C++
C++ Programming Language Quick Summary
C++ is a programming language of many different dialects, similar to the way
that each spoken language has many different dialects. In C++, dialects are not
because the speakers live in the North or South. Instead, it is because there
are many different compilers that support slightly different features. There are
several common compilers: in particular,
#include <iostream>
using namespace std;
int main()
{
cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";
cin.get();
}
Let's look at the elements of the program. The #include is a "preprocessor"
directive that tells the compiler to put code from the header called iostream
into our program before actually creating the executable. By including header
files, you an gain access to many different functions. For example, the cout
function requires iostream. Following the include is the statement, "using
namespace std;". This line tells the compiler to use a group of functions that
are part of the standard library (std). By including this line at the top of a
file, you allow the program to use functions such as cout. The semicolon is part
of the syntax of C and C++. It tells the compiler that you're at the end of a
command. You will see later that the semicolon is used to end most commands in
C++.
The next imporant line is int main(). This line tells the compiler that there is
a function named main, and that the function returns an integer, hence int. The
"curly braces" ({ and }) signal the beginning and end of functions and other
code blocks. If you have programmed in Pascal, you will know them as BEGIN and
END. Even if you haven't programmed in Pascal, this is a good way to think about
their meaning.
The next line of the program may seem strange. If you have programmed in another
language, you might expect that print would be the function used to display
text. In C++, however, the cout object is used to display text. It uses the <<
symbols, known as "insertion operators", to indicate what to output. cout<<
results in a function call with the ensuing text as an argument to the function.
The quotes tell the compiler that you want to output the literal string as-is.
The '\n' sequence is actually treated as a single character that stands for a
newline (we'll talk about this later in more detail). It moves the cursor on
your screen to the next line. Again, notice the semicolon: it is added onto the
end of all, such as function calls, in C++.
The next command is cin.get(). This is another function call: it reads in input
and expects the user to hit the return key. Many compiler environments will open
a new console window, run the program, and then close the window. This command
keeps that window from closing because the program is not done yet because it
waits for you to hit enter. Including that line gives you time to see the
program run.
Upon reaching the end of main, the closing brace, our program will return the
value of 0 (and integer, hence why we told main to return an int) to the
operating system. This return value is important as it can be used to tell the
OS whether our program succeeded or not. A return value of 0 means success and
is returned automatically (but only for main, other functions require you to
manually return a value), but if we wanted to return something else, such as 1,
we would have to do it with a return statement:
#include <iostream>
using namespace std;
int main()
{
cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";
cin.get();
return 1;
}
The final brace closes off the function. You should try compiling this program
and running it. You can cut and paste the code into a file, save it as a .cpp
(or whatever extension your compiler requires) file. If you are using a
command-line compiler, such as Borland C++ 5.5, you should read the compiler
instructions for information on how to compile. Otherwise compiling and running
should be as simple as clicking a button with your mouse.
You might start playing around with the cout function and get used to writing
C++.
Comments are critical for all but the most trivial programs and this tutorial
will often use them to explain sections of code. When you tell the compiler a
section of text is a comment, it will ignore it when running the code, allowing
you to use any text you want to describe the real code. To create a comment use
either //, which tells the compiler that the rest of the line is a comment, or
/* and then */ to block off everything between as a comment. Certain compiler
environments will change the color of a commented area, but some will not. Be
certain not to accidentally comment out code (that is, to tell the compiler part
of your code is a comment) you need for the program. When you are learning to
program, it is useful to be able to comment out sections of code in order to see
how the output is affected.
So far you should be able to write a simple program to display information typed
in by you, the programmer and to describe your program with comments. That's
great, but what about interacting with your user? Fortunately, it is also
possible for your program to accept input. The function you use is known as cin,
and is followed by the insertion operator >>.
Of course, before you try to receive input, you must have a place to store that
input. In programming, input and data are stored in variables. There are several
different types of variables; when you tell the compiler you are declaring a
variable, you must include the data type along with the name of the variable.
Several basic types include char, int, and float.
A variable of type char stores a single character, variables of type int store
integers (numbers without decimal places), and variables of type float store
numbers with decimal places. Each of these variable types - char, int, and float
- is each a keyword that you use when you declare a variable.
