MESSAGE
| DATE | 2010-02-02 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [NYLXS - HANGOUT] C++ Workshop - Introduction
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Conceptial Ideas:
In organizing the concepts of C++ which are being introduced we can view
each of them separately, but studying them it is essential to keep in
mind that they are designed to function together in the construction of
programs which are easier to debug, which allow for cleaner overall
syntax, and which encourage creation of reusable code.
We shall explore:
Data Types (build in and user created)
Pointers and References (and their subtle deference)
Manual Memory Allocation and the 'new' and delete keywords
Class Declaration
Class Definition
Private data
Public data
Object instantiation and access, Class typedef
Class Constructors
Class Destructor
Function or Method Overloading
Operator Overloading
External extension of definitions
Copy Constructors
at the end we'll try to actually make the example class, which is an
extended array.
1) Data Types: DATA DATA DATA, everything is DATA
As we know, C and C++ have built in data type which each variable and
object the language needs to be defined as. Both C and C++ are typed
language.
We have char, int, double, float, pointer, and so on, a complete list
of which is available around the net. A pointer stores an address of
another C++ object. Different hardware and software platforms also
have different sizes that it allocates for these data types, creating an
inconsistency which, as an aside, the Free Software community has tried
to address with the creation of the glib library, part of the GTK
project.
What is less apparent is that with regard to a computer, everything is
data. We have the data that we create and manipulated with
instructions, but the instructions themselves are a form data which we can package
up and save for later use, and feed to the CPU at will. When the CPU
runs out instructions the resulting actions are further instructions
that can be packaged and saved as data. In a word, everything is data,
and how we package that data is what separates one programming language
from the next.
This concept is covered extensively in NYLXS ?Introduction Programming
with Perl? class and since this is an advanced topic, we won't get
much further into this. But we will review the basics of data types and
then look at the new feature that C++ gives which C didn't have in such
a generous way, the ability to easily create new data types easily and
in a reusable fashion.
An integer in the C family on the 32 bit Intel clone architecture is
defined as a marked space in memory of 32 bits in size to represent both
positive and negitive numbers. On the new 64 bit architecture that many
of you might have, I don't know if this still holds true since the word
size of a those machines is 64 bits or 8 bytes.
when you use the declaration
int a = 3456;
The computer your program sets aside 32 bits, 4 bytes, of space in ram
and puts the binary representation of that value in that space. The leftmost
bit is usually the signed bit determining whether the number represented
within is either positive or negative. A signed integer can therefor
have a maximum value of 2,147,483,647 positive or negative. beyond that
you must use a long int, which on 32 bit architecture actually won't
help you, or to use external libraries with other data types defined.
By default, C allows certain syntax with a data typeis. It will
automatically translate it, for example, and on demand, into a char that will print
its representation for functions such as printf or in C++ the cout
object:
printf (?%d\n?, myint);
cout << myint;
It can be combined with operators it can be used with the assignment
operator to fill or initialize its space with data.
int myans,myx = 6,myy =12;
It can be combined with arithmetic operators and have results assigned
accordingly.
myans = myy + myx;
Two of them can be compared.
while (myy < myx){
... ...
}
They can be auto incremented
myy++; ++myy;
and so on....
One data type is actually a serial arraignment of data, that is an
array.
int myarray[100];
This defines an array of 100 elements indexed from zero to ninety-nine.
myy = myarray[4];
assigns the fifth element of our array to our integer variable myy.
One thing you can not do with an array data type is use an assignment
operator on the entire array object.
myarray[] = myarray2[]; //THIS IS AN ERROR
C++ allows up to define our own data types that have all the properties
of the built in ones. It uses the class mechanize, operator
overloading,
and the ?new? keyword to accomplish this.
2) Pointers and References ? Where did I PUT THAT!
When we create data for our program, we ask the program to insert memory
into RAM and to retrieve or assign the data from that memory location
for use. Internally the program keeps track of the symbols and the
memory locations. In fact is you run the program ?nm? on a C or C++
binary it will tell you all the symbols that it has in that binary.
ruben-at-www2:~/cplus> nm file3|less
0804a210 A __bss_start
08048a84 t call_gmon_start
0804a29c b completed.1
0804a0bc d __CTOR_END__
0804a0b4 d __CTOR_LIST__
U __cxa_atexit-at--at-GLIBC_2.1.3
0804a204 D __data_start
0804a204 W data_start
08048ed0 t __do_global_ctors_aux
08048ab0 t __do_global_dtors_aux
0804a208 D __dso_handle
......
But we can also create memory locations that are assignable, and store
a representation of that memory location directly into a variable that
only stores the memory location as data, not the data itself. In c
and C++ this is called pointers and we can use the following syntax to
create them.
int *pt = &myint;
This declares the pointer to an int variable called pt which stores the
address for myint. The syntax int * in a declaration (and ONLY in a
declaration) says make a pointer to an int. The & syntax in front of a
variable myint says don't return the value of the variable, but return
the address of the data stored in the variable itself.
There are functions that return only pointer data. Those functions make
it possible to access memory without the declaration of variables at
all.
There are also declarations that can be made in C and C++ which can
create variables without variable names either.
int (*pt)[10];
This declares a pointer (pt) to an array of 10 integers.
char (*p)[10][100]
This is a pointer which addresses an array of 10 pointers (implied)
to arrays of 100 chars each. Commonly this is know as a point to an
array of 10 strings. The symbolic variable name for an array often
gets automatically cast as a point type.
Most string functions in C return a char pointer for example
char * strtok(char *s1, const char *s2);
This would return an address of a char, which in theory would represent
an array of chars. In use it would look like this
char * spt; char wd1[100] = 'hello world', wd2[100] = ' ';
spt = strtok(wd1,wd2); printf(?%s\n?, spt);
Manual Memory Allocation and the 'new' and delete keywords
C++ makes it very convent to create dynamically allocated memory which
is accessed by pointers. We might call these anonymous pointers because
they do not point to any variables, just defined memory. We do this
with the key word ?new?.
int *pt = new int(124);
This creates a new int pointer called pt and assigns to the memory
pointed to by pt with the integer value 124.
delete pt;
deletes the anonymous pointer pt.
int *pt = new int[100];
This declaration creates a new int pointer to an array of 100 integers,
with no data assigned yet to that block of memory.
delete [] pt;
deletes the entire array pointed to by pt and then undefines pt.
Ruben
--
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