Full C++ Tutorial

Instructions for use Published by No-Danger


Last update on Mar 11, 2007 at 5:04pm








To whom is this tutorial directed?


This tutorial is for those people who want to learn programming in C++
and do not necessarily have any previous knowledge of other programming
languages. Of course any knowledge of other programming languages or
any general computer skill can be useful to better understand this
tutorial, although it is not essential.


It is also suitable for those who need a little update on the new features the language has acquired from the latests standards.




If you are familiar with the C language, you can take the first 3
parts of this tutorial as a review of concepts, since they mainly
explain the C part of C++. There are slight differences in the C++
syntax for some C features, so I recommend you its reading anyway.





The 4th part describes object-oriented programming.





The 5th part mostly describes the new features introduced by ANSI-C++ standard.








Structure of this tutorial

The tutorial is divided in 6 parts and each part is divided on its
turn into different sections covering a topic each one. You can access
any section directly from the section index available on the left side
bar, or begin the tutorial from any point and follow the links at the
bottom of each section.

Many sections include examples that describe the use of the newly
acquired knowledge in the chapter. It is recommended to read these
examples and to be able to understand each of the code lines that
constitute it before passing to the next chapter.




A good way to gain experience with a programming language is by
modifying and adding new functionalities on your own to the example
programs that you fully understand. Don't be scared to modify the
examples provided with this tutorial, that's the way to learn!








Compatibility Notes

The ANSI-C++ standard acceptation as an international standard is
relatively recent. It was first published in November 1997, and revised
in 2003. Nevertheless, the C++ language exists from a long time before
(1980s). Therefore there are many compilers which do not support all
the new capabilities included in ANSI-C++, specially those released
prior to the publication of the standard.

This tutorial is thought to be followed with modern compilers that
suport -at least on some degree- ANSI-C++ specifications. I encourage
you to get one if yours is not adapted. There are many options, both
commercial and free.








Compilers

The examples included in this tutorial are all console programs.
That means they use text to communicate with the user and to show their
results.

All C++ compilers support the compilation of console programs.
Check the user's manual of your compiler for more info on how to
compile them.

Structure of a program Published by No-Danger


Last update on Mar 1, 2007 at 9:53am





Probably the best way to start learning a programming language is by writing a program. Therefore, here is our first program:


// my first program in C++





#include <iostream>


using namespace std;





int main ()


{


cout << "Hello World!";


return 0;


}


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 pound 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.








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 (.
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++ 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




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 (
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.





Let us add an additional instruction to our first program:





// my second program in C++





#include <iostream>





using namespace std;





int main ()


{


cout << "Hello World! ";


cout << "I'm a C++ program";


return 0;


}


Hello World! I'm a C++ program










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:





int main () { cout << " Hello World! "; cout << " I'm a C++ program "; return 0; }











We were also free to divide the code into more lines if we considered it more convenient:





int main ()


{


cout <<


"Hello World!";


cout


<< "I'm a C++ program";


return 0;


}











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
discarded by the preprocessor and do not produce any code by
themselves. Preprocessor directives must be specified in their own line
and do not have to end with a semicolon (.








Comments


Comments are parts of the source code disregarded by 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++


with more comments */





#include <iostream>


using namespace std;





int main ()


{


cout << "Hello World! "; // prints Hello World!


cout << "I'm a C++ program"; // prints I'm a C++ program


return 0;


}


Hello World! I'm a C++ program










If you include comments within the source code of your programs
without using the comment characters combinations //, /* or */, the
compiler will take them as if they were C++ expressions, most likely
causing one or several error messages when you compile it.

Variables. Data Types. Published by No-Danger


Last update on Apr 17, 2007 at 4:14am





The usefulness of the "Hello World" programs shown in the previous
section is quite questionable. We had to write several lines of code,
compile them, and then execute the resulting program just to obtain a
simple sentence written on the screen as result. It certainly would
have been much faster to type the output sentence by ourselves.
However, programming is not limited only to printing simple texts on
the screen. In order to go a little further on and to become able to
write programs that perform useful tasks that really save us work we
need to introduce the concept of variable.

Let us think that I ask you to retain the number 5 in your mental
memory, and then I ask you to memorize also the number 2 at the same
time. You have just stored two different values in your memory. Now, if
I ask you to add 1 to the first number I said, you should be retaining
the numbers 6 (that is 5+1) and 2 in your memory. Values that we could
now for example subtract and obtain 4 as result.




The whole process that you have just done with your mental memory
is a simile of what a computer can do with two variables. The same
process can be expressed in C++ with the following instruction set:




a = 5;


b = 2;


a = a + 1;


result = a - b;











Obviously, this is a very simple example since we have only used two
small integer values, but consider that your computer can store
millions of numbers like these at the same time and conduct
sophisticated mathematical operations with them.





Therefore, we can define a variable as a portion of memory to store a determined value.




Each variable needs an identifier that distinguishes it from the
others, for example, in the previous code the variable identifiers were
a, b and result, but we could have called the variables any names we
wanted to invent, as long as they were valid identifiers.