Sometimes it can be confusing to have multiple variable types when it seems like
some variable types are redundant. Using the right variable size can be
important for making your code readable and for efficiency--some variables
require more memory than others. For now, suffice it to say that the different
variable types will almost all be used!
To declare a variable you use the syntax type <name>. It is permissible to
declare multiple variables of the same type on the same line; each one should be
separated by a comma. The declaration of a variable or set of variables should
be followed by a semicolon (Note that this is the same procedure used when you
call a function). If you attempt to use an undefined variable, your program will
not run, and you will receive an error message informing you that you have made
a mistake. Don't forget that variables, just like keywords, are case-sensitive,
so it's best to use a consistent capitalization scheme to avoid these errors.
Here are some variable declaration examples:
int x;
int a, b, c, d;
char letter;
float the_float;
While you can have multiple variables of the same type, you cannot have multiple
variables with the same name. Moreover, you cannot have variables and functions
with the same name.
Here is a sample program demonstrating the use a a variable:
#include <iostream>
using namespace std;
int main()
{
int thisisanumber;
cout<<"Please enter a number: ";
cin>> thisisanumber;
cin.ignore();
cout<<"You entered: "<< thisisanumber <<"\n";
cin.get();
}
Let's break apart this program and examine it line by line. The keyword int
declares thisisanumber to be an integer. The function cin>> reads a value into
thisisanumber; the user must press enter before the number is read by the
program. cin.ignore() is another function that reads and discards a character.
Remember that when you type intput into a program, it takes the enter key too.
We don't need this, so we throw it away. Keep in mind that the variable was
declared an integer; if the user attempts to type in a decimal number, it will
be truncated (that is, the decimal component of the number will be ignored). Try
typing in a sequence of characters or a decimal number when you run the example
program; the response will vary from input to input, but in no case is it
particularly pretty. Notice that when printing out a variable quotation marks
are not used. Were there quotation marks, the output would be "You Entered:
thisisanumber." The lack of quotation marks informs the compiler that there is a
variable, and therefore that the program should check the value of the variable
in order to replace the variable name with the variable when executing the
output function. Do not be confused by the inclusion of two separate insertion
operators on one line. Including multiple insertion operators on one line is
perfectly acceptable and all of the output will go to the same place. In fact,
you must separate string literals (strings enclosed in quotation marks)
and variables by giving each its own insertion operators (<<). Trying to put two
variables together with only one << will give you an error message, do not try
it. Do not forget to end functions and declarations with a semicolon. If you
forget the semicolon, the compiler will give you an error message when you
attempt to compile the program.
Of course, no matter what type you use, variables are uninteresting without the
ability to modify them. Several operators used with variables include the
following: *, -, +, /, =, ==, >, <. The * multiplies, the - subtracts, and the +
adds. It is of course important to realize that to modify the value of a
variable inside the program it is rather important to use the equal sign. In
some languages, the equal sign compares the value of the left and right values,
but in C++ == is used for that task. The equal sign is still extremely useful.
It sets the left input to the equal sign, which must be one, and only one,
variable equal to the value on the right side of the equal sign. The operators
that perform mathematical functions should be used on the right side of an equal
sign in order to assign the result to a variable on the left side.
Here are a few examples:
a = 4 * 6; // (Note use of comments and of semicolon) a is 24
a = a + 5; // a equals the original value of a with five added to it
a == 5 // Does NOT assign five to a. Rather, it checks to see if a equals 5.
The other form of equal, ==, is not a way to assign a value to a variable.
Rather, it checks to see if the variables are equal. It is useful in other areas
of C++; for example, you will often use == in such constructions as conditional
statements and loops. You can probably guess how < and > function. They are
greater than and less than operators.
For example:
a < 5 // Checks to see if a is less than five
a > 5 // Checks to see if a is greater than five
a == 5 // Checks to see if a equals five, for good measure
Without a conditional statement such as the if statement, programs would run
almost the exact same way every time. If statements allow the flow of the
program to be changed, and so they allow algorithms and more interesting code.
