Structure of a program
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 6 7 |
// 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++
Two slash signs indicate that the rest of the line is a
comment inserted by the programmer but which has 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 of the code.
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 whenmain 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
1
2 3 4 |
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:
1
2 3 4 5 6 7 8 |
// 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:
1
2 3 4 5 6 7 |
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 (;).
Comments
As noted above, comments do not affect the operation of the
program; however, they provide an important tool to document directly within
the source code what the program does and how it operates.
C++ supports two ways of commenting code:
1
2 |
// line comment
/* block comment */
|
The first of them, known as line comment, discards everything from
where the pair of slash signs (//) are 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 multiple lines.
Let's add comments to our second program:
1
2 3 4 5 6 7 8 9 10 |
/* my second program in C++
with more comments */
#include <iostream>
int main ()
{
std::cout << "Hello World!
"; // prints Hello World!
std::cout << "I'm a C++
program"; // prints I'm a C++
program
}
|
Hello World! I'm a C++ program
|
If comments are included within the source code of a program without using the
comment characters combinations //,/* or */,
the compiler takes them as if they were C++ expressions, most likely causing
the compilation to fail with one, or several, error messages.
Using
namespace std
If you have seen C++ code before, you may have seen cout being
used instead of std::cout. Both name the same object: the first one uses its unqualified
name (cout), while the second qualifies it directly within the namespace std (asstd::cout).
cout is part of the standard library, and all the elements
in the standard C++ library are declared within what is a called a namespace:
the namespace std.
In order to refer to the elements in the std namespace a program shall
either qualify each and every use of elements of the library (as we have done
by prefixing cout with std::), or introduce visibility of its
components. The most typical way to introduce visibility of these components is
by means of using declarations:
using namespace std;
|
The above declaration allows all elements in the std namespace
to be accessed in an unqualified manner (without thestd:: prefix).
With this in mind, the last example can be rewritten to make unqualified uses
of cout as:
1
2 3 4 5 6 7 8 9 |
// my second program in C++
#include <iostream>
using namespace std;
int main ()
{
cout << "Hello World!
";
cout << "I'm a C++
program";
}
|
Hello World! I'm a C++ program
|
Both ways of accessing the elements of the std namespace (explicit qualification and using declarations)
are valid in C++ and produce the exact same behavior. For simplicity, and to
improve readability, the examples in these tutorials will more often use this
latter approach with using declarations, although note
that explicit qualification is the only way to guarantee that
name collisions never happen.
Namespaces are explained in more detail in a later chapter.
Comments
Post a Comment