Identifiers

A valid identifier is a sequence of one or more letters, digits or
underscore characters (_). Neither spaces nor punctuation marks or
symbols can be part of an identifier. Only letters, digits and single
underscore characters are valid. In addition, variable identifiers
always have to begin with a letter. They can also begin with an
underline character (_ ), but in some cases these may be reserved for
compiler specific keywords or external identifiers, as well as
identifiers containing two successive underscore characters. In no case
they can begin with a digit.

Another rule that you have to consider when inventing your own
identifiers is that they cannot match any keyword of the C++ language
or your compiler's specific ones since they could be confused with
these. The standard reserved keywords are:




asm, auto, bool, break, case, catch, char, class, const,
const_cast, continue, default, delete, do, double, dynamic_cast, else,
enum, explicit, export, extern, false, float, for, friend, goto, if,
inline, int, long, mutable, namespace, new, operator, private,
protected, public, register, reinterpret_cast, return, short, signed,
sizeof, static, static_cast, struct, switch, template, this, throw,
true, try, typedef, typeid, typename, union, unsigned, using, virtual,
void, volatile, wchar_t, while







Additionally, alternative representations for some operators cannot be
used as identifiers since they are reserved words under some
circumstances:




and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq








Your compiler may also include some additional specific reserved keywords.





Very important: The C++ language is a "case sensitive" language. That
means that an identifier written in capital letters is not equivalent
to another one with the same name but written in small letters. Thus,
for example, the RESULT variable is not the same as the result variable
or the Result variable. These are three different variable identifiers.








Fundamental data types

When programming, we store the variables in our computer's memory,
but the computer has to know what we want to store in them, since it is
not going to occupy the same amount of memory to store a simple number
than to store a single letter or a large number, and they are not going
to be interpreted the same way.

The memory in our computers is organized in bytes. A byte is the
minimum amount of memory that we can manage in C++. A byte can store a
relatively small amount of data: one single character or a small
integer (generally an integer between 0 and 255). In addition, the
computer can manipulate more complex data types that come from grouping
several bytes, such as long numbers or non-integer numbers.




Next you have a summary of the basic fundamental data types in C++,
as well as the range of values that can be represented with each one:





Name Description Size* Range*


char Character or small integer. 1byte signed: -128 to 127


unsigned: 0 to 255


short int (short) Short Integer. 2bytes signed: -32768 to 32767


unsigned: 0 to 65535


int Integer. 4bytes signed: -2147483648 to 2147483647


unsigned: 0 to 4294967295


long int (long) Long integer. 4bytes signed: -2147483648 to 2147483647


unsigned: 0 to 4294967295


bool Boolean value. It can take one of two values: true or false. 1byte true or false


float Floating point number. 4bytes 3.4e +/- 38 (7 digits)


double Double precision floating point number. 8bytes 1.7e +/- 308 (15 digits)


long double Long double precision floating point number. 8bytes 1.7e +/- 308 (15 digits)


wchar_t Wide character. 2bytes 1 wide character







* The values of the columns Size and Range depend on the system the
program is compiled for. The values shown above are those found on most
32-bit systems. But for other systems, the general specification is
that int has the natural size suggested by the system architecture (one
word) and the four integer types char, short, int and long must each
one be at least as large as the one preceding it, with char being
always 1 byte long. The same applies to the floating point types float,
double and long double, where each one must provide at least as much
precision as the preceding one.








Declaration of variables

In order to use a variable in C++, we must first declare it
specifying which data type we want it to be. The syntax to declare a
new variable is to write the specifier of the desired data type (like
int, bool, float...) followed by a valid variable identifier. For
example:


int a;


float mynumber;










These are two valid declarations of variables. The first one
declares a variable of type int with the identifier a. The second one
declares a variable of type float with the identifier mynumber. Once
declared, the variables a and mynumber can be used within the rest of
their scope in the program.




If you are going to declare more than one variable of the same
type, you can declare all of them in a single statement by separating
their identifiers with commas. For example:





int a, b, c;











This declares three variables (a, b and c), all of them of type int, and has exactly the same meaning as:





int a;


int b;


int c;










The integer data types char, short, long and int can be either
signed or unsigned depending on the range of numbers needed to be
represented. Signed types can represent both positive and negative
values, whereas unsigned types can only represent positive values (and
zero). This can be specified by using either the specifier signed or
the specifier unsigned before the type name. For example:




unsigned short int NumberOfSisters;


signed int MyAccountBalance;











By default, if we do not specify either signed or unsigned most
compiler settings will assume the type to be signed, therefore instead
of the second declaration above we could have written:





int MyAccountBalance;











with exactly the same meaning (with or without the keyword signed)




An exception to this general rule is the char type, which exists by
itself and is considered a different fundamental data type from signed
char and unsigned char, thought to store characters. You should use
either signed or unsigned if you intend to store numerical values in a
char-sized variable.