Before discussing the actual structure of the if statement, let us examine the
meaning of TRUE and FALSE in computer terminology. A true statement is one that
evaluates to a nonzero number. A false statement evaluates to zero. When you
perform comparison with the relational operators, the operator will return 1 if
the comparison is true, or 0 if the comparison is false. For example, the check
0 == 2 evaluates to 0. The check 2 == 2 evaluates to a 1. If this confuses you,
try to use a cout statement to output the result of those various comparisons
(for example cout<< ( 2 == 1 );)
When programming, the aim of the program will often require the checking of one
value stored by a variable against another value to determine whether one is
larger, smaller, or equal to the other.
There are a number of operators that allow these checks.
Here are the relational operators, as they are known, along with examples:
> greater than 5 > 4 is TRUE
< less than 4 < 5 is TRUE
>= greater than or equal 4 >= 4 is TRUE
<= less than or equal 3 <= 4 is TRUE
== equal to 5 == 5 is TRUE
!= not equal to 5 != 4 is TRUE
It is highly probable that you have seen these before, probably with slightly
different symbols. They should not present any hindrance to understanding. Now
that you understand TRUE and FALSE in computer terminology as well as the
comparison operators, let us look at the actual structure of if statements.
The structure of an if statement is as follows:
if ( TRUE )
Execute the next statement
To have more than one statement execute after an if statement that evaluates to
true, use braces, like we did with the body of a function. Anything inside
braces is called a compound statement, or a block.
For example:
if ( TRUE ) {
Execute all statements inside the braces
}
There is also the else statement. The code after it (whether a single line or
code between brackets) is executed if the if statement is FALSE.
It can look like this:
if ( TRUE ) {
// Execute these statements if TRUE
}
else {
// Execute these statements if FALSE
}
One use for else is if there are two conditional statements that may both
evaluate to true, yet you wish only one of the two to have the code block
following it to be executed. You can use an else if after the if statement; that
way, if the first statement is true, the else if will be ignored, but if the if
statement is false, it will then check the condition for the else if statement.
If the if statement was true the else statement will not be checked. It is
possible to use numerous else if statements.
Let's look at a simple program for you to try out on your own.
#include <iostream>
using namespace std;
int main() // Most important part of the program!
{
int age; // Need a variable...
cout<<"Please input your age: "; // Asks for age
cin>> age; // The input is put in age
cin.ignore(); // Throw away enter
if ( age < 100 ) { // If the age is less than 100
cout<<"You are pretty young!\n"; // Just to show you it works...
}
else if ( age == 100 ) { // I use else just to show an example
cout<<"You are old\n"; // Just to show you it works...
}
else {
cout<<"You are really old\n"; // Executed if no other statement is
}
cin.get();
}
Boolean operators allow you to create more complex conditional statements. For
example, if you wish to check if a variable is both greater than five and less
than ten, you could use the boolean AND to ensure both var > 5 and var < 10 are
true. In the following discussion of boolean operators, I will capitalize the
boolean operators in order to distinguish them from normal english. The actual
C++ operators of equivalent function will be described further into the tutorial
- the C++ symbols are not: OR, AND, NOT, although they are of equivalent
function.
When using if statements, you will often wish to check multiple different
conditions. You must understand the Boolean operators OR, NOT, and AND. The
boolean operators function in a similar way to the comparison operators: each
returns 0 if evaluates to FALSE or 1 if it evaluates to TRUE.
NOT: The NOT operator accepts one input. If that input is TRUE, it returns
FALSE, and if that input is FALSE, it returns TRUE. For example, NOT (1)
evalutes to 0, and NOT (0) evalutes to 1. NOT (any number but zero) evaluates to
0. In C and C++ NOT is written as !. NOT is evaluated prior to both AND and OR.
AND: This is another important command. AND returns TRUE if both inputs are TRUE
(if 'this' AND 'that' are true). (1) AND (0) would evaluate to zero because one
of the inputs is false (both must be TRUE for it to evaluate to TRUE). (1) AND
(1) evaluates to 1. (any number but 0) AND (0) evaluates to 0. The AND operator
is written && in C++. Do not be confused by thinking it checks equality between
numbers: it does not. Keep in mind that the AND operator is evaluated before the
OR operator.
