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Structure of a program

Structure of a program

C++ uses notations that may appear strange to nonprogrammers. We now consider a simple program that prints a line of text. This program illustrates several important features of the C++ language. We consider each line in detail.

 1  // Fig. 2.1: fig02_01.cpp
 2  // Text-printing program.
 3  #include <iostream> // allows program to output data to the screen
 4
 5  // function main begins program execution
 6  int main()
 7  {
 8     std::cout << "Welcome to C++!\n"; // display message
 9
10     return 0; // indicate that program ended successfully
11
12  } // end function main
 Welcome to C++!

Lines 1 and 2

Section 2.2.  First Program in C++: Printing a Line of Text

// Fig. 2.1: fig02_01.cpp
// Text-printing program.

each begin with //, indicating that the remainder of each line is a comment. Programmers insert comments to document programs and also help people read and understand them. Comments do not cause the computer to perform any action when the program is runthey are ignored by the C++ compiler and do not cause any machine-language object code to be generated. The comment Text-printing program describes the purpose of the program. A comment beginning with // is called a single-line comment because it terminates at the end of the current line. [Note: C++ programmers also may use C’s style in which a commentpossibly containing many linesbegins with the pair of characters /* and ends with */.]

The best way to learn a programming language is by writing programs. Typically, the first program beginners write is a program called “Hello World”, which simply prints “Hello World” to your computer screen. Although it is very simple, it contains all the fundamental components C++ programs have:

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// my first program in C++
#include <iostream>

int main()
{
  std::cout << "Hello World!";
}
Hello World!

The left panel above shows the C++ code for this program. The right panel shows the result when the program is executed by a computer. The grey numbers to the left of the panels are line numbers to make discussing programs and researching errors easier. They are not part of the program.

Let’s examine this program line by line:

Line 1: // my first program in C++
Lines beginning with two slash signs (//) are comments by the programmer and have no effect on the behavior of the program. Programmers use them to include short explanations or observations concerning the code or program. In this case, it is a brief introductory description of the program.
Line 2: #include <iostream>
Lines beginning with a hash sign (#) are directives read and interpreted by what is known as the preprocessor. They are special lines interpreted before the compilation of the program itself begins. In this case, the directive #include <iostream>, instructs the preprocessor to include a section of standard C++ code, known as header iostream, that allows to perform standard input and output operations, such as writing the output of this program (Hello World) to the screen.
Line 3: A blank line.
Blank lines have no effect on a program. They simply improve readability.
Line 4: int main ()
This line initiates the declaration of a function. Essentially, a function is a group of code statements which are given a name: in this case, this gives the name “main” to the group of code statements that follow. Functions will be discussed in detail in a later chapter, but essentially, their definition is introduced with a succession of a type (int), a name (main) and a pair of parentheses (()), optionally including parameters.The function named main is a special function in all C++ programs; it is the function called when the program is run. The execution of all C++ programs begins with the main function regardless of where the function is actually located within the code.
Lines 5 and 7: { and }
The open brace ({) at line 5 indicates the beginning of main‘s function definition, and the closing brace (}) at line 7, indicates its end. Everything between these braces is the function’s body that defines what happens when main is called. All functions use braces to indicate the beginning and end of their definitions.
Line 6: std::cout << "Hello World!";
This line is a C++ statement. A statement is an expression that can actually produce some effect. It is the meat of a program, specifying its actual behavior. Statements are executed in the same order that they appear within a function’s body.This statement has three parts: First, std::cout, which identifies the standard character output device (usually, this is the computer screen). Second, the insertion operator (<<), which indicates that what follows is inserted into std::cout. Finally, a sentence within quotes (“Hello world!”), is the content inserted into the standard output.Notice that the statement ends with a semicolon (;). This character marks the end of the statement, just as the period ends a sentence in English. All C++ statements must end with a semicolon character. One of the most common syntax errors in C++ is forgetting to end a statement with a semicolon.

You may have noticed that not all the lines of this program perform actions when the code is executed. There is a line containing a comment (beginning with //). There is a line with a directive for the preprocessor (beginning with #). There is a line that defines a function (in this case, the main function). And, finally, a line with a statements ending with a semicolon (the insertion into cout), which was within the block delimited by the braces ( { } ) of the main function.

The program has been structured in different lines and properly indented, in order to make it easier to understand for the humans reading it. But C++ does not have strict rules on indentation or on how to split instructions in different lines. For example, instead of

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int main ()
{
  std::cout << " Hello World!";
}

We could have written:

 
int main () { std::cout << "Hello World!"; }

all in a single line, and this would have had exactly the same meaning as the preceding code.

In C++, the separation between statements is specified with an ending semicolon (;), with the separation into different lines not mattering at all for this purpose. Many statements can be written in a single line, or each statement can be in its own line. The division of code in different lines serves only to make it more legible and schematic for the humans that may read it, but has no effect on the actual behavior of the program.

Now, let’s add an additional statement to our first program:

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// my second program in C++
#include <iostream>

int main ()
{
  std::cout << "Hello World! ";
  std::cout << "I'm a C++ program";
}
Hello World! I'm a C++ program

In this case, the program performed two insertions into std::cout in two different statements. Once again, the separation in different lines of code simply gives greater readability to the program, since main could have been perfectly valid defined in this way:

 
int main () { std::cout << " Hello World! "; std::cout << " I'm a C++ program "; }

The source code could have also been divided into more code lines instead:

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int main ()
{
  std::cout <<
    "Hello World!";
  std::cout
    << "I'm a C++ program";
}

And the result would again have been exactly the same as in the previous examples.

Preprocessor directives (those that begin by #) are out of this general rule since they are not statements. They are lines read and processed by the preprocessor before proper compilation begins. Preprocessor directives must be specified in their own line and, because they are not statements, do not have to end with a semicolon (;).

The best way to learn a programming language is by writing programs. Typically, the first program beginners write is a program called “Hello World”, which simply prints “Hello World” to your computer screen. Although it is very simple, it contains all the fundamental components C++ programs have:

1
2
3
4
5
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7
// my first program in C++
#include <iostream>

int main()
{
  std::cout << "Hello World!";
}

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