short and long can be used alone as type specifiers. In this case,
they refer to their respective integer fundamental types: short is
equivalent to short int and long is equivalent to long int. The
following two variable declarations are equivalent:





short Year;


short int Year;










Finally, signed and unsigned may also be used as standalone type
specifiers, meaning the same as signed int and unsigned int
respectively. The following two declarations are equivalent:




unsigned NextYear;


unsigned int NextYear;











To see what variable declarations look like in action within a program,
we are going to see the C++ code of the example about your mental
memory proposed at the beginning of this section:





// operating with variables





#include <iostream>


using namespace std;





int main ()


{


// declaring variables:


int a, b;


int result;





// process:


a = 5;


b = 2;


a = a + 1;


result = a - b;





// print out the result:


cout << result;





// terminate the program:


return 0;


}


4










Do not worry if something else than the variable declarations
themselves looks a bit strange to you. You will see the rest in detail
in coming sections.







Scope of variables


All the variables that we intend to use in a program must have been
declared with its type specifier in an earlier point in the code, like
we did in the previous code at the beginning of the body of the
function main when we declared that a, b, and result were of type int.

A variable can be either of global or local scope. A global
variable is a variable declared in the main body of the source code,
outside all functions, while a local variable is one declared within
the body of a function or a block.











Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration.




The scope of local variables is limited to the block enclosed in
braces ({}) where they are declared. For example, if they are declared
at the beginning of the body of a function (like in function main)
their scope is between its declaration point and the end of that
function. In the example above, this means that if another function
existed in addition to main, the local variables declared in main could
not be accessed from the other function and vice versa.











Initialization of variables

When declaring a regular local variable, its value is by default
undetermined. But you may want a variable to store a concrete value at
the same moment that it is declared. In order to do that, you can
initialize the variable. There are two ways to do this in C++:

The first one, known as c-like, is done by appending an equal sign
followed by the value to which the variable will be initialized:





type identifier = initial_value ;




For example, if we want to declare an int variable called a
initialized with a value of 0 at the moment in which it is declared, we
could write:





int a = 0;










The other way to initialize variables, known as constructor
initialization, is done by enclosing the initial value between
parentheses (()):




type identifier (initial_value) ;





For example:





int a (0);











Both ways of initializing variables are valid and equivalent in C++.





// initialization of variables





#include <iostream>


using namespace std;





int main ()


{


int a=5; // initial value = 5


int b(2); // initial value = 2


int result; // initial value undetermined





a = a + 3;


result = a - b;


cout << result;





return 0;


}


6














Introduction to strings


Variables that can store non-numerical values that are longer than one single character are known as strings.


The C++ language library provides support for strings through the
standard string class. This is not a fundamental type, but it behaves
in a similar way as fundamental types do in its most basic usage.




A first difference with fundamental data types is that in order to
declare and use objects (variables) of this type we need to include an
additional header file in our source code: <string> and have
access to the std namespace (which we already had in all our previous
programs thanks to the using namespace statement).





// my first string


#include <iostream>


#include <string>


using namespace std;





int main ()


{


string mystring = "This is a string";


cout << mystring;


return 0;


}


This is a string










As you may see in the previous example, strings can be initialized
with any valid string literal just like numerical type variables can be
initialized to any valid numerical literal. Both initialization formats
are valid with strings:





string mystring = "This is a string";


string mystring ("This is a string");










Strings can also perform all the other basic operations that
fundamental data types can, like being declared without an initial
value and being assigned values during execution:





// my first string


#include <iostream>


#include <string>


using namespace std;





int main ()


{


string mystring;


mystring = "This is the initial string content";


cout << mystring << endl;


mystring = "This is a different string content";


cout << mystring << endl;


return 0;


}





This is the initial string content


This is a different string content

Constants Published by No-Danger


Last update on Apr 4, 2007 at 1:45pm





Constants are expressions with a fixed value.





Literals


Literals are used to express particular values within the source code
of a program. We have already used these previously to give concrete
values to variables or to express messages we wanted our programs to
print out, for example, when we wrote:


a = 5;











the 5 in this piece of code was a literal constant.





Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values.








Integer Numerals


1776


707


-273










They are numerical constants that identify integer decimal values.
Notice that to express a numerical constant we do not have to write
quotes (") nor any special character. There is no doubt that it is a
constant: whenever we write 1776 in a program, we will be referring to
the value 1776.




In addition to decimal numbers (those that all of us are used to
use every day) C++ allows the use as literal constants of octal numbers
(base
and hexadecimal numbers (base 16). If we want to express an octal
number we have to precede it with a 0 (zero character). And in order to
express a hexadecimal number we have to precede it with the characters
0x (zero, x). For example, the following literal constants are all
equivalent to each other:




75 // decimal


0113 // octal


0x4b // hexadecimal











All of these represent the same number: 75 (seventy-five) expressed as
a base-10 numeral, octal numeral and hexadecimal numeral, respectively.