OR: Very useful is the OR statement! If either (or both) of the two values it
checks are TRUE then it returns TRUE. For example, (1) OR (0) evaluates to 1.
(0) OR (0) evaluates to 0. The OR is written as || in C++. Those are the pipe
characters. On your keyboard, they may look like a stretched colon. On my
computer the pipe shares its key with \. Keep in mind that OR will be evaluated
after AND.
It is possible to combine several boolean operators in a single statement; often
you will find doing so to be of great value when creating complex expressions
for if statements. What is !(1 && 0)? Of course, it would be TRUE. It is true is
because 1 && 0 evaluates to 0 and !0 evaluates to TRUE (ie, 1).
Try some of these - they're not too hard. If you have questions about them, feel
free to stop by our forums.
A. !( 1 || 0 ) ANSWER: 0
B. !( 1 || 1 && 0 ) ANSWER: 0 (AND is evaluated before OR)
C. !( ( 1 || 0 ) && 0 ) ANSWER: 1 (Parenthesis are useful)
Loops are used to repeat a block of code. Being able to have your program repeatedly execute a block of code is one of the most basic but useful tasks in programming -- many programs or websites that produce extremely complex output (such as a message board) are really only executing a single task many times. (They may be executing a small number of tasks, but in principle, to produce a list of messages only requires repeating the operation of reading in some data and displaying it.) Now, think about what this means: a loop lets you write a very simple statement to produce a significantly greater result simply by repetition.
One Caveat: before going further, you should understand the concept of C++'s
true and false, because it will be necessary when working with loops (the
conditions are the same as with if statements). There are three types of loops:
for, while, and do..while. Each of them has their specific uses. They are all
outlined below.
FOR - for loops are the most useful type. The syntax for a for loop is
for ( variable initialization; condition; variable update ) {
Code to execute while the condition is true
}
The variable initialization allows you to either declare a variable and give it
a value or give a value to an already existing variable. Second, the condition
tells the program that while the conditional expression is true the loop should
continue to repeat itself. The variable update section is the easiest way for a
for loop to handle changing of the variable. It is possible to do things like
x++, x = x + 10, or even x = random ( 5 ), and if you really wanted to, you
could call other functions that do nothing to the variable but still have a
useful effect on the code. Notice that a semicolon separates each of these
sections, that is important. Also note that every single one of the sections may
be empty, though the semicolons still have to be there. If the condition is
empty, it is evaluated as true and the loop will repeat until something else
stops it.
Example:
#include <iostream>
using namespace std; // So the program can see cout and endl
int main()
{
// The loop goes while x < 10, and x increases by one every loop
for ( int x = 0; x < 10; x++ ) {
// Keep in mind that the loop condition checks
// the conditional statement before it loops again.
// consequently, when x equals 10 the loop breaks.
// x is updated before the condition is checked.
cout<< x <<endl;
}
cin.get();
}
This program is a very simple example of a for loop. x is set to zero, while x
is less than 10 it calls cout<< x <<endl; and it adds 1 to x until the condition
is met. Keep in mind also that the variable is incremented after the code in the
loop is run for the first time.
WHILE - WHILE loops are very simple. The basic structure is
while ( condition ) { Code to execute while the condition is true } The true
represents a boolean expression which could be x == 1 or while ( x != 7 ) (x
does not equal 7). It can be any combination of boolean statements that are
legal. Even, (while x ==5 || v == 7) which says execute the code while x equals
five or while v equals 7. Notice that a while loop is the same as a for loop
without the initialization and update sections. However, an empty condition is
not legal for a while loop as it is with a for loop.
Example:
#include <iostream>
using namespace std; // So we can see cout and endl
int main()
{
int x = 0; // Don't forget to declare variables
while ( x < 10 ) { // While x is less than 10
cout<< x <<endl;
x++; // Update x so the condition can be met eventually
}
cin.get();
}
This was another simple example, but it is longer than the above FOR loop. The
easiest way to think of the loop is that when it reaches the brace at the end it
jumps back up to the beginning of the loop, which checks the condition again and
decides whether to repeat the block another time, or stop and move to the next
statement after the block.
