know all the rule at first, then coding will be boring. Therefore, here is our first program:
// my first program in C++
#include <iostream>
using namespace std;
int main ( )
{
cout << " Hello World! ";
return 0 ;
}
The output is:
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.
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.
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.
Comments :
Comments are parts of the source code disregarded y the compiler. They simply do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code.
C++ supports two ways to insert comments:
// line comment
/* block comment */
The first of them, known as line comment, discards everything from where the pair of slash signs (//) is found up to the end of that same line. The second one, known as block comment, discards everything between the /* characters and the first appearance of the */ characters, with the possibility of including more than one line.
We are going to add comments to our second program:
// my second program in C++ Hello World! I'm a C++ program
#include <iostream>
using namespace std;
int main ( )
{
cout << " Hello World !! dear Coder! "<<endl;
cout << "I'm a C++ program, I wish to learn ."<<endl;
return 0;
}
The output is:
Hello World !! dear Coder!
I'm a C++ program, I wish to learn .
In this case, we performed two insertions into cout in two different statements. Once again, the separation in
different lines of code has been done just to give greater readability to the program, since main could have been perfectly valid defined this way. Here endl is used to make a new line sequence.