Literal constants, like variables, are considered to have a specific
data type. By default, integer literals are of type int. However, we
can force them to either be unsigned by appending the u character to
it, or long by appending l:





75 // int


75u // unsigned int


75l // long


75ul // unsigned long











In both cases, the suffix can be specified using either upper or lowercase letters.








Floating Point Numbers

They express numbers with decimals and/or exponents. They can
include either a decimal point, an e character (that expresses "by ten
at the Xth height", where X is an integer value that follows the e
character), or both a decimal point and an e character:


3.14159 // 3.14159


6.02e23 // 6.02 x 1023


1.6e-19 // 1.6 x 10-19


3.0 // 3.0










These are four valid numbers with decimals expressed in C++. The
first number is PI, the second one is the number of Avogadro, the third
is the electric charge of an electron (an extremely small number) -all
of them approximated- and the last one is the number three expressed as
a floating-point numeric literal.




The default type for floating point literals is double. If you
explicitly want to express a float or long double numerical literal,
you can use the f or l suffixes respectively:





3.14159L // long double


6.02e23f // float










Any of the letters than can be part of a floating-point numerical
constant (e, f, l) can be written using either lower or uppercase
letters without any difference in their meanings.








Character and string literals


There also exist non-numerical constants, like:


'z'


'p'


"Hello world"


"How do you do?"










The first two expressions represent single character constants, and
the following two represent string literals composed of several
characters. Notice that to represent a single character we enclose it
between single quotes (') and to express a string (which generally
consists of more than one character) we enclose it between double
quotes (").




When writing both single character and string literals, it is necessary
to put the quotation marks surrounding them to distinguish them from
possible variable identifiers or reserved keywords. Notice the
difference between these two expressions:




x


'x'











x alone would refer to a variable whose identifier is x, whereas 'x'
(enclosed within single quotation marks) would refer to the character
constant 'x'.




Character and string literals have certain peculiarities, like the
escape codes. These are special characters that are difficult or
impossible to express otherwise in the source code of a program, like
newline (\n) or tab (\t). All of them are preceded by a backslash (\).
Here you have a list of some of such escape codes:




\n newline


\r carriage return


\t tab


\v vertical tab


\b backspace


\f form feed (page feed)


\a alert (beep)


\' single quote (')


\" double quote (")


\? question mark (?)


\\ backslash (\)








For example:





'\n'


'\t'


"Left \t Right"


"one\ntwo\nthree"











Additionally, you can express any character by its numerical ASCII code
by writing a backslash character (\) followed by the ASCII code
expressed as an octal (base-
or hexadecimal (base-16) number. In the first case (octal) the digits
must immediately follow the backslash (for example \23 or \40), in the
second case (hexadecimal), an x character must be written before the
digits themselves (for example \x20 or \x4A).




String literals can extend to more than a single line of code by
putting a backslash sign (\) at the end of each unfinished line.





"string expressed in \


two lines"










You can also concatenate several string constants separating them
by one or several blank spaces, tabulators, newline or any other valid
blank character:




"this forms" "a single" "string" "of characters"











Finally, if we want the string literal to be explicitly made of wide
characters (wchar_t), instead of narrow characters (char), we can
precede the constant with the L prefix:





L"This is a wide character string"











Wide characters are used mainly to represent non-English or exotic character sets.








Boolean literals

There are only two valid Boolean values: true and false. These can
be expressed in C++ as values of type bool by using the Boolean
literals true and false.





Defined constants (#define)

You can define your own names for constants that you use very often
without having to resort to memory-consuming variables, simply by using
the #define preprocessor directive. Its format is:


#define identifier value





For example:





#define PI 3.14159265


#define NEWLINE '\n'










This defines two new constants: PI and NEWLINE. Once they are
defined, you can use them in the rest of the code as if they were any
other regular constant, for example:




// defined constants: calculate circumference





#include <iostream>


using namespace std;





#define PI 3.14159


#define NEWLINE '\n'





int main ()


{


double r=5.0; // radius


double circle;





circle = 2 * PI * r;


cout << circle;


cout << NEWLINE;





return 0;


}


31.4159











In fact the only thing that the compiler preprocessor does when it
encounters #define directives is to literally replace any occurrence of
their identifier (in the previous example, these were PI and NEWLINE)
by the code to which they have been defined (3.14159265 and '\n'
respectively).




The #define directive is not a C++ statement but a directive for
the preprocessor; therefore it assumes the entire line as the directive
and does not require a semicolon ( at its end. If you append a semicolon character ( at the end, it will also be appended in all occurrences within the body of the program that the preprocessor replaces.








Declared constants (const)


With the const prefix you can declare constants with a specific type in the same way as you would do with a variable:


const int pathwidth = 100;


const char tabulator = '\t';











Here, pathwidth and tabulator are two typed constants. They are treated
just like regular variables except that their values cannot be modified
after their definition.