DO..WHILE - DO..WHILE loops are useful for things that want to loop at least
once. The structure is
do {
} while ( condition );
Notice that the condition is tested at the end of the block instead of the
beginning, so the block will be executed at least once. If the condition is
true, we jump back to the beginning of the block and execute it again. A
do..while loop is basically a reversed while loop. A while loop says "Loop while
the condition is true, and execute this block of code", a do..while loop says
"Execute this block of code, and loop while the condition is true".
Example:
#include <iostream>
using namespace std;
int main()
{
int x;
x = 0;
do {
// "Hello, world!" is printed at least one time
// even though the condition is false
cout<<"Hello, world!\n";
} while ( x != 0 );
cin.get();
}
Keep in mind that you must include a trailing semi-colon after the while in the above example. A common error is to forget that a do..while loop must be terminated with a semicolon (the other loops should not be terminated with a semicolon, adding to the confusion). Notice that this loop will execute once, because it automatically executes before checking the condition.
Functions that a programmer writes will generally require a prototype. Just like
a blueprint, the prototype tells the compiler what the function will return,
what the function will be called, as well as what arguments the function can be
passed. When I say that the function returns a value, I mean that the function
can be used in the same manner as a variable would be. For example, a variable
can be set equal to a function that returns a value between zero and four.
For example:
#include <cstdlib> // Include rand()
using namespace std; // Make rand() visible
int a = rand(); // rand is a standard function that all compilers have
Do not think that 'a' will change at random, it will be set to the value
returned when the function is called, but it will not change again.
The general format for a prototype is simple:
return-type function_name ( arg_type arg1, ..., arg_type argN );
arg_type just means the type for each argument -- for instance, an int, a float,
or a char. It's exactly the same thing as what you would put if you were
declaring a variable.
There can be more than one argument passed to a function or none at all (where
the parentheses are empty), and it does not have to return a value. Functions
that do not return values have a return type of void. Lets look at a function
prototype:
int mult ( int x, int y );
This prototype specifies that the function mult will accept two arguments, both
integers, and that it will return an integer. Do not forget the trailing
semi-colon. Without it, the compiler will probably think that you are trying to
write the actual definition of the function.
When the programmer actually defines the function, it will begin with the
prototype, minus the semi-colon. Then there should always be a block with the
code that the function is to execute, just as you would write it for the main
function. Any of the arguments passed to the function can be used as if they
were declared in the block. Finally, end it all with a cherry and a closing
brace. Okay, maybe not a cherry.
Lets look at an example program:
#include <iostream>
using namespace std;
int mult ( int x, int y );
int main()
{
int x;
int y;
cout<<"Please input two numbers to be multiplied: ";
cin>> x >> y;
cin.ignore();
cout<<"The product of your two numbers is "<< mult ( x, y ) <<"\n";
cin.get();
}
int mult ( int x, int y )
{
return x * y;
}
This program begins with the only necessary include file and a directive to make
the std namespace visible. Everything in the standard headers is inside of the
std namespace and not visible to our programs unless we make them so. Next is
the prototype of the function. Notice that it has the final semi-colon! The main
function returns an integer, which you should always have to conform to the
standard. You should not have trouble understanding the input and output
functions. It is fine to use cin to input to variables as the program does. But
when typing in the numbers, be sure to separate them by a space so that cin can
tell them apart and put them in the right variables.
Notice how cout actually outputs what appears to be the mult function. What is
really happening is cout is printing the value returned by mult, not mult
itself. The result would be the same as if we had use this print instead
cout<<"The product of your two numbers is "<< x * y <<"\n";
The mult function is actually defined below main. Due to its prototype being
above main, the compiler still recognizes it as being defined, and so the
compiler will not give an error about mult being undefined. As long as the
prototype is present, a function can be used even if there is no definition.
However, the code cannot be run without a definition even though it will
compile. The prototype and definition can be combined into one also. If mult
were defined before it is used, we could do away with the prototype because the
definition can act as a prototype as well.
