Structure of a C++ Program
The structure of a C++ program is as described below:-
1. At the top of the program header files are declared.
2. A C++ prgram starts executing from main() function. The body of the main function is included with surly braces.
3. Each of the statment of the C++ program must end with a semicilon. a semicilon is also known as statement terminator.
So my first program would probably be:-
// my first program in C++
#include <iostream.h>
int main ()
{
cout << "Hello World!";
return 0;
}
This is the source code to which a file name has to be assigned, for e.g. hello.cpp. Below is the output of the program once compiled and executed.
Output:
Hello World!
Explanation of the Source code :-
// my first program in C++
This is a comment line. All the lines beginning with two slash signs (//) are considered comments and do not have any effect on the behavior of the program. They can be used by the programmer to include short explanations or observations within the source itself.
The header file <iostream.h>
Sentences that begin with a pound sign (#) are directives for the preprocessor. They are not executable code lines but indications for the compiler. In this case the sentence #include <iostream.h> tells the compiler's preprocessor to include the iostream standard header file. This specific file includes the declarations of the basic standard input-output library in C++, and it is included because its functionality is used later in the program.
main ()
This line corresponds to the beginning of the main function declaration. The main function is the point where all C++ programs begin their execution. It is independent of whether it is at the beginning, at the end or in the middle of the code - its content is always the first to be executed when a program starts. In addition, for that same reason, it is essential that all C++ programs have a main function.
main is followed by a pair of parenthesis () because it is a function. In C++ all functions are followed by a pair of parenthesis () that, optionally, can include arguments within them. The content of the main function immediately follows its formal declaration and it is enclosed between curly brackets ({}), as in our example. cout << "Hello World";
This instruction does the most important thing in this program. cout is the standard output stream in C++ (usually the screen), and the full sentence inserts a sequence of characters (in this case "Hello World") into this output stream (the screen). cout is declared in the iostream.h header file, so in order to be able to use it that file must be included.
Notice that the sentence ends with a semicolon character (;). This character signifies the end of the instruction and must be included after every instruction in any C++ program (one of the most common errors of C++ programmers is indeed to forget to include a semicolon ; at the end of each instruction).
return 0;
The return instruction causes the main() function finish and return the code that the instruction is followed by, in this case 0. This it is most usual way to terminate a program that has not found any errors during its execution. As you will see in coming examples, all C++ programs end with a sentence similar to this.
CommentsComments are pieces of source code discarded from the code by the compiler. They 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, the line comment, discards everything from where the pair of slash signs (//) is found up to the end of that same line. The second one, the block comment, discards everything between the /* characters and the next appearance of the */ characters, with the possibility of including several lines.