Operators Published by No-Danger


Last update on Feb 2, 2007 at 6:15am





Once we know of the existence of variables and constants, we can begin
to operate with them. For that purpose, C++ integrates operators.
Unlike other languages whose operators are mainly keywords, operators
in C++ are mostly made of signs that are not part of the alphabet but
are available in all keyboards. This makes C++ code shorter and more
international, since it relies less on English words, but requires a
little of learning effort in the beginning.

You do not have to memorize all the content of this page. Most
details are only provided to serve as a later reference in case you
need it.







Assignment (=)


The assignment operator assigns a value to a variable.


a = 5;











This statement assigns the integer value 5 to the variable a. The part
at the left of the assignment operator (=) is known as the lvalue (left
value) and the right one as the rvalue (right value). The lvalue has to
be a variable whereas the rvalue can be either a constant, a variable,
the result of an operation or any combination of these.

The most important rule when assigning is the right-to-left rule:
The assignment operation always takes place from right to left, and
never the other way:





a = b;










This statement assigns to variable a (the lvalue) the value
contained in variable b (the rvalue). The value that was stored until
this moment in a is not considered at all in this operation, and in
fact that value is lost.




Consider also that we are only assigning the value of b to a at the
moment of the assignment operation. Therefore a later change of b will
not affect the new value of a.




For example, let us have a look at the following code - I have
included the evolution of the content stored in the variables as
comments:





// assignment operator





#include <iostream>


using namespace std;





int main ()


{


int a, b; // a, b


a = 10; // a:10, b


b = 4; // a:10, b:4


a = b; // a:4, b:4


b = 7; // a:4, b:7





cout << "a:";


cout << a;


cout << " b:";


cout << b;





return 0;


}


a:4 b:7










This code will give us as result that the value contained in a is 4
and the one contained in b is 7. Notice how a was not affected by the
final modification of b, even though we declared a = b earlier (that is
because of the right-to-left rule).




A property that C++ has over other programming languages is that
the assignment operation can be used as the rvalue (or part of an
rvalue) for another assignment operation. For example:




a = 2 + (b = 5);











is equivalent to:





b = 5;


a = 2 + b;











that means: first assign 5 to variable b and then assign to a the value
2 plus the result of the previous assignment of b (i.e. 5), leaving a
with a final value of 7.





The following expression is also valid in C++:





a = b = c = 5;











It assigns 5 to the all the three variables: a, b and c.








Arithmetic operators ( +, -, *, /, % )


The five arithmetical operations supported by the C++ language are:


+ addition


- subtraction


* multiplication


/ division


% modulo







Operations of addition, subtraction, multiplication and division
literally correspond with their respective mathematical operators. The
only one that you might not be so used to see may be modulo; whose
operator is the percentage sign (%). Modulo is the operation that gives
the remainder of a division of two values. For example, if we write:





a = 11 % 3;











the variable a will contain the value 2, since 2 is the remainder from dividing 11 between 3.

Compound assignment (+=, -=, *=, /=, %=, >>=, <<=, &=, ^=, |=)


When we want to modify the value of a variable by performing an
operation on the value currently stored in that variable we can use
compound assignment operators:





expression is equivalent to


value += increase; value = value + increase;


a -= 5; a = a - 5;


a /= b; a = a / b;


price *= units + 1; price = price * (units + 1);








and the same for all other operators. For example:





// compound assignment operators





#include <iostream>


using namespace std;





int main ()


{


int a, b=3;


a = b;


a+=2; // equivalent to a=a+2


cout << a;


return 0;


}


5














Increase and decrease (++, --)

Shortening even more some expressions, the increase operator (++)
and the decrease operator (--) increase or reduce by one the value
stored in a variable. They are equivalent to +=1 and to -=1,
respectively. Thus:


c++;


c+=1;


c=c+1;











are all equivalent in its functionality: the three of them increase by one the value of c.




In the early C compilers, the three previous expressions probably
produced different executable code depending on which one was used.
Nowadays, this type of code optimization is generally done
automatically by the compiler, thus the three expressions should
produce exactly the same executable code.




A characteristic of this operator is that it can be used both as a
prefix and as a suffix. That means that it can be written either before
the variable identifier (++a) or after it (a++). Although in simple
expressions like a++ or ++a both have exactly the same meaning, in
other expressions in which the result of the increase or decrease
operation is evaluated as a value in an outer expression they may have
an important difference in their meaning: In the case that the increase
operator is used as a prefix (++a) the value is increased before the
result of the expression is evaluated and therefore the increased value
is considered in the outer expression; in case that it is used as a
suffix (a++) the value stored in a is increased after being evaluated
and therefore the value stored before the increase operation is
evaluated in the outer expression. Notice the difference:





Example 1 Example 2


B=3;


A=++B;


// A contains 4, B contains 4 B=3;


A=B++;


// A contains 3, B contains 4







In Example 1, B is increased before its value is copied to A. While
in Example 2, the value of B is copied to A and then B is increased.







Relational and equality operators ( ==, !=, >, <, >=, <= )


In order to evaluate a comparison between two expressions we can use
the relational and equality operators. The result of a relational
operation is a Boolean value that can only be true or false, according
to its Boolean result.