Return is the keyword used to force the function to return a value. Note that it
is possible to have a function that returns no value. If a function returns
void, the retun statement is valid, but only if it does not have an expression.
In otherwords, for a function that returns void, the statement "return;" is
legal, but redundant.
The most important functional (Pun semi-intended) question is why do we need a
function? Functions have many uses. For example, a programmer may have a block
of code that he has repeated forty times throughout the program. A function to
execute that code would save a great deal of space, and it would also make the
program more readable. Also, having only one copy of the code makes it easier to
make changes. Would you rather make forty little changes scattered all
throughout a potentially large program, or one change to the function body? So
would I.
Another reason for functions is to break down a complex program into logical
parts. For example, take a menu program that runs complex code when a menu
choice is selected. The program would probably best be served by making
functions for each of the actual menu choices, and then breaking down the
complex tasks into smaller, more manageable tasks, which could be in their own
functions. In this way, a program can be designed that makes sense when read.
And has a structure that is easier to understand quickly. The worst programs
usually only have the required function, main, and fill it with pages of jumbled
code.
Switch case statements are a substitute for long if statements that compare a variable to several "integral" values ("integral" values are simply values that can be expressed as an integer, such as the value of a char). The basic format for using switch case is outlined below. The value of the variable given into switch is compared to the value following each of the cases, and when one value matches the value of the variable, the computer continues executing the program from that point.
switch ( <variable> ) {
case this-value:
Code to execute if <variable> == this-value
break;
case that-value:
Code to execute if <variable> == that-value
break;
...
default:
Code to execute if <variable> does not equal the value following any of the cases
break;
}
The condition of a switch statement is a value. The case says that if it has the value of whatever is after that case then do whatever follows the colon. The break is used to break out of the case statements. Break is a keyword that breaks out of the code block, usually surrounded by braces, which it is in. In this case, break prevents the program from falling through and executing the code in all the other case statements. An important thing to note about the switch statement is that the case values may only be constant integral expressions. Sadly, it isn't legal to use case like this:
int a = 10;
int b = 10;
int c = 20;
switch ( a ) {
case b:
// Code
break;
case c:
// Code
break;
default:
// Code
break;
}
The default case is optional, but it is wise to include it as it handles any
unexpected cases. Switch statements serves as a simple way to write long if
statements when the requirements are met. Often it can be used to process input
from a user.
Below is a sample program, in which not all of the proper functions are actually
declared, but which shows how one would use switch in a program.
#include <iostream>
using namespace std;
void playgame();
void loadgame();
void playmultiplayer();
int main()
{
int input;
cout<<"1. Play game\n";
cout<<"2. Load game\n";
cout<<"3. Play multiplayer\n";
cout<<"4. Exit\n";
cout<<"Selection: ";
cin>> input;
switch ( input ) {
case 1: // Note the colon, not a semicolon
playgame();
break;
case 2: // Note the colon, not a semicolon
loadgame();
break;
case 3: // Note the colon, not a semicolon
playmultiplayer();
break;
case 4: // Note the colon, not a semicolon
cout<<"Thank you for playing!\n";
break;
default: // Note the colon, not a semicolon
cout<<"Error, bad input, quitting\n";
break;
}
cin.get();
}
This program will compile, but cannot be run until the undefined functions are given bodies, but it serves as a model (albeit simple) for processing input. If you do not understand this then try mentally putting in if statements for the case statements. Default simply skips out of the switch case construction and allows the program to terminate naturally. If you do not like that, then you can make a loop around the whole thing to have it wait for valid input. You could easily make a few small functions if you wish to test the code.
Pointers are aptly named: they "point" to locations in memory. Think of a row of
safety deposit boxes of various sizes at a local bank. Each safety deposit box
will have a number associated with it so that the teller can quickly look it up.
These numbers are like the memory addresses of variables. A pointer in the world
of safety deposit box would simply be anything that stored the number of another
safety deposit box. Perhaps you have a rich uncle who stored valuables in his
safety deposit box, but decided to put the real location in another, smaller,
safety deposit box that only stored a card with the number of the large box with
the real jewelery. The safety deposit box with the card would be storing the
location of another box; it would be equivalent to a pointer. In the computer,
pointers are just variables that store memory addresses, usually the addresses
of other variables.