We may want to compare two expressions, for example, to know if
they are equal or if one is greater than the other is. Here is a list
of the relational and equality operators that can be used in C++:




== Equal to


!= Not equal to


> Greater than


< Less than


>= Greater than or equal to


<= Less than or equal to








Here there are some examples:





(7 == 5) // evaluates to false.


(5 > 4) // evaluates to true.


(3 != 2) // evaluates to true.


(6 >= 6) // evaluates to true.


(5 < 5) // evaluates to false.











Of course, instead of using only numeric constants, we can use any
valid expression, including variables. Suppose that a=2, b=3 and c=6,





(a == 5) // evaluates to false since a is not equal to 5.


(a*b >= c) // evaluates to true since (2*3 >= 6) is true.


(b+4 > a*c) // evaluates to false since (3+4 > 2*6) is false.


((b=2) == a) // evaluates to true.










Be careful! The operator = (one equal sign) is not the same as the
operator == (two equal signs), the first one is an assignment operator
(assigns the value at its right to the variable at its left) and the
other one (==) is the equality operator that compares whether both
expressions in the two sides of it are equal to each other. Thus, in
the last expres​sion((b=2) == a), we first assigned the value 2 to b
and then we compared it to a, that also stores the value 2, so the
result of the operation is true.








Logical operators ( !, &&, || )

The Operator ! is the C++ operator to perform the Boolean operation
NOT, it has only one operand, located at its right, and the only thing
that it does is to inverse the value of it, producing false if its
operand is true and true if its operand is false. Basically, it returns
the opposite Boolean value of evaluating its operand. For example:





!(5 == 5) // evaluates to false because the expression at its right (5 == 5) is true.


!(6 <= 4) // evaluates to true because (6 <= 4) would be false.


!true // evaluates to false


!false // evaluates to true.










The logical operators && and || are used when evaluating
two expressions to obtain a single relational result. The operator
&& corresponds with Boolean logical operation AND. This
operation results true if both its two operands are true, and false
otherwise. The following panel shows the result of operator &&
evaluating the expression a && b:





&& OPERATOR a b a && b


true true true


true false false


false true false


false false false







The operator || corresponds with Boolean logical operation OR. This
operation results true if either one of its two operands is true, thus
being false only when both operands are false themselves. Here are the
possible results of a || b:





|| OPERATOR a b a || b


true true true


true false true


false true true


false false false








For example:





( (5 == 5) && (3 > 6) ) // evaluates to false ( true && false ).


( (5 == 5) || (3 > 6) ) // evaluates to true ( true || false ).














Conditional operator ( ? )

The conditional operator evaluates an expression returning a value
if that expression is true and a different one if the expression is
evaluated as false. Its format is:




condition ? result1 : result2








If condition is true the expression will return result1, if it is not it will return result2.





7==5 ? 4 : 3 // returns 3, since 7 is not equal to 5.


7==5+2 ? 4 : 3 // returns 4, since 7 is equal to 5+2.


5>3 ? a : b // returns the value of a, since 5 is greater than 3.


a>b ? a : b // returns whichever is greater, a or b.











// conditional operator





#include <iostream>


using namespace std;





int main ()


{


int a,b,c;





a=2;


b=7;


c = (a>b) ? a : b;





cout << c;





return 0;


}


7











In this example a was 2 and b was 7, so the expression being evaluated
(a>b) was not true, thus the first value specified after the
question mark was discarded in favor of the second value (the one after
the colon) which was b, with a value of 7.








Comma operator ( , )

The comma operator (,) is used to separate two or more expressions
that are included where only one expression is expected. When the set
of expressions has to be evaluated for a value, only the rightmost
expression is considered.


For example, the following code:





a = (b=3, b+2);










Would first assign the value 3 to b, and then assign b+2 to
variable a. So, at the end, variable a would contain the value 5 while
variable b would contain value 3.








Bitwise Operators ( &, |, ^, ~, <<, >> )


Bitwise operators modify variables considering the bit patterns that represent the values they store.





operator asm equivalent description


& AND Bitwise AND


| OR Bitwise Inclusive OR


^ XOR Bitwise Exclusive OR


~ NOT Unary complement (bit inversion)


<< SHL Shift Left


>> SHR Shift Right











Explicit type casting operator

Type casting operators allow you to convert a datum of a given type
to another. There are several ways to do this in C++. The simplest one,
which has been inherited from the C language, is to precede the
expression to be converted by the new type enclosed between parentheses
(()):


int i;


float f = 3.14;


i = (int) f;










The previous code converts the float number 3.14 to an integer
value (3), the remainder is lost. Here, the typecasting operator was
(int). Another way to do the same thing in C++ is using the functional
notation: preceding the expression to be converted by the type and
enclosing the expression between parentheses:





i = int ( f );











Both ways of type casting are valid in C++.








sizeof()

This operator accepts one parameter, which can be either a type or
a variable itself and returns the size in bytes of that type or object:


a = sizeof (char);











This will assign the value 1 to a because char is a one-byte long type.


The value returned by sizeof is a constant, so it is always determined before program execution.








Other operators

Later in these tutorials, we will see a few more operators, like
the ones referring to pointers or the specifics for object-oriented
programming. Each one is treated in its respective section.





Precedence of operators

When writing complex expressions with several operands, we may have
some doubts about which operand is evaluated first and which later. For
example, in this expression:


a = 5 + 7 % 2











we may doubt if it really means:





a = 5 + (7 % 2) // with a result of 6, or


a = (5 + 7) % 2 // with a result of 0










The correct answer is the first of the two expressions, with a
result of 6. There is an established order with the priority of each
operator, and not only the arithmetic ones (those whose preference come
from mathematics) but for all the operators which can appear in C++.
From greatest to lowest priority, the priority order is as follows:




Level Operator Description Grouping


1 :: scope Left-to-right


2 () [] . -> ++ -- dynamic_cast static_cast reinterpret_cast const_cast typeid postfix Left-to-right


3 ++ -- ~ ! sizeof new delete unary (prefix) Right-to-left


* & indirection and reference (pointers)


+ - unary sign operator


4 (type) type casting Right-to-left


5 .* ->* pointer-to-member Left-to-right


6 * / % multiplicative Left-to-right


7 + - additive Left-to-right


8 << >> shift Left-to-right


9 < > <= >= relational Left-to-right


10 == != equality Left-to-right


11 & bitwise AND Left-to-right


12 ^ bitwise XOR Left-to-right


13 | bitwise OR Left-to-right


14 && logical AND Left-to-right


15 || logical OR Left-to-right


16 ?: conditional Right-to-left


17 = *= /= %= += -= >>= <<= &= ^= != assignment Right-to-left


18 , comma Left-to-right








Grouping defines the precedence order in which operators are evaluated
in the case that there are several operators of the same level in an
expression.




All these precedence levels for operators can be manipulated or
become more legible by removing possible ambiguities using parentheses
signs ( and ), as in this example:





a = 5 + 7 % 2;











might be written either as:





a = 5 + (7 % 2);





or


a = (5 + 7) % 2;











depending on the operation that we want to perform.




So if you want to write complicated expressions and you are not
completely sure of the precedence levels, always include parentheses.
It will also become a code easier to read.

Basic Input/Output Published by No-Danger


Last update on Jan 26, 2007 at 12:03pm





Until now, the example programs of previous sections provided very
little interaction with the user, if any at all. Using the standard
input and output library, we will be able to interact with the user by
printing messages on the screen and getting the user's input from the
keyboard.

C++ uses a convenient abstraction called streams to perform input
and output operations in sequential media such as the screen or the
keyboard. A stream is an object where a program can either insert or
extract characters to/from it. We do not really need to care about many
specifications about the physical media associated with the stream - we
only need to know it will accept or provide characters sequentialy.





The standard C++ library includes the header file iostream, where the standard input and output stream objects are declared.








Standard Output (cout)


By default, the standard output of a program is the screen, and the C++ stream object defined to access it is cout.


cout is used in conjunction with the insertion operator, which is written as << (two "less than" signs).





cout << "Output sentence"; // prints Output sentence on screen


cout << 120; // prints number 120 on screen


cout << x; // prints the content of x on screen










The << operator inserts the data that follows it into the
stream preceding it. In the examples above it inserted the constant
string Output sentence, the numerical constant 120 and variable x into
the standard output stream cout. Notice that the sentence in the first
instruction is enclosed between double quotes (") because it is a
constant string of characters. Whenever we want to use constant strings
of characters we must enclose them between double quotes (") so that
they can be clearly distinguished from variable names. For example,
these two sentences have very different results:





cout << "Hello"; // prints Hello


cout << Hello; // prints the content of Hello variable











The insertion operator (<<) may be used more than once in a single statement:





cout << "Hello, " << "I am " << "a C++ statement";










This last statement would print the message Hello, I am a C++
statement on the screen. The utility of repeating the insertion
operator (<<) is demonstrated when we want to print out a
combination of variables and constants or more than one variable:





cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode;










If we assume the age variable to contain the value 24 and the
zipcode variable to contain 90064 the output of the previous statement
would be:




Hello, I am 24 years old and my zipcode is 90064











It is important to notice that cout does not add a line break after its
output unless we explicitly indicate it, therefore, the following
statements:





cout << "This is a sentence.";


cout << "This is another sentence.";











will be shown on the screen one following the other without any line break between them:





This is a sentence.This is another sentence.







even though we had written them in two different insertions into
cout. In order to perform a line break on the output we must explicitly
insert a new-line character into cout. In C++ a new-line character can
be specified as \n (backslash, n):





cout << "First sentence.\n ";


cout << "Second sentence.\nThird sentence.";











This produces the following output:





First sentence.


Second sentence.


Third sentence.








Additionally, to add a new-line, you may also use the endl manipulator. For example:





cout << "First sentence." << endl;


cout << "Second sentence." << endl;











would print out:





First sentence.