The cool thing is that once you can talk about the address of a variable, you'll
then be able to go to that address and retrieve the data stored in it. If you
happen to have a huge piece of data that you want to pass into a function, it's
a lot easier to pass its location to the function than to copy every element of
the data! Moreover, if you need more memory for your program, you can request
more memory from the system--how do you get "back" that memory? The system tells
you where it is located in memory; that is to say, you get a memory address
back. And you need pointers to store the memory address.
A note about terms: the word pointer can refer either to a memory address
itself, or to a variable that stores a memory address. Usually, the distinction
isn't really that important: if you pass a pointer variable into a function,
you're passing the value stored in the pointer--the memory address. When I want
to talk about a memory address, I'll refer to it as a memory address; when I
want a variable that stores a memory address, I'll call it a pointer. When a
variable stores the address of another variable, I'll say that it is "pointing
to" that variable.
Pointers require a bit of new syntax because when you have a pointer, you need
the ability to request both the memory location it stores and the value stored
at that memory location. Moreover, since pointers are somewhat special, you need
to tell the compiler when you declare your pointer variable that the variable is
a pointer, and tell the compiler what type of memory it points to.
The pointer declaration looks like this:
<variable_type> *<name>;
For example, you could declare a pointer that stores the address of an integer with the following syntax:
int *points_to_integer;
Notice the use of the *. This is the key to declaring a pointer; if you add it directly before the variable name, it will declare the variable to be a pointer. Minor gotcha: if you declare multiple pointers on the same line, you must precede each of them with an asterisk:
// one pointer, one regular int
int *pointer1, nonpointer1;
// two pointers
int *pointer1, *pointer2;
As I mentioned, there are two ways to use the pointer to access information: it is possible to have it give the actual address to another variable. To do so, simply use the name of the pointer without the *. However, to access the actual memory location, use the *. The technical name for this doing this is dereferencing the pointer; in essence, you're taking the reference to some memory address and following it, to retrieve the actual value. It can be tricky to keep track of when you should add the asterisk. Remember that the pointer's natural use is to store a memory address; so when you use the pointer:
call_to_function_expecting_memory_address(pointer);
then it evaluates to the address. You have to add something extra, the asterisk, in order to retrieve the value stored at the address. You'll probably do that an awful lot. Nevertheless, the pointer itself is supposed to store an address, so when you use the bare pointer, you get that address back.
Pointing to Something: Retrieving an Address
In order to have a pointer actually point to another variable it is necessary to
have the memory address of that variable also. To get the memory address of a
variable (its location in memory), put the & sign in front of the variable name.
This makes it give its address. This is called the address-of operator, because
it returns the memory address. Conveniently, both ampersand and address-of start
with a; that's a useful way to remember that you use & to get the address of a
variable.
For example:
#include <iostream>
using namespace std;
int main()
{
int x; // A normal integer
int *p; // A pointer to an integer
p = &x; // Read it, "assign the address of x to p"
cin>> x; // Put a value in x, we could also use *p here
cin.ignore();
cout<< *p <<"\n"; // Note the use of the * to get the value
cin.get();
}
The cout outputs the value stored in x. Why is that? Well, let's look at the
code. The integer is called x. A pointer to an integer is then defined as p.
Then it stores the memory location of x in pointer by using the address-of
operator (&) to get the address of the variable. Using the ampersand is a bit
like looking at the label on the safety deposit box to see its number rather
than looking inside the box, to get what it stores. The user then inputs a
number that is stored in the variable x; remember, this is the same location
that is pointed to by p.
The next line then passes *p into cout. *p performs the "dereferencing"
operation on p; it looks at the address stored in p, and goes to that address
and returns the value. This is akin to looking inside a safety deposit box only
to find the number of (and, presumably, the key to ) another box, which you then
open.
Notice that in the above example, pointer is initialized to point to a specific
memory address before it is used. If this was not the case, it could be pointing
to anything. This can lead to extremely unpleasant consequences to the program.
For instance, the operating system will probably prevent you from accessing
memory that it knows your program doesn't own: this will cause your program to
crash. If it let you use the memory, you could mess with the memory of any
running program--for instance, if you had a document opened in Word, you could
change the text! Fortunately, Windows and other modern operating systems will
stop you from accessing that memory and cause your program to crash. To avoid
crashing your program, you should always initialize pointers before you use
them.
It is also possible to initialize pointers using free memory. This allows
dynamic allocation of array memory. It is most useful for setting up structures
called linked lists. This difficult topic is too complex for this text. An
understanding of the keywords new and delete will, however, be tremendously
helpful in the future.
The keyword new is used to initialize pointers with memory from free store (a
section of memory available to all programs). The syntax looks like the example:
int *ptr = new int;
It initializes ptr to point to a memory address of size int (because variables
have different sizes, number of bytes, this is necessary). The memory that is
pointed to becomes unavailable to other programs. This means that the careful
coder should free this memory at the end of its usage.
The delete operator frees up the memory allocated through new. To do so, the
syntax is as in the example.
delete ptr;
After deleting a pointer, it is a good idea to reset it to point to 0. When 0 is
assigned to a pointer, the pointer becomes a null pointer, in other words, it
points to nothing. By doing this, when you do something foolish with the pointer
(it happens a lot, even with experienced programmers), you find out immediately
instead of later, when you have done considerable damage.
In fact, the concept of the null pointer is frequently used as a way of
indicating a problem--for instance, some functions left over from C return 0 if
they cannot correctly allocate memory (notably, the malloc
function).
You want to be sure to handle this correctly if you ever use malloc or other C
functions that return a "NULL pointer" on failure.
In C++, if a call to new fails because the system is out of memory, then it will
"throw an exception". For the time being, you need not worry too much about this
case, but you can read
more about what happens when new fails.
Pointers may feel like a very confusing topic at first but I think anyone can come to appreciate and understand them. If you didn't feel like you absorbed everything about them, just take a few deep breaths and re-read the lesson. You shouldn't feel like you've fully grasped every nuance of when and why you need to use pointers, though you should have some idea of some of their basic uses.
Lesson 7: Structures (Printable
Version)
Before discussing classes, this lesson will be an introduction to data
structures similar to classes. Structures are a way of storing many different
values in variables of potentially different types under the same name. This
makes it a more modular program, which is easier to modify because its design
makes things more compact. Structs are generally useful whenever a lot of data
needs to be grouped together--for instance, they can be used to hold records
from a database or to store information about contacts in an address book. In
the contacts example, a struct could be used that would hold all of the
information about a single contact--name, address, phone number, and so forth.
The format for defining a structure is
struct Tag {
Members
};
Where Tag is the name of the entire type of structure and Members are the variables within the struct. To actually create a single structure the syntax is
struct Tag name_of_single_structure;
To access a variable of the structure it goes
name_of_single_structure.name_of_variable;
For example:
struct example {
int x;
};
struct example an_example; //Treating it like a normal variable type
an_example.x = 33; //How to access it's members
Here is an example program:
struct database {
int id_number;
int age;
float salary;
};
int main()
{
database employee; //There is now an employee variable that has modifiable
// variables inside it.
employee.age = 22;
employee.id_number = 1;
employee.salary = 12000.21;
}
The struct database declares that database has three variables in it, age, id_number, and salary. You can use database like a variable type like int. You can create an employee with the database type as I did above. Then, to modify it you call everything with the 'employee.' in front of it. You can also return structures from functions by defining their return type as a structure type. For instance:
database fn();
I
will talk only a little bit about unions as well. Unions are like structures
except that all the variables share the same memory. When a union is declared
the compiler allocates enough memory for the largest data-type in the union. Its
like a giant storage chest where you can store one large item, or a small item,
but never the both at the same time.
The '.' operator is used to access different variables inside a union also.
As a final note, if you wish to have a pointer to a structure, to actually
access the information stored inside the structure that is pointed to, you use
the -> operator in place of the . operator. All points about pointers still
apply.
A quick example:
#include <iostream>
using namespace std;
struct xampl {
int x;
};
int main
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