Second sentence.







The endl manipulator produces a newline character, exactly as the
insertion of '\n' does, but it also has an additional behavior when it
is used with buffered streams: the buffer is flushed. Anyway, cout will
be an unbuffered stream in most cases, so you can generally use both
the \n escape character and the endl manipulator in order to specify a
new line without any difference in its behavior.








Standard Input (cin).

The standard input device is usually the keyboard. Handling the
standard input in C++ is done by applying the overloaded operator of
extraction (>>) on the cin stream. The operator must be followed
by the variable that will store the data that is going to be extracted
from the stream. For example:


int age;


cin >> age;










The first statement declares a variable of type int called age, and
the second one waits for an input from cin (the keyboard) in order to
store it in this integer variable.




cin can only process the input from the keyboard once the RETURN
key has been pressed. Therefore, even if you request a single
character, the extraction from cin will not process the input until the
user presses RETURN after the character has been introduced.




You must always consider the type of the variable that you are
using as a container with cin extractions. If you request an integer
you will get an integer, if you request a character you will get a
character and if you request a string of characters you will get a
string of characters.




// i/o example





#include <iostream>


using namespace std;





int main ()


{


int i;


cout << "Please enter an integer value: ";


cin >> i;


cout << "The value you entered is " << i;


cout << " and its double is " << i*2 << ".\n";


return 0;


}


Please enter an integer value: 702


The value you entered is 702 and its double is 1404.











The user of a program may be one of the factors that generate errors
even in the simplest programs that use cin (like the one we have just
seen). Since if you request an integer value and the user introduces a
name (which generally is a string of characters), the result may cause
your program to misoperate since it is not what we were expecting from
the user. So when you use the data input provided by cin extractions
you will have to trust that the user of your program will be
cooperative and that he/she will not introduce his/her name or
something similar when an integer value is requested. A little ahead,
when we see the stringstream class we will see a possible solution for
the errors that can be caused by this type of user input.





You can also use cin to request more than one datum input from the user:





cin >> a >> b;











is equivalent to:





cin >> a;


cin >> b;










In both cases the user must give two data, one for variable a and
another one for variable b that may be separated by any valid blank
separator: a space, a tab character or a newline.








cin and strings


We can use cin to get strings with the extraction operator (>>) as we do with fundamental data type variables:


cin >> mystring;










However, as it has been said, cin extraction stops reading as soon
as if finds any blank space character, so in this case we will be able
to get just one word for each extraction. This behavior may or may not
be what we want; for example if we want to get a sentence from the
user, this extraction operation would not be useful.





In order to get entire lines, we can use the function getline, which is the more recommendable way to get user input with cin:





// cin with strings


#include <iostream>


#include <string>


using namespace std;





int main ()


{


string mystr;


cout << "What's your name? ";


getline (cin, mystr);


cout << "Hello " << mystr << ".\n";


cout << "What is your favorite team? ";


getline (cin, mystr);


cout << "I like " << mystr << " too!\n";


return 0;


}


What's your name? Juan Soulié


Hello Juan Soulié.


What is your favorite team? The Isotopes


I like The Isotopes too!










Notice how in both calls to getline we used the same string
identifier (mystr). What the program does in the second call is simply
to replace the previous content by the new one that is introduced.








stringstream

The standard header file <sstream> defines a class called
stringstream that allows a string-based object to be treated as a
stream. This way we can perform extraction or insertion operations
from/to strings, which is especially useful to convert strings to
numerical values and vice versa. For example, if we want to extract an
integer from a string we can write:


string mystr ("1204");


int myint;


stringstream(mystr) >> myint;










This declares a string object with a value of "1204", and an int
object. Then we use stringstream's constructor to construct an object
of this type from the string object. Because we can use stringstream
objects as if they were streams, we can extract an integer from it as
we would have done on cin by applying the extractor operator (>>)
on it followed by a variable of type int.





After this piece of code, the variable myint will contain the numerical value 1204.





// stringstreams


#include <iostream>


#include <string>


#include <sstream>


using namespace std;





int main ()


{


string mystr;


float price=0;


int quantity=0;





cout << "Enter price: ";


getline (cin,mystr);


stringstream(mystr) >> price;


cout << "Enter quantity: ";


getline (cin,mystr);


stringstream(mystr) >> quantity;


cout << "Total price: " << price*quantity << endl;


return 0;


}


Enter price: 22.25


Enter quantity: 7


Total price: 155.75










In this example, we acquire numeric values from the standard input
indirectly. Instead of extracting numeric values directly from the
standard input, we get lines from the standard input (cin) into a
string object (mystr), and then we extract the integer values from this
string into a variable of type int (myint).




Using this method, instead of direct extractions of integer values,
we have more control over what happens with the input of numeric values
from the user, since we are separating the process of obtaining input
from the user (we now simply ask for lines) with the interpretation of
that input. Therefore, this method is usually preferred to get
numerical values from the user in all programs that are intensive in
user input.